I’m preparing to construct a timber-framed screen porch addition on my house. The posts will bear on concrete piers, which will extend well below the frost line (about 3 feet here in southern NH). Simple enough, but here’s the complication: I want to build a mortared stone wall two feet high around the perimeter (screened walls will rise from the top of the stone). The concrete piers will be poured first, contained within plastic forms, to the height of the 2-ft wall. The mortared wall will then be laid around the piers, so they’ll be more or less invisible once the posts have been set on them.
My plan is to dig an 18″ deep trench under the wall, place 4″ drainage pipe at the bottom of the trench (draining to daylight downslope), cover with compacted crushed stone, then a 6″ reinforced slab to serve as the base for the mortared wall. This design assumes that the structure is frost-protected because the piers extend below frost level and are not tied to the wall (even though they’re encapsulated in it). I’m seeking to protect the mortared wall from major movement with the drainage pipe and crushed stone, but that doesn’t extend below frost level. I’ve been advised to line the trench with rigid foam insulation. Subsoil is gravel (very little clay here in NH), with excellent drainage.
I’d be grateful for any advise on whether this seems like adequate frost protection.
Thanks.
Replies
I know very little about this sort of thing, but am thinking that unless the porch is heated (and I assume since it's screened it will not be), the foam will do nothing. What I have seen is putting the foam on the outside of the wall that's below grade and that insulated it from the outside frost, plus it hold in any heat that is escaping into below grade from the adjacent heated structure (the actual house). Then the natural heat of the earth (always about 55 degrees if you get below the frost line) will keep the area above it from freezing because the foam will sort of encapsulate that heat and the heat from the house. But if you put foam under the drain tiles, seems like it would insulate it from the heat, while letting the frost penetrate and letting the house heat out. I've even seen where they put a layer of foam just under the soil at the perimeter of the structure and sloping down slightly into the ground to keep frost out--but I think it only works with a heated structure. Hope this makes some sense. The rest of your ideas sound fine.
Yeah, I didn't think the foam was likely to do much. The advise, by the way, was not to lay the foam in the bottom, but around the perimeter. My question was intended to be less about the foam detail, and more about the base for the mortared wall.
your plan is trouble waiting to happen.
you want to put in a proper foundation (below frostline) for the concrete piers but an improper foundation (above frostline) for the wall foundation.
you then want the stone which is resting on a foundation which will move with frost heave to connect and wrap around a pier which won't move with frost heave. this is going to cause cracking and real mess.
to do this properly, you need to excavate for full footers below frostline on all three sides of the addition. concrete block or poured concrete foundation walls can then be installed as a base for your stonework. this will provide a better (broader) foundation for your timber posts so that the post load is not just on a 2 x 2 pier. if you want a concrete slab, this can be poured on a shelf created by the foundation walls.
it sounds like you are about to invest a lot of money in this addition. save yourself some headaches early on.
is your footer depth really only 36" in northern NH ? that's what we use in southern PA.........
carpenter in transition
GT,
The ship I was on used to tie up in Portsmouth at the wire and cable factory. Beautiful place, NH.
What ya wanna do is dig a trench to the frost line and fill it with "gravel (very little clay here in NH), with excellent drainage", compact it to the same density as the "gravel (very little clay here in NH), with excellent drainage" ya dug out.
If your lazy, just pretend ya did all that work.
LOL
You already have an excelent base for a small freestanding wall like that.
Make sure there is no top soil on top of this gravel, then lay your stone wall.
If there is no water in your frost zone, there will be no effective frost heave. Moisture doesn't count with gravel. You do have some fines in the mix, so I would leave an inch clear between the wall and the timber. Any local soil engineer can tell you what the normal heave height is. Leave that clearance.
I'm with Danno re: foam.
SamT
There are standard frosted protected shallow foundation designs for heated homes. The IRC has them.
However, in googling there I have seen references to frost protectected shallow foundation designs for unheated buildings.
The concept is valid, but I don't know how well it will work in your application.
Basically all the insualtion does is to put artifical "dirt" over the foundation and it is "heated" by the earth.
But with the masonary wall on top that might be too big a heat source.
But try a google on frosted protected shallow foundation unheated or garage.