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.
This Monday the Pole Barn Guru answers questions about use of faced or unfaced insulation, the correct screw pattern, and viability of connecting two buildings together.
DEAR POLE BARN GURU: Should I use faced or unfaced insulation in my pole barn attic w/ ridge vent? DAINE in PALMER
DEAR DAINE: In order for your ridge vent to be effective, it does need to have an intake – ideally from vented soffits. Make sure there is at least an inch of clear space above any attic insulation, to allow unobstructed airflow from eave to ridge. You should be using blown in attic insulation. Your need for a vapor barrier below your blown in attic insulation depends upon your number of heating degree days (please see link in this article: https://www.hansenpolebuildings.com/2018/10/ceiling-vapor-barriers-in-post-frame-construction/).
DEAR POLE BARN GURU: I am building a house on 24″ truss centers and want to know if this is the correct way to install the medal rood? 7/16 OSB synthetic felt and 1X4 purlins to mail the medal to will this sweat or have problems and is this the correct way to go? KENNY in OSAGE CITY
DEAR KENNY: I would recommend you use 2×4 purlins placed wide face towards your OSB sheathing. You want to make certain you have securely fastened purlins with nails long enough to penetrate through OSB and 1-1/2 inches into each truss top chord.
Reasoning for 2×4 is you should be using 1-1/2 inch long screws, placed in flats of roofing and you want entirety of your screw shanks to be firmly into solid wood (OSB will not hold your screws). Here is your correct pattern for screw placement:
DEAR POLE BARN GURU: If I buy two units, can I connect them in anyway or turn two units into 1 long unit? NATHAN in SAN LUIS
DEAR NATHAN: Yes to both – however please let us and our third-party engineer design all of your project together. When buildings become lengthy in relationship to width structural design challenges can occur in relationship to an ability to adequately transfer wind loads from roofs to endwalls. By doing an entire structural design, we can insure your finished product will remain standing and useful for a lifetime.
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′ 4-5/8″. I like taller rooms, so you might want to experiment with eave heights of 11′ 4-5/8″ and 12’4-5/8″ (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.