How far below slab should footings be?
timrowledge
| Posted in Construction Techniques on
I’m trying to work out how many courses of ICF (16.5″ & 7.5″ heights available locally) I need to use for a walk-out basement foundation wall. The wall height is 9’3.25″ from the top of concrete to top of slab (small variation would be tolerable).
7 courses of full height ICFs would provide 115.5″ which would put the top of the footing 4.25″ below the top of the slab. Given that the slab is expected to be 4″ concrete over 2.5 rigid foam and 6″ gravel, is this sensible? Should the footing top be below the foam, for example? Should it be below the gravel?
Various combinations of the 16.5 & 7.5 blocks can produce a lot of total heights but obviously it’s less hassle to use 7 courses of full-height than 15 of half-height, for example.
Replies
I don't know about ICFs, but for an eight-foot joist-bottom height, we pour about 8'6" over the footing. Fill the depth of the footing with gravel and two inches on top of it, then pour the 4" slab.
If you want 2" of foam, I'd put it inside the footings, not on top of them. You don't want the foam supported by two substrates with different support capacities.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
we pour right on top of footer, so footer elevation, then four inch slab
I have done it that way when I could; but here we're on bedrock most of the time and it's common to have 'steps' in the footers, so....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
A couple of reasons not to do it that way: The fill material under the rest of the slab will compact leaving the footings to hold it up. Also, the perimeter drains are at the footing level so any water that penetrates between the footing and slab is now likely to flood the basement rather than stay in the 3 to 4 inches of fill covering the footings that is generally recommended.
tim
I suspect that your footings need to be protected against frost if you are putting in 2.5" of foam.
What Dinosaur said is right - you want consistent support under your slab - no hard edges, but I'd deepen the whole thing some, putting stone, insulation and slab above the footing. You will also need to deepen the footing or add sub soil insulation under the walk out to get frost protection there.
If you need to do it, cutting a course of ICF block for the top is no big deal anyway.
Save all your foam scrap and lay it into the attic before any other insulation goes in.
Ron
Hmm, interesting answers - thanks.I don't *think* the footings need too much frost protection where I live; mid-Vancouver Island gets snow some years but isn't anything like Edmonton for freezing. Stepping the footings down at the front for the walk-out sounds very sensible though. It's proving tricky to find numbers for the frostline depth in this area right now - is anyone familiar with the area?If I used 7 courses of full-hieght + 1 course of half-height I'd end up with a little over 11" from the planned top of slab to top of footing, which seems plenty to allow stone, foam and slab.And of course you're right - cutting foam is no big deal in the end. Though with a vaulted SIP roof, there won't be any attic to throw the scraps in. I might have to save them for cutting model aircraft wings or something like that!
tim
May I ask why you chose ICF in what I think of as a mild climate? I know you can find snow there in August (I lived in Comox) , but it's unusual to approach extremes of heat or cold.
Ron
Why ICFs? Just seems like a nice combination of good insulation (the aboveground walls & roof will be SIP on a timber frame), interesting technology, simple construction and local availability of parts and labour.
Unless you are in a high elevation location, like Mt Washington, you can assume your frost level is a foot below grade.
I've been told by a local contractor that we should use 18" here, part way up Mt. Arrowsmith, for frost line depth. Just in case anyone else ends up searching for the info...
Uh, what's code in your area? Generally you're going to have to go down below the frost line.
"Uh, what's code in your area?"Ah, well that's an interesting question. Y'see where I live is a part of Vancouver Island known officially as 'Area F'. There are assorted 'regions' in the geopolitical organisation of B.C. and subdivisions etc. Area F was until not that many years ago almost unoccupied, with just a couple of tiny villages. There still isn't a lot of population to be honest. And there aren't any building inspections..... weird, huh?So part of the answer to your question is 'Canadian Building Code' and part is 'who cares, nobody will inspect for it'! Sometimes I think this is still trying to be the Wild West. A year or so ago there were moves to introduce an inspection regime and you should have heard the shrieks of protest - in fact you might have, all the way across to MN.Personally, I will be insisting on at least meeting code requirements and if necessary I will employ a private inspector to check things before signing off on job completion.PS - I like and concur with your Carter quote. Best ex-Pres you guys have had.
Frost Depth
This an old thread but the question still seems to come up. The Code is vague on this point as it says foundations should be below frost depth but doesn't give any guidance as to how this should be determined and it usually seems to come down to local practice.
Vancouver Island and Lower Mainland are off the Freezing Degree Days map from Environment Canada as it stops at 500 (in C) and all of the Island is less than this. I did finally find a list of Freezing Indices from Dow for use in selecting their foam insulation. It indicates that Comox is 596 Freezing Degree Days in F snd 331 Freezing Degree Days in C. Caution - some of the graphs are in F and some in C.
It looks like Frost depth is about 1 to 2 feet for Comox bareground or paving and about 6" with turf and no snow and 0 feet with turf and 12" of snow. Still somewhat discretionary and we usually have a bit of snow when we have the hard freezes but I would be pretty comfortable using 12" near the Strait of Georgia, 18" for most the Valley and 24" on up for higher elevations. Check with your Inspector if you are in an area that requires it. It seems like inspectors usually look for about 24" in the Comox Valley.