Tag Archives: roof loads

Stretching Stick Frame Construction

Post frame (pole building) construction is popular due to efficiencies of materials (ability to do more with less) and speed of construction.

Reader RAYMOND in BARLING is trying to find a way to make stick framing cheaper, he writes:

“24×64 pole barn in question. 4 pitch.  I am just comparing the cost of alternate designs.

Using 2×6 rafters with purlins across top for metal. Can I part from the standard 24 OC of rafters and expand to 30 OC (since more support from purlins)?

Furthermore, is it possible to use 30 OC studs all around, instead of poles (since more support from purlins on walls)

I would really appreciate your wisdom.

Thanks!!”

Mike the Pole Barn Guru says:

Let’s begin with, “since more support from purlins on walls”. Studs in stick framed walls will not resist wind loads perpendicular to a wall any better due to lateral support from purlins (actually girts) installed horizontally.

Your rafters are also going to be unable to support greater roof loads due to purlins being attached.

Building Codes have prescriptive requirements limiting what can and cannot be done with conventional (stud wall) framing, without having to have a fully engineered building. This would include studs and rafters being no greater than 24 inches on center. They also preclude wall heights of over 12 feet (you did not mention any heights however it should be kept in mind).

International Residential Code (IRC) Table R8702.4.1(1) provides rafter spans for common lumber species with a roof live load of 20 psf (this happens to be Code minimum whether snow is present or not). Being as you are in Arkansas, we will assume the minimum load as well as no ceiling being attached to rafters. With rafters 24 inches on center your rafters would need to be 2×8 #2 Southern Pine at a minimum. You would also need to provide ceiling joists or rafter ties to resist outward push of rafters on bearing walls. In order to get full value from rafters, ratio of rafter ties measured vertically above the top of stud walls to the height of roof ridge would need to be 1/7.5 or less. At a 4/12 slope ridge height would be 55.64″ meaning rafter ties could be located no more than 7-3/8″ above top of stud wall, so plan on then being at least 20 feet in length. A ridge board must also be provided as well as a collar tie, gusset plate or ridge strap (please refer to IRC R802.4.2).

Stud walls also mean you would need to make provisions for structural headers above any opening in any load bearing exterior wall. With post frame construction openings can be placed between columns in exterior walls, eliminating structural headers (this assumes trusses are placed aligned with wall columns with roof purlins on edge).

For stud wall construction, your concrete slab on grade will need to have an appropriately thickened edge in order to support weight of walls, or a continuous footing and foundation will need to be poured.

Ultimately post frame construction, not stick wall construction, is most probably going to be Raymond’s best route to go when considering investment and ease of construction.

Splash Boards, Roof Loads, and Truss Spacing

This week the Pole Barn Guru answers reader questions about shrinkage of splash boards installed wet, roof load capacity, and truss spacing for an RV storage building.

DEAR POLE BARN GURU: I have two questions (related) regarding splash boards and concrete floor top. My splash boards have been in place for quite a while, and have actually shrunk (they were quite wet when nailed in place). There are several that are not over 7 inches wide. I’m going to end up with only 3-1/2 inches of splash board above the concrete. Is this sufficient? The tops of the splash boards are very level, and I’m thinking of attaching treated 2x4s on the inside to screed against. It would be easier (and probably more accurate/consistent) for me to measure 3-1/2 inches down from the top rather than up from the bottom of the splash boards.

Also, wondering if there is a benefit to placing 3 x 1/4 inch galvanized lag bolts, 1 inch into the splash boards from the interior side 1 or 2 feet apart, to “anchor” the splash boards to the slab?

Thanks so much for sharing your experience and insights! GREG in COLVILLE

DEAR GREG: As long as you are measuring from a level point and top of your concrete slab will be below bottom of your base trim you will be all good with measuring down 3-1/2″ from splash board tops. While I have not done it personally, I know more than one person who has used a pressure preservative treated 2×4 to screed against as you describe. At a minimum it should be rated UC-4A (ground contact) for treatment.

There might be some small benefit to be gained by using a mechanical attachment of splash boards to your slab. As to how much, I have not seen any studies to verify.

Thank you for your kind words and please remember to send me progress photos!

 

DEAR POLE BARN GURU: Do you know how much weight per square foot the roof of a pole building can hold?  My building is 64’ x 36’, if that matters. ROBERT

DEAR ROBERT: Weight per square foot (psf) will be dependent upon what your building was engineered to support. Every set of engineer sealed building plans is required to list all loads to be supported. Usually this will be specified by a value for sloped roof snow load (Ps) for roof plane live loads. Dead loads (actual weight of structure and supported materials) should also be listed.

 

DEAR POLE BARN GURU: I’m wanting to do a pole barn to park my RV under for the winter. My question is can I use a single truss spaced at 10’? Do I need to use two trusses per post? ERIC in BONNERS FERRY

DEAR ERIC: Without knowing how far you intend to span with your trusses it is difficult to provide a definitive answer. Boundary County does not require building inspections, so even though you are in an area of extremely high snow loads – risks end up being upon you as a new building owner.

While a single ply truss may work, in most instances your investment into true double trusses (nailed face-to-face as a pair) is minimal. Double trusses provide greater reliability as your probability of having two adjacent trusses having a same ‘weak link’ is small. Bracing requirements are also reduced when a pair of trusses are utilized.

Even though you may not need a building permit, I would strongly encourage you to only erect a fully engineered building. Protection for your RV is only going to be as good as what your building is designed for.

 

Considering a Pole Barn, Roof Loads, and Proper Ventilation

DEAR POLE BARN GURU: Good morning.

I am considering building a pole barn on our land in northwest Georgia and wanted to know the following:

1) On your website, you list links for residential, agricultural, and commercial buildings.  What is the difference between a those three types of buildings?  Are they different because of design or do they each involve different construction materials?  Do the commercial buildings use a lower gauge (thicker) sheet of metal for siding than a residential building?

2) Do you have any product comparison documentation between your kits and the other pole barn kits on the market (DIY, Menards, etc.)?  Interested specifically in design, material, and construction comparisons.

3) Would your pole barn kits be able to accommodate a chimney/stove pipe if I wanted to use a wood burning stove for heat?

Thanks! CHRIS in RISING FAWN

About Hansen BuildingsDEAR CHRIS: The differences for residential, agricultural and commercial buildings shown on our website are for the convenience of those who are looking for a particular end use, it keeps from having to browse through a plethora of photos of buildings which may not be what one is looking for. The construction materials and methods used are going to be individually tailored to the ultimate end needs of each client, as well as the climactic conditions of a particular site.

Our goal is to custom design for you a building which best meets your wants, needs and budget. We are so confident in our ability to provide the best possible value for your post frame building investment, once this is done, we would happily shop this building for you with any other provider or providers you so desire. How easy is this?

(BTW – Menards might be a bit geographically challenging as their nearest location to you is in Owensboro, KY)

Actually any post frame building (not just a Hansen Pole Building) can accommodate a chimney/stove pipe with the use of a Dektite® (read more here: https://www.hansenpolebuildings.com/2012/09/dektite/).

 

DEAR POLE BARN GURU: I have a 30 year old pole barn that is 30’ x 40’ x 9’ tall. It has a metal roof, trusses are 4’ on center. Can I tear off the metal on the roof and put down OSB and shingles? JIM in LAWTON

DEAR JIM: Chances are excellent your existing roof system is not designed to support the weight of OSB and shingles, as most pole barn (post frame) trusses are designed for a dead load of only 3 to 5 psf (pounds per square foot) which includes the weight of the trusses themselves plus the roof purlins. Steel roofing weighs in at under one pound per square foot. 7/16″ OSB comes in at roughly 1.5 psf, 15# felt and shingles 2.5 psf making the weight combination more than four times greater than the steel.

The big question is – why? Even “lifetime” shingles will usually last only about 15 years and you know the steel roofing you have had made it twice as long. Steel is far more impervious to weather (especially hail) and readily sheds snow, unlike shingles. For my money, if I had to re-roof I would invest in steel roofing with a high quality paint system like Kynar. Read more about Kynar here: https://www.hansenpolebuildings.com/2014/05/kynar/.

 

DEAR POLE BARN GURU: I have a pole building with all metal sheeting. The interior walls are framed and insulated R13 batt. The ceiling is insulated with 1/2 foam a 3/4″ air gap then R19 on top. The underside of the roof is not insulated. I have eave ridge vent. Building is heated in winter. Can I exhaust fumes (paint,lawnmower,etc.) into the attic space and let it vent out the ridge or will I be causing a condensation problem? I will use a standard box fan to blow exhaust into the attic space. I’m also hoping to do this to help melt snow off the roof.
Thanks. RICH in LEHIGHTON

crash-test-dummy-symbolDEAR RICH: I’ve seriously struggled with your question for several weeks now. It lead me to spend hours researching the International Mechanical Code (I am proficient in the IRC and IBC, but not the IMC), looking for backup as to your scenario. In the end it all comes down to this – WHY would you want to dump toxic fumes and their waste into your attic? At some point this has got to be just plain unhealthy.

Whether you do or do not blow exhausts into your attic, your building has the strong potential for a condensation problem because there is no thermal break below the roof steel. You should look at having closed cell spray foam installed on the underside of the roof steel.

As to the heat from the exhaust helping to melt snow off the roof – do not count on it, by the time it gets into your attic, the heat generated will be minimal at best.

Collateral Loads and Why They Matter

Collateral loads are additional dead loads hung from the building, such as sprinkler systems; drop ceilings, HVAC equipment, lighting etc. These are the loads which are most often overlooked or neglected in the design of a new pole building. Many times, it is just no one having made the client aware. Most building sales people and even designers and engineers, do not go beyond present day use to discuss what the building may be used for in the future.

Generally, the actually dead load weight of roof truss bottom chords and the minimal lateral bracing needed to restrain them from movement is about one-half of a pound, per square foot (psf). Most pole building trusses are designed using a one psf load, which will allow for the weight of standard electrical wiring and lights. As an example, for a 30’ x 40’ building approximately 600 pounds of wiring and lighting could be evenly distributed across the roof truss bottom chords, without the need to increase the load carrying capacity.

The Metal Building Manufacturers Association (MBMA) Low Rise Systems Manual recommends the following collateral loads, per square foot:
Sprinkler system – dry 1.5  wet  3
Ceilings – Suspended Acoustical Fiber 1
Gypsum Board – 1/2″ 2 – 5/8″ 3
Lighting 0.1 to 1
HVAC Ducts 1

2×6 ceiling joists at two foot on center will add about ¾ psf.

These loads are cumulative. If you have more than one, you add them together. In most cases, a ceiling load of 5 psf will be adequate to handle a layer of 5/8″ drywall, ceiling joists and blown in insulation.

Many of us are packrats. We accumulate “stuff” and the space in the trusses looks like a good place to put it.  When I first visited the home of my now bride of 12 years, I took one look “up” in her garage and quickly ran for cover!  The trusses just weren’t designed for the “stuff” she had stored up there.  Her defiant assertion of “if it ain’t broke…” finally gave way to “ok, if you insist” and we cleaned it all out…ten years later!

The International Building Codes have a provision to add 20 psf of live load onto the bottom chord of trusses where a box 42” tall and 24” in width would fit between the top and bottom chords of prefabricated roof trusses, in areas where a truss web would not interfere. This same provision mandates an increase in the bottom chord dead load to 10 psf.

The roof system of your new building is not a place to become penny wise and pound foolish. Overloading a roof system, beyond its design capacity, can result in catastrophic failure.

Planning a new building? Discuss not only your future needs, but also what other owners of your building might do with the building, with your Building Designer.

Some folks are pushing budget just to get the basic building but have plans for future interior build out. From a liability position, the building owner bears the risk if ordering a building which assumes no collateral loads, then later turns it into living space with joists, drywall and insulation.

Always look towards solving future needs. It is fairly inexpensive to add ceiling loading to trusses, and saves a world of potential grief later.

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