Building a floor structure on helical piles
Hello – looking to get some advice on a general direction for my floor build.
I’m building an 8×12, freestanding office on top of helical piles. I’m in Ontario Canada (zone 7), so cold and wet weather is a concern. I’ve found 2 or 3 posts on this site with mostly the same advice, but I’ve been given supplemental suggestions and want to see if they fit.
I have put down a gravel base and it was suggested that I fill the void between the gravel and the floor joists with foam board insulation. The logic here is foam is used in concrete floor construction. I’m not sure how sound this idea is. I’ll skirt the outside of the building, but the foam will still be exposed to the elements to a degree. The foam doesn’t take the load, but does need to be tight to the bottom.
As for the floor, I’ve read here that a mix of foam and traditional insulation is a good approach. I’m thinking that the sandwich will look like (top down)
-subfloor
-2” foam insulation
-rockwool
-1/2” PT ply
Is this correct?
I came across some suggestions on putting an air gap in there, but it sounds like that might be a bad approach?
Does any sort of house wrap (tyvek) make any sense for the ‘sandwich’?
And I realize the floor will never really be “warm” without some sort of floor heating system. I just want to construct something that’ll keep the moisture out and follows best practices for efficiency.
thanks!
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Replies
Keveh,
I've done these small buildings a few ways, but this summer build a 350 sq ft recording studio on piers using a floor assembly featured in FHB, and from now on it's the one I will use.
Based on it, here is what I would suggest for the floor (from the bottom up):
- PT 2"x4" @ 24"oc, with a 1/2" pt sub-floor, taped and sealed.
- 1 1/2" foam board, with a 2"x4" (on the flat) perimeter.
- 2"x6" @ 16"oc, with the joist cavities filled with batts, and a 5/8" t&g sub-floor.
The advantages are:
- All the work can be done from above, so it can be kept close to, or set directly on grade.
- You don't care what happens underneath, there is nothing exposed for pests to get at.
- Want more insulation? Double up the 2"x4" perimeter and add another layer of foam board.
I'm glad my near simultaneous response yielded something that seems vaguely Canadian friendly. :-)
> I’m building an 8×12, freestanding office on top of helical piles. I’m in Ontario Canada (zone 7), so cold and wet weather is a concern
This is a *tiny* structure; why do you need helical piles? Why not put a PT frame directly on grade, or a couple of PT beams on grade and joists on top? Does the applicable Canadian building code preclude this? (For reference to US readers, the IRC allows this.)
Whether directly on grade or resting on beams on grade, I'd likely use mineral wool between the joists, a layer of plywood for air sealing above the joists, rigid foam, and then a subfloor layer. There are other ways to mix this up, but this strikes me as pretty simple for such a structure.
Patrick,
Great minds and all that...
I'm with you on just setting it on gravel. Canadian building codes don't usually apply to structures under 100 sf. Sometimes they do depending on what the occupancy is, but then the requirements are typically based around that. So a small bunkhouse might need smoke detectors and a code compliant exit, but how it's supported shouldn't trigger code issues.
We had a guy in Ottawa last week doing a building on helicals -- https://www.greenbuildingadvisor.com/question/how-do-i-create-a-crawl-space-skirt-for-helical-piles -- and he said the building inspector wouldn't let him do a skirt because it wasn't protected from frost movement.
That said, where I am anything under 100 square feet doesn't require a foundation or inspection, I would just build that on treated skids directly on the ground.
Thanks you Malcolm, Patrick and DC — these were the thoughts I was hoping to have validated.
Some quick points - under 100 sq ft doesn’t require a building permit (you all identified this) No building permit means you don’t have to have an extremely picky, zone 7, capital of Canada, get involved. If you catch my drift. A permitted building on helicals would have meant an extra $500 engineering report.
I’ve already got the piers in the ground, and yes, I realize it’s probably well above and beyond what I needed to the tune of $1200 more, but now I don’t worry about the ground heaving, too much rain, etc etc. it gives me peace of mind. So my path forward is pretty decided at this point.
The point about doing the work from the bottom up has been my major challenge point while thinking about this. I’m not sure I can go that route exactly. I was going to go 2x10 across the piers with 2x8 joists. Short of building the perimeter beside the helicals on top of the 1/2” ply and mustering up some big strength to move the whole deal, I feel like I need to build in place.
Keveh,
Whether built on grade, or supported by the piers, I would use the assembly I described. here is a link to the original article I modified it from: https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2021/12/16130718/Built-on-Piers.pdf
Thanks, Malcolm. I like the “sandwich” you’ve described. I just need to figure out how to build in on the piers. (Securing the bottom with a tight space being the problem)
Keveh,
There is no bottom. It all gets built in layers starting with the 2"x4" framing. Then a pt sub-floor, foam, a second layer of 2"x6" framing and another sub-floor.
Thanks for the diagram. That makes much more sense. You’re building across the joists on the helicals. I pictured hanging them across the joists, not on top.
If you go with 2x12 rim joists and 2x8 joists you can build a bottom below the 2x8 joists out of 2x4 with 1/2" plywood on top. The bottom will be 4" high, exactly the difference between 2x12 and 2x8. You build the bottom from above first and then put the 2x8's in.
Since it's 8x12 I assume you'll run the joists the 8' direction. The bottom will be three pieces of plywood that you can run the same way so the joints are over the joists, no exposed joints anywhere. That gives you a nice air-tight, rodent-proof bottom to your floor.
Thanks for this tip. Since I was using 2x10 beams and 2x8 joists, this was actually much easier than I thought. I used 2x2 around the inside perimeter with some 2x4s where the bottom PT ply seams ran. It was enough to support that bottom floor. Slide in the hangers and joists, and a few screws toenailed down to grab the ply and it’s as sturdy as the bottom needs to be. Finished off with Malcolm’s suggested sandwich and I think it’ll work pretty well.
Thanks both!
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In such a small space I doubt you'll be running any plumbing lines out there so freezing pipes won't be an issue.
I dislike a stick built floor system whenever it's up on piers. All those voids between joists that are (at best) filled with batt insulation that does nothing to stop air or vapor movement within the cavity.
I would contact a SIP manufacturer and get an 8' by 12' panel (probably 12" thick) and lay it on top of your pier beams. The panel will probably cost you about $500 and your structure and insulation is done in one shot (you can even ask the manufacturer to router out some foam at the edges and install the rim boards for you). Build your walls on top of the panel and if you want you can install a 2x4 raised floor system (with plywood on top) within the walls so you have a service cavity for under floor electrical wiring.
Under the SIP panel use a fluid applied waterproofing coating , some furring strips and the finish material of your choice. You shouldn't need any skirting at this point.