Tag Archives: slab on grade

Allowable Variations in Concrete Slabs on Grade

Allowable Variations in Concrete Slabs on Grade

Growing up, I witnessed my Father forming and pouring four foot squares of concrete for our backyard patio. He would alternate them between smooth finish and exposed aggregate to create a variation in appearance. Oh how he made it look all so easy.

Me – I know very little about concrete finishing, other than I try to avoid it like a plague. Those perfectly finished squares by my Father, I could seriously bungle. If I need concrete finished, I hire a professional.

When hiring a professional, how does one determine if an outcome meets accepted practice? ACI 117-10 American Concrete Institute “Specification for Tolerances for Concrete Construction and Materials” should be included in contract documents with your finisher.

Here are some relevant sections:

2.2.1 In slabs four inches or less in thickness rebar placement from top or bottom of slab shall be within ¼” of specified depth. Over four inches thick, deviation can be 3/8”

2.2.2 Concrete cover of rebar from concrete surface can be no more than 3/8” less than specified.

3.3.2 Drilled piers (poured for wet set brackets) can be as much as 1” too tall or three inches too low.

Here is a big one:

4.4.1 Top surface of slabs (slabs-on-ground) can vary by ¾” up or down. Seems like a lot to me personally.

Paying to have fine grading done?

4.4.5 Fine grade of soil immediately below slabs-on-ground can also vary ¾” up or down.

4.5.4 Thickness of slabs-on-ground, average of four samples for every 500 square feet poured, can vary no more than 3/8”, with maximum variation for any one sample being ¾”.

Table R4.8.6.1 gives methods to determine flatness of a given slab on grade. In simple terms, placing a 10 foot straight edge anywhere on slab should yield a gap no larger than 3/4” to meet a conventional application. For “moderately flat” maximum gap would be 5/8” and for ‘flat’ 3/8”.

Post Frame Footings, Delivery Limitations, and Foundation Types

This Wednesday the Pole Barn Guru addresses reader questions about common gable post frame footings, weight limitations for a building delivery and the possible solutions, and what types of foundations Hansen Buildings can design for in Weld County Colorado.

DEAR POLE BARN GURU: What is a common gable post footing compared to a main truss post footing, where are these located on a building? ELLIOT in PINEHURST

DEAR ELLIOT: Most post frame buildings are rectangular, with peaks (a point or gable) on opposite ends. Building codes require a minimum footing thickness of six inches, or an ICC-ESR approved alternative (like these https://www.hansenpolebuildings.com/2014/05/footingpad/). Main truss post footings will be located along the eave (side snow slides off of) and support roof trusses (hence ‘main truss post’). As they carry more weight than endwall (peak of roof ends) columns, they will typically be larger in diameter. Your building’s engineer sealed plans will specify locations and diameters required to adequately support weight of building (including applied loads). This is not a place to guess or scrimp, as you really wouldn’t be happy with any of your columns settling.

 

DEAR POLE BARN GURU: 50x 50 x 16 pole barn approximate weight, limited by bridge on property? NEIL in TULSA

DEAR NEIL: A far greater issue than weight of your building package (roughly 20,000 pounds total depending upon features) will be weight of trucks making deliveries (many weigh 32-40,000 pounds when empty). We have had many clients in a similar situation to yours and materials can often be offloaded onto a flat trailer you can pull behind a pickup, or similar, in order to get into challenging jobsites. Biggest concern will be 50 foot long roof trusses, as truss truck is going to be a semi pulling usually a 48 foot long trailer. You might want to consider making a donation to your local high school’s football team in order to have them physically pick up and carry individual trusses across bridge and to your site.

 

DEAR POLE BARN GURU: What type of foundations are used in Weld County, CO for pole barn homes? Insulated slab on grade? Crawl space? BRENT in KERSEY

DEAR BRENT: We have provided roughly 300 fully engineered post frame buildings to our clients in Colorado (many of these in Weld County). Types of foundations for post frame homes are nearly as varied as are our clients. We’ve done full or partial basements (including walkout or daylight) in block, poured concrete or ICF; crawl spaces (both conditioned and non-conditioned) as well as slabs on grade (both with heated slabs and under floor insulation or unheated slabs with perimeter insulation). Embedded columns are going to be least expensive and strongest, however we can also design and provide for cases with ICC-ESR approved wet set brackets. With most sites in Colorado, it is beneficial to involve a Geotechnical engineer to do a proper assessment of your site’s soil conditions and bearing capacity in order to assure best outcome. Often jurisdictions will make this a requirement. Here is some extended reading on slabs vs. crawl spaces: https://www.hansenpolebuildings.com/2019/03/slab-on-grade-or-crawl-space/

A House on a Hill, Slab on Grade, and Post Brackets

This week the Pole Barn Guru answers reader questions about building a house on a hill lakeside, building a slab on grade instead or embedded posts due to rocky soil, and use of dry set brackets.

DEAR POLE BARN GURU: Hey. We have a lot on the lake that is on a hill (not super steep but does have a decent slope). Is a post frame house an option and if so how would be the best way to build it as far as ‘foundation’. ASHLEY in BOWLING GREEN

DEAR ASHLEY: A post frame home is certainly an excellent design solution for a slope building site. You do have some options (both of these I used on a personal building site of my own) – you can cut to create a ‘daylight’ or ‘walk-out’, or build on stilts.

Here is how I handled cutting into a hillside: https://www.hansenpolebuildings.com/2012/02/grade-change/ as well as a little reading on stilt houses: https://www.hansenpolebuildings.com/2017/09/stilt-houses/.

 

DEAR POLE BARN GURU: I’m finding it prohibitive with local excavators to set poles in this part of rocky NH. It makes a slab on grade look better than ‘poles’. However – I assume I lose lateral support if I build on a slab will horizontal girts provide enough stiffness to the building or would bottom sills/sheathing be required. I’m looking at a 30×20 shed and would like 10′ bays and metal roofing, local green lumber vertical b&b siding. DAVID in GRANTHAM

DEAR DAVID: Even building with a slab on grade is going to require excavation, as you need to have footings either extending below frost line, or thermally isolated to prevent frost heaving. Horizontal girts, by themself, provide little or no resistance to racking. Properly engineered, your steel roofing and board and batten siding can provide adequate shear resistance.

If it were my own building, I would probably consider excavating holes, place and properly backfill sonotubes, and use ICC-ESR approved wet set brackets to mount columns. I would avoid use of green lumber, in any circumstance: https://www.hansenpolebuildings.com/2011/09/499green-lumber-vs-dry-lumber/.

 

DEAR POLE BARN GURU: I have an existing 35′ x 50′ slab with 16″ beams around the perimeter. Is it possible to construct a pole barn on it that meets windstorm qualifications for my area? ALAN in LAKE JACKSON

DEAR ALAN: There are plenty of folks out there who would gladly be willing to use dry set brackets to mount pole barn columns to your slab. In most instances, this is a less than adequate design solution as these brackets are not designed to withstand moment (bending) loads. (For extended reading, please visit https://www.hansenpolebuildings.com/2014/12/dry-set-column-anchors/).

My preference would be to use embedded columns – either by placing them outside of your existing slab perimeter, or by saw cutting through edges of your existing slab.

 

Unheated Post Frame Building Slabs on Grade

Are Unheated Post Frame Building Slabs on Grade Required to Be Frost Protected?

Reader BILL in CLAYTON writes:

“I’m in early planning for a post frame garage – just over 1000 sf but will reduce it if it solves a code problem for “private garages” in IBC. Ignoring that, where does the code permit a slab on ground floor in a post frame building to not be frost protected? Is it not a part of the “building and structure”? Obviously, the floor in most unheated post frame buildings with slabs are not frost protected. In IRC (which Hansen says does not apply to post frame) R301.1 says “Buildings and structures, and parts thereof…” shall be on a foundation and R403.1.4.1 “Except where otherwise protected from frost, foundation walls, piers and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:…:” Is a slab on ground floor excluded from “foundation walls, piers and other permanent supports of buildings and structures”? The slab on the ground floor is not a part of the building and structure? Thank you!”

IRC R301.1.3 Engineered design.

“When a building of otherwise conventional construction contains structural elements exceeding the limits of Section R301 or otherwise not conforming to this code, these elements shall be designed in accordance with accepted engineering practice. The extent of such design need only demonstrate compliance of nonconventional elements with other applicable provisions and shall be compatible with the performance of the conventional framed system. Engineered design in accordance with the International Building Code is permitted for all buildings and structures, and parts thereof, included in the scope of this code.”

Unless your site is precluded from having a detached accessory building of over 1000 square feet – my recommendation is to erect the largest building you can afford and fit on your property. Whatever size you build, it will not be large enough. Being over 1000 square feet just means you have an S-2 rather than U classification building and is not going to affect structural design unless your Building Official deems your structure to be Risk Category II, rather than I.

Foundations of most post frame buildings are either embedded columns or columns anchored by approved wet set brackets to concrete piers. A slab on grade, in a post frame building with foundation as described, has no weight of building placed upon it, therefore is not a permanent support of structure.

With this said, Jefferson County is in Climate Zone 6A. As such I personally would follow International Energy Code Table R402.1.2 and place R-10 rigid insulation inside of my splash plank from top of slab (3-1/2″ up from bottom of splash plank) extending downward 48 inches. This can easily be done by trenching at time of construction and would be of benefit should building ever be heated (as most strictly non-agricultural buildings usually are at some point) and be a point in eventual resale.

Must Do’s for a Worry Free Barndominium

My Facebook friend RICK in MALDEN messaged me:

“I have never built a building like this. I have seen many bad experiences with concrete, poor quality metal work and many more issues. I would just like to know if there is a list of things to make sure I get a quality home. I saw the other day you said osb under the roof metal could help with condensation issues. I’m also worried about gaps where the metal meets together. I don’t want to just shoot spray insulation and call it good. You have said that is not the way to do it. I guess I’m looking for something to tell me the quality method for the most common mistakes people or contractors make. I am using a contractor because I don’t have the skill or experience to DIY. I also don’t want to rely on the contractor to tell me the right way. If I had it my way I’d have you do it. You are the most knowledgeable and in-depth person I have found on the internet about the Barndo building subject.”

Thank you for all of your kind words. 

There are days when I think what a joy it would be to be out building. I do truly love to build, smells of earth from freshly dug holes, lumber being taken out of a wrapped unit, sawdust – all of these give me warm, fuzzy nostalgic feelings.

I have kept myself in great physical condition and at 63 years-old I could certainly be out building. And I do know there are folks who would gladly pay my rate to have me do their construction. However this would allow me to properly assist only one client at a time. What I do now allows me to help thousands of people every year to get better buildings.

Enough of me waxing poetic – let’s get down to business!

(Side note – much of this advice is expounded upon in detail in previously written articles. Please visit www.HansenPoleBuildings.com, navigate to SEARCH at upper right corner, click on it, type in a word or phrase and ENTER)

Plan tips – consider these factors:

Direction of access (you don’t want to have to drive around your house to get to garage doors).

‘Curb appeal’ – what will people see as they drive up?

Any views? If so, take advantage of them.

North-south alignment – place no or few windows on north wall, lots on south wall.

Overhang on south wall to shade windows from mid-day summer sun If your AC bill is far greater than your heating bill, reverse this and omit or minimize north overhangs.

Slope of site.

Work from inside out – do not try to fit what you need within a pre-ordained box just because someone said using a “standard” size might be cheaper. Differences in dimensions from “standard” are pennies per square foot, not dollars.

Put up the largest building you can economically justify and fit on your property.

Plan for accessibility – 4′ or wider hallways and stairs, an ADA bathroom with a roll in shower. 3′ wide interior doors.

Walk-in (roll-in) closets for bedrooms, even secondary ones.

Consider if you truly want to live on a concrete floor. Crawl spaces are roughly the same investment.

Kitchen – two dishwashers, two microwaves, two ovens, trash compactor. Separate side-by-side refrigerator and freezer units. A good sized pantry.

9′ or 10′ finished ceilings in living areas.

Onto building construction itself….

Probably most important (and most often neglected) is proper site prep.

Make sure there is a vapor barrier under any slab-on-grade (and use 10-15 mil).

For slab-on-grade at least have pex-al-pex tubes run.

Personally I like flash & batt for walls – two inches of closed cell spray foam ideally with BIBs insulation to fill balance of insulation cavity. In this circumstance, you do not want a WRB, however you do want to order inside closures for top and bottom of every wall panel.

Order raised heel trusses so you can blow in fiberglass insulation to full depth from wall-to-wall.

For condensation control – use steel panels with Integral Condensation Control.

Vent sidewall overhangs and ridge.

Use all 5/8″ Type X sheetrock.

Make sure you and your contractor have a written agreement covering everything – it keeps feelings from being hurt and clearly outlines expectations. I will have a series of articles soon outlining some important inclusions for agreements, please watch for them.

How Tall Should My Eave Height Be for Two Stories?

How Tall Should My Eave Height Be for Two Stories?

I have learned a couple of things in 40 years of post frame building construction. One amongst these is – most people are dimensionally challenged (no offense intended).

As much as some folks would like to believe, you cannot legitimately put two full height finished floors in a 16 foot eave height post frame building.

Now fear is a strong motivating force. Perhaps it is fear of a building “appearing” too tall or of OMG it will be too expensive keeping people from considering what it actually takes to create a Building Code conforming two story building.

Back in my early roof truss selling days, I had two clients who had relocated from New York state to North Idaho and were building new homes on adjacent properties. Both of them (and their spouses) were relatively short of stature and had decided to build their homes to Code minimum ceiling heights of seven feet. Their reasoning was it would be less space to heat and cool and they could chop two studs out of 14 foot long materials.

Missed in all of this was how much sheetrock waste would be created!

Sidebar – modern Building Codes allow seven foot ceilings under International Residential Code (IRC), however IBC (International Building Code) requires six more inches.

Now I am vertically challenged at 6’5” and would feel very uncomfortable with seven foot ceilings. In my own personal shouse, most ceilings on both floors are 16 feet high!

In today’s exciting episode we will learn together how tall eave heights should actually be to give reasonable ceilings in post frame buildings.

Setting a “zero point” at exterior grade (and assuming slab on grade for lower floor), top of slab will be at +3.5 inches.

To create eight foot finished ceilings requires 8’ 1-1/8” (allows for 5/8” sheetrock on ceilings).

In order to be able to run utilities (e.g. plumbing and ductwork) through second floor supports, I highly recommend prefabricated wood floor trusses (https://www.hansenpolebuildings.com/2020/01/floor-trusses-for-barndominiums/). Generally truss height will be about an inch for every foot of clearspan with a 12 inch minimum. 

In my own shouse, we have a 48 foot clearspan floor over our basketball court. And yes, those trusses are four feet deep!

Allow ¾ inch for OSB floor sheeting.

6-1/16″ for heel height of trusses with 2×6 top chord at 4/12 slope (provided you are using closed cell spray foam insulation between purlins)

If using blown-in insulation truss heel height should be R value of insulation divided by 3 plus 2″ to allow plenty of eave to ridge air flow above insulation.

At a bare minimum an eave height of 18’ 0-9/16” will be needed to create those eight foot ceilings.

Meeting Barndominium Perimeter Slab Insulation Requirements

Meeting Barndominium Perimeter Slab Insulation Requirements

Our world (at least my world) of post frame buildings has evolved quickly into residential construction of barndominiums, shouses (shop/houses) and post frame homes. Having built two shouses for myself, I have learned a lot about what to do and not to do, as well as receiving helpful contributions from thousands upon thousands of loyal readers such as JOE in BEDFORD who writes:
“Long time reader, first time poster. I’m in the middle of planning & prepping to build a post frame house (48′ x 60′ x 10′) for myself & I have some basic questions on how
to meet both the IRC & IECC codes for the foundation/floor systems. In PA (climate Zone 5) how is it possible to continuously insulate the “footings” (down 3′ – 4′) of my barndominium to prevent frost heave/moisture intrusion/etc? Wouldn’t that require digging a continuous “footing” thus defeating the main purpose of a post frame design?

To add to that thought, most “floors” of post frame houses are slab on grade concrete (with radiant heat in slab I assume), which to meet the IECC code for a heated on grade slab, it requires R-15 down 2′ on the slab edge (plus R-10 for the underslab insulation). See link below:
https://www.phrc.psu.edu/assets/docs/Webinars/SlabInsulation.pdf

>From my understanding, the savings & efficiency of post frame houses comes from not having to excavate, pour & then backfill a continuous footer + stem wall (or footer with a slab on grade floor). How is it possible to meet these challenges & codes with a post frame design method? If you have to excavate a continuous footing & then insulate the footing & the house floor is going to be insulated & poured either way, wouldn’t the “stick frame” method be more cost effective at that point then?

Thanks for the help & clarification!”

Mike the Pole Barn Guru responds:

Appreciate your being a long time reader, hopefully you have found my articles to be informative and entertaining.

Thanks to glories of rigid board insulation, you can still do standard embedded columns, pour a slab on grade and meet insulation requirements to prevent both frost heave and to keep from having to heat ground outside and underneath your building (see drawing). Requirements for insulation and thickness can be found here: https://www.huduser.gov/publications/pdf/fpsfguide.pdf.

Even if you were to opt to pour a continuous footing, post frame construction will still prove to be more cost effective due to elimination of redundant members and structural headers inherent to stick construction. Post frame is easier to super insulate (fewer members touch both exterior and interior surfaces), you can create some unique architectural features not easily done with stick frame construction and you can easily DIY it should you be so inclined.

Top of Barndominium Slab

Where Should the Top of Barndominium Slab Be?

Loyal reader DANIEL in OWENSVILLE writes:

“Mike,

First I want to say thanks for all that I have learned from your Blog. I am confused on a couple of points you made concerning floor height…

“Occasionally we have clients who ask why they can’t run the concrete to the top of the splash plank, as they want to use the splash plank to “screed” the concrete slab top. Using any other measure for the concrete slab top, will result in wall steel and doors not properly fitting, as well as possible interior clear height loss.”

This really is not answering the question… the building could be designed with the door openings, ceiling heights, etc. to compensate for a higher floor height/thicker floor. Request it in the design and build it to the plan.

Also, “Your new Hansen Pole Building has as the bottom horizontal framing member, connecting pressure treated column to pressure treated column, is a pressure preservative treated splash plank. The building design is such so the top of any concrete floor is set at 3-1/2″ above the bottom of the splash plank.” and, In another post you stated the splash plank rests on the finished grade. That would put the finished concrete floor only 3-1/2″ above the finished grade. And below the weep screed, rat guard, any water being shed on the outside of the sheathing, and what codes require for an occupied building.

Please explain if there is any “real” reason for not raising the interior floor to 6 inches or more above grade (as is required for a house)?”

Daniel ~

Thank you for your kind words. Certainly any building could be designed for door openings, ceiling heights, etc., to be adjusted for top of slab on grade to be at any point. This would entail leaving greater amounts of splash plank exposed on exterior beneath siding in order to prevent concrete aprons, sidewalks, driveways, etc., from being poured up against wall steel. Some people find great amounts of splash plank being exposed to be aesthetically unpleasant however. By being consistent in design, it also allows for one set of assembly instructions to be used – rather than having to rely upon making adjustments for whatever custom situation individuals (or their builders) deemed their particular case.

I went back and read through both IRC (International Residential Code) and IBC (International Building Code) codes and there is no requirement for an interior concrete floor to be at six inches or more above grade for an occupied building or a house.

From 2018 IRC R506.1 “Concrete slab-on-ground floors shall be designed and constructed in accordance with the provisions of this section or ACI 332. Floors shall be a minimum 3-1/2 inches thick.”

From 2018 IBC 1907.1 “The thickness of concrete floor slabs supported directly on the ground shall not be less than 3-1/2”

Both of these imply top of concrete floor at 3-1/2″ above ground (grade) is totally acceptable. 

Having been involved in tens of thousands of post frame buildings successfully engineer designed and approved in structural plan reviews leads me to believe how we are doing it both works and is code conforming.

For extended reading on this subject: https://www.hansenpolebuildings.com/2016/05/concrete-floor/ and https://www.hansenpolebuildings.com/2012/02/where-is-the-top-of-the-concrete-slab/.

Slab on Grade or Crawl Space?

Slab on Grade or Crawlspace?

Long-time readers of this column recall seeing a profuse number of articles written in regards to crawl spaces. These articles have been on a gradual increase since this first one six years ago: https://www.hansenpolebuildings.com/2013/03/crawl-space/.

With residential post frame construction becoming rapidly more popular as more people discover this system’s benefits, this debate of slab on grade versus crawl space will continue.

Hansen Pole Buildings’ Senior Designer Wayde recently had a client order a new post frame building kit package with an elevated wood floor (to create a crawl space). After client has placed their building order, Wayde came back to me with this, “Can you tell me the Pros and cons of building this as we designed and sold it vs. lowering it three feet and adding a radiant concrete floor?”

I happen to be a big fan of hydronic radiant floor heat in concrete slabs, we have it in our own building: https://www.hansenpolebuildings.com/2012/08/radiant-floor-heating/.

Biggest pro of “as is” – living upon a wood floor will be so much more comfortable than upon concrete. Wayde’s client could still do radiant floor heat, should they opt to not go with a forced air HVAC system.

Slab on grade the client will have to (or should) do a post frame shallow frost protected foundation: https://www.hansenpolebuildings.com/2019/02/minimizing-excavation-in-post-frame-buildings/. This perimeter rigid board insulation must be covered with rodent proof material.

If I went to slab on grade, I would recommend a minimum R-60 for ceiling, taking a 22 inch deep raised heel truss to allow for adequate depths of blown in insulation. (Read more about raised heel trusses here: https://www.hansenpolebuildings.com/2012/07/raised-heel-trusses/).

For an 8′ finished ceiling, they would then need an eave height of 10′ 2-5/8″. I like taller rooms, so you might want to experiment with eave heights of 11′ 2-5/8″ and 12’2-5/8″ (latter of these will be easier to drywall and will result in least waste).

Making a choice between living on concrete or wood will be one only able to be made prior to time of construction and should not be taken lightly. All factors should be taken into consideration most importantly being what creates a most comfortable living space.

Pole Barn Ignorance

Gordon Miller photo

Pole Barn Ignorance

After spending pretty well my entire adult life in some facet of my industry, it becomes amazing to me to find pole barn ignorance. Other than recent immigrants to our country, and maybe those who have never traveled outside of urban areas most everyone should have some inkling as to what a pole barn or post frame building can be.

Even more astounding to me – when a contractor and designer appear to have very little grasp of “pole barn” construction.

Here’s an excerpt from an Oct. 18, 2018 Waterbury Record article by Caleigh Cross:

“Phil Sweet, a contractor and designer in Waitsfield, planned the barn raising, and designed a barn based off the design of a pole barn.

“Technically, a pole barn would probably be put onto bare land,” and they’re built by using salvage lumber such as old telephone poles, Sweet said.

Instead, Rosina’s new barn has both a literal and metaphorical foundation — it’s built atop the concrete foundation her grandfather had put in two generations ago, Sweet said.

“She wanted to incorporate those historic elements that were still there, and that’s what we did,” he said.

Sweet and the nine other contractors used conventional lumber, instead of salvage, and since the new barn incorporates a foundation, he’s reluctant to call it a pole barn.”

Mike the Pole Barn Guru writes:

By definition (ANSI/ASABE S618 DEC 2010 Post Frame Building System Nomenclature):

“3.1 Post-frame building system: A building characterized by primary structural frames of wood posts as columns and trusses or rafters as roof framing. Roof framing is attached to the posts, either directly or indirectly through girders. Posts are embedded in the soil and supported on isolated footings, or are attached to the top of piers, concrete or masonry walls, or slabs-on-grade.”

Our modern pole building industry wouldn’t consider using of salvage lumber or old telephone poles unless they were being used to satisfy some aesthetic needs of a client.

Raised Floors in Post Frame Homes

Raised Floors Are an Opportunity for Post Frame Homes

Three months after Hurricane Harvey churned through Texas, dumping 51 inches of rain and damaging an estimated 150,000 homes, the state’s most populous county took a bureaucratic step which has huge implications for how it will deal with the risk of future flooding.

On December 5, Harris County, which surrounds the City of Houston, approved an overhaul of its flood rules expanding them from 100-year floodplains—which have a one percent change of flooding in a given year—to 500-year floodplains. The new rules (which don’t apply inside Houston city limits) will compel people building houses in some areas to elevate them up to eight feet higher than before.

“We had 30,000 houses that flooded” from Harvey, said county engineer John Blount, who put forward the rule changes. Before the floodwaters even subsided, hundreds of county employees fanned out to survey the damage. “We went to every one of those houses and figured out how much water got in them, and then we did a statistical analysis,” Blount said.
The data was geocoded, factoring in location and neighborhood conditions, and one result was the increased elevation rule. (The county is also buying out 200 of the most vulnerable homes and hopes to buy out thousands more, but those represent a small fraction of the homes inside the floodplain.)

Harris County’s new rules are the most stringent flood-related development restrictions anywhere in the United States, according to Blount. If a future Harvey-sized deluge comes, almost all the homes in the area will be safe, he said: “Had that same event happened, at the same location but [with houses] built to the new standard, 95 percent or more would not have flooded.”

For a structure, standing water is a fearsome enemy. Even a small amount of flooding in a home can exile its inhabitants for weeks and require costly repairs. After Harvey, tens of thousands of evacuees lived in hotels or with friends as workers in their homes tore out drywall to prevent the spread of mold, which can sicken residents. And more Harveys are coming: As Robinson Meyer reported, a new MIT study concludes Harvey-scale flooding in Texas is six times as likely now as it was in the late 20th century, and will only get more likely as this century wears on.

More than a million people live in the 100- and 500-year flood zones across the Houston area, and hundreds of thousands more do in other U.S. cities, including Miami and New York. Harris County’s move conforms with the advice of building engineers, climate experts, and the insurance industry. If you live in an area which is prone to flooding—or will be soon—getting off the ground is the best way to avoid recurring, expensive, and heart-rending damage to your house.

“There’s no real substitute for elevation. That’s your best bet,” said Tim Reinhold, senior vice president and chief engineer of the Insurance Institute for Business and Home Safety (IBHS), a research organization based in Tampa and funded by insurers.
Houses don’t have an engineered safety margin for avoiding flooding the way they do for wind resistance, Reinhold points out; even a few inches of water can be devastating. His advice: “Build that margin in by going higher.” The IBHS recommends elevating houses three or more feet above the 100-year floodplain.

Yet three feet is nowhere near standard. The City of Houston requires one foot of elevation above the 100-year floodplain. Many jurisdictions in Texas and other states require none. What seems like a simple, obvious safeguard raises tricky questions: How high is high enough? Who has to pay for it? And at what point does it no longer make sense to build in a place at all?
Nationwide, according to Census data, 59 percent of new single-family homes are “slab-on-grade,” as it’s known in the construction industry. The technique is pretty much what it sounds like: Concrete is poured into a mold set shallowly into the ground, forming a slab several inches thick. Because a ground freeze can crack the slab, the method is mainly used in warmer climates. It’s straightforward and cheap. But it results in a house with a low elevation, which is obviously not ideal in a flood zone. “I don’t understand why you would ever build a house on a slab on grade that could be in a flood-prone area,” Barcik said.
Building a house with a raised foundation isn’t cheap. “If you’re starting with a home that’s slab-on-grade right now, and want to raise it by using fill, it could be $13,000 to $14,000 to do that just one foot,” said Gary Ehrlich, the director of construction codes and standards for the National Association of Home Builders.

Mike the Pole Barn Guru comments:
The fill method—trucking in soil and resting the slab on top of it—is cheaper than the pier-and-beam or stem-wall options. But it’s not adequate for raising a house’s height by several feet. The eight-foot-elevated homes now required in parts of Harris County would carry significant added costs, which builders would pass on to homebuyers.

However, elevating a post frame home, even as much as eight feet, is a negligible investment compared to the costs of stick frame construction. Considering constructing a new home? If so, look towards post frame construction as an affordable design solution.

Can I Build a Pole Barn on my Concrete Slab?

Can I Build a Pole Barn on My Concrete Slab?

I dove off from the turnip truck a long time ago, so I have seen a lot of strange things constructed over my nearly 60 year lifetime. Sometimes strange is good, usually not so good. What is remarkable are the structures which are constructed directly upon nothing more than a four inch thick concrete slab on grade. This includes post frame (pole barn) buildings.
Reader TIM in TYLER gets credit for triggering this article when he wrote:

“I have a newly poured 24 x 40 slab which I intended to build a convention style garage but budget restraints have rerouted me. I want to build a simply pole barn style carport with a metal roof. We have no snow to speak of in TX so weight should be no issue. I also want to do a storage area in the back like 12 feet x 24. Can I use the brackets and anchors I see to build on top of the slab? I’m thinking the loads points should be minimal due to a steel roof. I am in a rural area with no inspection requirements.”

Mike the Pole Barn Guru writes:
To begin with, in my humble opinion, just because one is going to build where there happen to be no inspection requirements does not mean one should exclude themselves from following the Building Codes. The Building Codes are only put into place after exhaustive discussions between construction professionals, Building Code officials, engineers, architects and product manufacturers. The Codes are perpetually changing, as new and better products arrive on the marketplace, practices are refined and more research is done into how buildings and materials perform.

While no snow may fall where your building will be located, one must still consider provisions for Code minimum loadings on the roof and the members below which carry them, including the concrete slab. Assuming a fairly standard roof overhang of say a foot, this leaves the perimeter of the slab being required to carry a load of over 13 tons! Using typical post frame construction, the point load at any one column could be over 4000 pounds, which could easily fracture just a slab on grade.

Standard post base brackets which you may see at your local hardware or big box store are not adequate to carry the loads to even an adequate concrete slab.

How I would do it?

I would use a concrete saw to cut out two foot by two foot squares at each column location, remove the concrete, auger holes as appropriately designed by the building engineer, set the columns in the hole per the plans then form and fill with premix around the top of the column to complete the slab.

If using brackets is your true desire, it may be possible to excavate under the floor at each column location and thicken the floor by pouring under the slab. It can be done, however it does take some effort.