Is cement really necessary in a foundation? If the foundation is protected from water and insulated from the cold and dries to the inside then wood could be used.
nilst
| Posted in Green Building Techniques on
I imagined this slab on grade for a ‘house to be’ in climate zone 7 (southern Quebec). Level site with appropriate drainage, six inches or more compacted gravel (3/4 inch), six to ten inches insulation, (R30 plus) , two layers of 4″x12″ pine around the perimeter to replace the cement footings and cement board furrings and pex piping for the heating underneath a hardwood finish floor. Also there will be an insulating skirt around the exterior. If cement slab can sit on insulation and support a building then so too can wood with its much greater tensile strength.
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Tensile strength doesn’t matter much if the material is just getting squished. You need compressive strength here. In the case of wood, tensile and compressive strength are about the same.
Concrete has the advantage that it doesn’t rot. Even treated wood is less durable in that regard. I personally would never use wood as a foundation material because I don’t trust it to last, and it can be VERY expensive to repair foundation damage. That said, I was only hours ago talking to another builder who is a big fan of pole barn buildings using treated wood posts.
Bill
I was inspired by an architect friend who replaced his crumbling stone foundation thirty years ago with a treated wood foundation. It was a beefed up stud wall (2x8) on one foot centres with a treated plywood exterior. This was resting on a shallow trench filled with ¾¨ gravel. He's had no problems with it. Concrete is expensive, has no tensile strength, and needs to be protected from water so as to avoid mould problems, has a high carbon cost; but it doesn't rot. In my project wood is there on site offering itself up for the taking.
Nils,
There are two distinct questions. One being how it would work, the other what the code would say about it.
The practical problems are probably mainly t0 do with the durability of the wood, not structural. That's going to be a hurdle you need to overcome to meet code too, which requires wood in close proximity to grade to be pressure treated.
I'd be interested to see a section of what you are proposing. There have been a couple of houses posted here in GBA that eliminate the slab in favour of wood framing on foam, but still kept the concrete foundation at the perimeter. I'd imagine that's because the problems at the interface between the ground and house are trickier to solve there.
I imagine that I'll need someone with letters to post after her name to help me get through institutional hurdles but I have put a lot of thought into the house/ground interface and think I have it figured out, but would appreciate any thoughts people have on how to deal with not only water but snow and ice accumulation plus water. I am curious to see the houses on GBA you refer to. Where are they?
Nils,
Here is one that Mike Maines linked to in an earlier discussion:
https://www.finehomebuilding.com/2019/02/27/minimizing-concrete-in-a-slab-on-grade-home
I'm interested to see that you come up with. I'm not comfortable burying a wood foundation, but what you are proposing may make a lot of sense. I was sceptical about eliminating the slab, but the more I think about it the less objections I can come up with.
I often do a simplified version of something quite similar when I build garden sheds, using a perimeter of pt 4"x4"s on a bed of clear-crushed gravel. The floor can be gravel or pavers. This is a woodshed I just finished:
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It's not clear to me, in this scenario is the wood foundation sitting on the foam, or the ground? If it's the foam, then you'd need to spread that load out over the whole floor. I'm not sure how you'd do that with wood, but maybe an engineer could do it. In that case, the wood is all within the conditioned space and should last as long as stick framed walls. If it's resting on the ground, then long term durability is something I'd worry about. A highly durable hardwood like black locust would be far superior to pressure treated lumber, but good luck getting that past a code official.
The raised bed of gravel plus the styrofoam in theory would put the wood foundation one foot above grade. If the 8 mil plastic radon and vapour barrier on top of the styrofoam go under the grade beam then up the exterior wall behind the sheathing two feet, then a continuous waterproof sheathing covers the intersection of the wall/grade beam down to the insulating skirt. That should keep water away from the wood. Assuming that my guesstimate of the width of the grade beam is correct there is the question of whether the beam will bow out in the middle over time.
There is such a thing as a wood foundation, but it's very different from what you describe here.
If you just want to avoid concrete, you could use those screw piles and build on that. No concrete needed there.
Nils,
If you Google "permanent wood foundations" you'll find that there are lots of proponents of these systems and you'll find lots of information on how they suggest detailing them.
Check out:
http://cwc.ca/wp-content/uploads/2015/07/PermanentWoodFoundationsCWC_2015_.pdf
The biggest challenge for long term durability is keeping all the water out. If it all wrapped in foam with water proofing similar to an inside-out pool, I can't see why it would not last indefinitely.
Akos, I concur the biggest challenge/issue for PWF foundations is keeping the water out. They are not suitable for every situation. There are prescriptive methods for building PWF foundations and from my understanding, they are accepted in most building codes and they can be made to last.
Nils, the part of your proposal to eliminate the concrete and use wood as a footing definitely has merit. However, the wood footings must be below grade. Backfill of the footings and foundation walls is a key structural component in PWF foundations to prevent the walls from bowing out is my understanding. A concrete or wood floor keeps the walls from bowing in.
Andy’s suggestion utilizing helical piles to support a PWF foundation is a good one. The piles can be capped above grade or below grade. Laminated beams would span the piles. Saddle style pile caps prevent lateral movement of the beam. Build your PWF foundation on the beams. Clean stone below the assembly is an important part of keeping the water issue at bay. Drainage from the site is also a key component for durability.
Here's two ideas to counter the bowing out issue. Steel cable to secure squares of greater than sixteen to twenty feet, or simply make the foundation into squares not more than twenty feet.
I don’t think steel cables would help much. Steel cables that are set under tension to stiffen things (like pretensioned cables in cast concrete beams) are under tensions that a normal wood structure wouldn’t be able to handle.
You’d probably be better off putting some horizontal beams across the wall — think of a sideways joist. This would help to stiffen the face of the wood foundation wall to keep it from bowing. You’d need to strap the horizontal beams using something like hurricane ties since most wood to wood connections are not able to work in tension very well.
I don’t see any reason why a wood foundation wall couldn’t be constructed to be able to handle the force of the ground on the other side. My only concern would be the long term durability of the structure. Concrete is happy to stay wet forever without problems, but even treated wood will eventually degrade. With suitable soil conditions, and the proper ground contact rated treated wood you could probably get a pretty decent life from such a wooden structure though. I think this would be an application for stainless fasteners too.
Bill
The foundation he proposed doesn't have footings. It's like a raft slab in which wood beam floor is substituted for the concrete slab. I'm sure there's a configuration that would satisfy the rigidity requirements as regards to bowing, I just don't know whether it would be practical.
AFIK pressure treated wood is nowhere near as good, or as durable as it was 20 years ago.
Imo it would end up being cheaper, and easier to build on helical piles.
I personally would never build or buy any building with a pwf foundation
Trevor,
In Canada arsenic-based CCA pt wood rated for direct ground contact was never banned and is still available. Because of public concern most lumberyards switched to ACQ and more recently to MPS, but you can still special order the CCA.
Like you, I wouldn't build a pwf.
Malcolm, you beat me to posting that info :)
Here in the US, the more nasty pressure treating compounds are available for some limited uses (like utility poles) where longevity is very important. Creosote is still available in poles. I’m not sure if the CCA treated wood can still be ordered in regular dimensional lumber anymore though. As far as I know, all of the ground contact rated treated lumber is using ACQ but with more of the treatment injected in the better grades that would be required for underground use.
I personally wouldn’t trust wood in a foundation either. Concrete is a much better option below ground, in my opinion.
Bill
If the wood foundation is above grade, then there is no sideways force in, the wood will not bow. The joist for the foor will do a good job of connecting everything up with a couple of simpson tension ties.
Even if you are not in Kansas, code will still require some way anchor the fundation to the ground, I think the screw pile suggestion above is probably the best. The one issue is that now you have something going through your water proofing layer, a detail which might not last the life of the house. Maybe if the screw pile is anchored to the outside to above grade upside down corbels would be better.
I hadn't thought about the possibility of wind moving the house. The steel screw piles would address the lateral movement and anchoring issue. I could place them inside the foundation beams at the corners and midway points, and detail them as I would plumbing or electrical entrances that pierce the "slab".