Screw piles instead of concrete foundation for two story addition
Nsgreenhome
| Posted in Building Code Questions on
Hello we’re building a two story addition to our home and have decided to use helical piles instead of concrete slab on grade foundation. Each unit is around 950sq. Now the city wants a a new cross section to show the size and ply of new beams, and how the ground floor will be constructed and insulated. I have no idea how to go about doing this. We live in Nova Scotia, Canada with cold long winters. Any help would be appreciated. Thank you.
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nsgreenhome,
Who did the drawings and specifications for your original building permit submission? Can they not alter the plans to show the new foundation?
We hired a designer but would have to pay her more $ in order to make changes to the plan she provided so would prefer to do this ourselves.
Essentially you can't.
Screw piles are not part of the prescriptive code, so you need somebody to stamp the design. You are paying for this no matter what.
You can try to limit the scope, say pick the beams from the prescriptive tables from the code and draw some of your own insulation details.
As for insulation, you are looking at an exposed floor. This is about what you want:
https://basc.pnnl.gov/code-compliance/sealing-and-insulating-existing-floors-above-unconditioned-spaces-code-compliance
The R value of the rigid plus fluffy should be what your code calls for exposed flooring.
Hi thanks for the reply. Yes the helical pile company we're going with will provide a plan stamped by their engineer but the city is looking for how the first floor will be constructed/insulated, which the helical pile company will not do. So you're saying we can do this portion our selves by looking at the building code site? Thanks again.
Nsgreenhome,
The first building section in the link Akos provided shows a good floor assembly. You just have to choose the joist sizing and amount of each type of insulation based on the NS building code.
If the floor is close to grade:
- The dropped beams may need to be pressure-treated.
- It may be very difficult to install the protective layer under the joists.
Hi thanks for the reply. The floor will be 8-12" off the grade. I saw your old comments and posts and saw that I should say something like this in the plan: framing, subfloor, R-31 Batt Insulation + R-10 CodeBord, framing, and another subfloor.
I'm just not sure what size for the framing and subfloor. I'm in zone 6, cold and long, wet springs and winters. I really really appreciate your help!
Nsgreenhome,
- The framing is typically spaced at 16"oc, the spans and depth of the joists depend on what loads they are taking up from above. The sub-floor is usually 5/8" t+g plywood.
- See what information they will accept for the changes. Here you could not just list them, the drawings would have to accurately reflect what you intended to build, and the changes be made to the plans, elevations and sections.
- You've got a real problem with a floor that low. You need to work out a detail to get a layer of protective plywood or gypsum board on the underside.
- Is there any plumbing in these two additions that will need to be brought through the floor?
Would R-10-2" board insulation be sufficient for the underside?
Yes, plumbing will be brought in from the existing house but I'm wondering if we should run them through the interior walls instead of the floor.
Also you mentioned previously that the dropped beams should be pressure treated. Do you mean the 2x4 framing?
Nsgreenhome,
- You want something to protect the foam from pests, especially as if they get in there you will have no access to the space to deal with them. Even if you just use foam you still have the problem of securing it to the underside of the floor. One way around that is to build a 2"x4" floor first, then the load-bearing one above that, so all the work can be done from above. This link shows that assembly. I used it on an outbuilding last summer and it works well: https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
- I would keep as much of the plumbing as you can out of the floor.
- I meant the beams that will be set on the piers that the floor joists will bear on. If the underside of the floor is only going to be 8" to 10" above grade, those beams will have to be flush, not dropped, but wouldn't need to be pressure treated.
In the northeast US, floor sheathing (aka subflooring) is usually 3/4" T+G, preferably Advantech. It's probably the same in nearby Nova Scotia.
Michael,
Thinking about it again, they are two storey additions. The specs for most of the assemblies should already be called out in the original permit set.
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Thank you so much, you've been a huge help!! I really appreciate it.
Nsgreenhome,
Good luck with your project.
Hi I drew a picture of the assembly for my city permit, do you think this is ok? Thanks again.
You haven't drawn the floor framing, just the beam and pier, as far as I can tell.
Burying the beam into the soil puts it at high risk of frost heaving and is not something I would do.
I described the sandwich assembly for the floor in the arrow, bottom to top. Is that what you're referring to?
NSgreenhome,
No. They don't make any sense. As Michael said, you need to show how everything goes together. How is that lower framing attached to anything. How does it transfer the loads from above. How do the beams connect with the framing, how does any of this relate to the plans, etc. And you can't have large black areas on sections.
Before you spend time on this you should talk to the city. As I said in an earlier comment, they may well want the whole drawing package amended to show the changes. If so that's something you can't do without the approval of the designer who did the drawings, as they hold copyright.
Oh sorry I was mistaking the first frame as just being able to sit in the brackets of the screw piles ... they will be on beams. but how, I don't know. sigh.
There's a reason we designers get the big bucks! (Mostly joking, but there is a lot to know about how things typically go together, alternative ways they can go together and how to relay them in a way that can be understood.)
NSgreenhome,
It's not just a few things on these sections. Unfortunately you just don't have the construction or architectural experience necessary to do the drawings they want, or an understanding of how all this goes together. And it's not just for approval. The drawings you do will be used by your contractor, so they need to work, or they will end up having to make up details on site, which can have very serious consequences.
Hello again!
We hired a designer who drafted a plan for us and he had recommended 1/2” marine plywood then R40 insulation then 6 mil poly film for the first floor construction but now that I’m about to order the lumber, I’m finding out that marine plywood is crazy expensive ($7000). I emailed him to see if there are any cheaper alternatives but no answer. Any recommendations? Would 1/2” PT plywood be sufficient? Thank you in advance.
Marine plywood is for building boats, it's designed for constant immersion in water. It's overkill for the underside of a house. If you look at the link that Malcolm posted in #10 it calls for 1/2" treated plywood as the bottom layer.
NSgreenhome,
As DC said, pt. plywood would work fine. You should minimize the amount of moisture coming up from the ground below by laying a vapour-barrier on grade and covering it with gravel.
What did the joists end up being? If they are solid lumber you want a layer of foam under the floor to reduce thermal bridging or it will feel cold.
You don't want or need poly in that floor stack-up. The vapour-drive is upwards, and you want to glue down the sub-floor.
How will the floor be built with no access from below? How does the designer propose to attach the plywood on the underside of the joists?
Thank you. I've attached a plan here and I think he's saying the plywood will be on top of the joists, nothing under. Is that a real problem? The joists are solid lumber.
Nsgreenhome,
No he is showing the plywood under the joists, but there is 18" clearance to grade, so it is tight but doable.
The section appears to call out the floor joists as TJIs not solid lumber.
I would urge you to consider my other suggestions about the poly, etc.
I think those are the worst leaders I have seen
will do for sure, thank you very much!
So vapour barrier/gravel, 1/2" PT plywood then R40 insulation is sufficient? no poly?
NSgreenhome,
That would be my suggestion.
Thank you both. Michael, are you a designer? Can I pay you to design this? haha
I am a residential designer but I'm based in Maine and booked out about a year, so I'm not a good option for you, sorry!
I have a builder friend on Grand Manaan Island; I could ask him if he has any designers to recommend for you. Where in NS are you?
im in halifax! thanks!
(Replying to #29).
I read that plan as saying the floor assembly will be, from the top down, 3/4" T&G OSB, then poly film, then R40, then marine plywood.
You really want something under the insulation, both as an air barrier and to keep rodents out of the insulation. Go back to post #10, look at the link Malcolm provided, that double-layer floor is as good as it gets.
If it were me, the way I'd do it would be to hang joist hangers off of the beam and run treated 2x6's between them. Then I would deck over the whole thing with treated plywood, t&g if you can get it. Maybe even caulk or tape the seams. You want that layer to be air-tight. Then lay the joists over that deck, fill the whole thing with insulation, cover with the vapor barrier and deck over it.
You are going to struggle with two things during construction. One is keeping that insulation dry until you get a roof on. The other is keeping people from cutting holes in your floor assembly, the plumber and the electrician are all going to want to go under there with a sawzall, you don't want to let them. Before starting construction I would go over with the architect and GC if you can what mechanicals there are going to be and where they're expected to run.
As Malcolm suggested, you want a layer of plastic on the ground with gravel over it. You also want to let as much air flow as you can under there, Joe Lstiburek recommends no more than 50% coverage from any skirt walls with a vented crawl space like that.
DC,
"You are going to struggle with two things during construction. One is keeping that insulation dry until you get a roof on."
Maybe it's a climate specific problem, but here assemblies that have to be insulated before the roof is on would preclude you from building for a good part of the year that you could otherwise productively have worked. Even with the outbuilding I referred to in post #10 which was framed in late June, we still sweated about possible rain.
Why is there a step down from the house? That is a bad idea for a number of reasons.
Bringing the floor up also means more space which will make any crawlspace work much easier.