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low277
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# Posted: 2 May 2016 12:40pm
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I spent a good deal of time reading old posts of all kinds with special attention to foundations.
I worked on a house about 20 years ago that was set on a wood foundation with crushed rock/gravel as a footing. I remember it being about 4ft from the bottom plate to the bottom of the floor joists. I could clearly see the bottom plate and the gravel. I was installing electrical power for the sump pump, a short electric water heater and the forced air gas furnace ( it was a horizontal unit ).
The bottom of the foundation plate was only in the ground about 3 feet below outside grade.
I am thinking of trying a similar set up, I would insulate the foundation walls and keep it heated to at least 50 possibly higher, I wired another more conventional house on a crawl space which used the whole crawl space as a air plenum. The furnace was in the crawl space with return ducts connected to it, but the supply side just pressurized the entire crawl space and the floor registers were just cut through the flooring.
Anyone have any experience with this type of foundation?
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azgreg
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# Posted: 2 May 2016 05:04pm
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Here's one from another forum.
http://countryplans.com/smf/index.php?topic=10671.0
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MtnDon
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# Posted: 2 May 2016 07:27pm
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I have not personal experience with a PWF, but it is one of the foundations I would be considering if I was to build a new cabin. My other choice to consider would be a concrete slab. I like the PWF as I could do it myself with my neighbor's skid steer for the required excavation.
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Don_P
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# Posted: 2 May 2016 07:38pm
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It doesn't need to be below grade at all if you don't want, the bottom of the gravel trench needs to be below frost depth is all. If you poured a thin "rat slab" underneath to seal it well The plenum should work fine and will give you a conditioned crawl space, high tech and everything.
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Julie2Oregon
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# Posted: 2 May 2016 07:48pm
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Quoting: azgreg Here's one from another forum.http://countryplans.com/smf/index.php?topic=10671.0
That was really interesting.
One thing that had me scratching my head. Why did he let the cabin go unsided for so long? I mean, he was adding on a deck, the deck railing, bringing in the kitchen cabinets and everything while there was still no siding on the cabin. Is there a reason for that?
I saw he had mold inside the crawl space foundation, too. 
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bldginsp
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# Posted: 2 May 2016 08:00pm - Edited by: bldginsp
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Here in California they used to build houses on old growth redwood foundations. I had some friends who bought one of those, a 100 year old farm house which was still on the wood foundation. It needed replacement which they did, but 100 years ain't bad.
But the PT wood they have now won't last half as long as tight grained old growth redwood. The best they have is PT fir, and the fir is second growth. If I were doing this I would bend over backwards to get high quality wood of great resistance to rot. Around here it's possible to get Juniper, which supposedly is more rot resistant than old growth redwood. But Juniper is rare and not available outside of the West Coast.
Black Locust and Osage Orange are two common East Coast species that have extreme rot resistance. You might have to find a small local mill that can custom cut the dimensions you need, and you might have to wait a while to get it.
But I doubt that standard PT fir would last more than 20 years in a moist, humid environment, more in a dryer area.
I have to say I don't like the wood foundation shown in the link above. It has to rot in your lifetime, and when it does there is no easy fix. I don't see the sense in putting so much work into something that has to fail. IMO
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Don_P
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# Posted: 2 May 2016 10:05pm
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Black locust has the reputation for being extremely rot resistant. I've had posts rot in 7 years and some are over 30. In the heartwood of naturally decay resistant species the concentration of extractives varies tremendously. The Forest Products Labs have done tests with the same results.
The best treating wood is southern yellow pine, it has long, large, open cells that take the chemicals readily. You are looking for wood graded and treated for a permanent wood foundation, so we are not talking about standard treated here. It has no heartwood, which doesn't take the chemical and is treated to .60 lbs per cu ft or UC4A (one of the PWF guru's can double check that use category, going from memory). This is typically special order although in coastal areas up to 2.50 is often available.
I prefer this braced foundation to piers as it does stabilize the structure but do prefer full perimeter masonry for a foundation overall. Precast foundation walls, Superior Walls and others are crane set on a gravel trench, insulated and ready to build on the next day, another way, sort of the best of both if available.
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bldginsp
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# Posted: 2 May 2016 11:23pm
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I've seen descriptions of some fast growing tree species with such open pore structures that they can attach a pressure fitting on one end of the pole and force preservative through the entire length of the pole, displacing the water content of the cells.
I've heard of positive results with treated S Yellow Pine. How long they figure it's good for in ground contact?
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Don_P
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# Posted: 3 May 2016 07:45am
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Worth inquiring, off the top I believe the warranty is 40 years.
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azgreg
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# Posted: 3 May 2016 09:58am
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Quoting: Don_P Worth inquiring, off the top I believe the warranty is 40 years.
I believe the life span of a concrete/block foundation is a wee bit longer. 
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KinAlberta
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# Posted: 3 May 2016 12:27pm - Edited by: KinAlberta
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Yeah, anywhere, where wood is near ground contact I'd want a rot proof (not just rot resistant) separator. Concrete blocks, or maybe even a layer of composite decking or anything that will prevent moisture from wicking up into the wood, PT or not.
As an aside, note in that country plans thread, that the guy laid the plate flat on the gravel. I figure he missed an opportunity to build beams under the walls to stabilize and spread the load of the structure. Also, he created a 6' crawl space. Why not pay a bit more and create a more functional basement, even if it is only going to be used for storage?
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low277
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# Posted: 3 May 2016 02:14pm
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The house in the original post is in NW Minnesota about 20 miles east of the North Dakota line and 15 miles south of the Manitoba line. The charts say that frost line is between 5 and 7 feet although I do not think I have seen any footings for residential basements any deeper than 7 feet below grade.
I had asked the homeowner about his choice of foundations and he said it was a speed thing, his carpenters could build the foundation much faster than he could get a block or concrete one made and being he was heating the crawl space frost wasn't an issue.
The bottom plate of the foundation wall couldn't have been more than 3 feet below grade.
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Julie2Oregon
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# Posted: 3 May 2016 08:11pm
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Quoting: KinAlberta Also, he created a 6' crawl space. Why not pay a bit more and create a more functional basement, even if it is only going to be used for storage?
He wrote that he didn't want a basement for code and tax reasons. If he had an actual basement, it would have been classified as habitable space and add to the square footage/tax bill. As a crawl space foundation, he can still use the area for storage and his mechanical systems but it wouldn't be habitable space.
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Don_P
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# Posted: 3 May 2016 09:27pm
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The flat plate on gravel is the prescriptive method, that is a minimum though, you can always do more. Part of what they are trying for is affordable. I agree that a block foundation is better as would be stainless I beams, you're looking down the road the wrong way though... it is far superior to piers.
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NorthRick
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# Posted: 4 May 2016 01:53am
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Quoting: KinAlberta Yeah, anywhere, where wood is near ground contact I'd want a rot proof (not just rot resistant) separator. Concrete blocks, or maybe even a layer of composite decking or anything that will prevent moisture from wicking up into the wood, PT or not.
There are a about a bazillion utility poles that are made from wood and set in the ground. Many are 30, 40, 50, or more, years old. Properly treated, wood will last a lifetime in the ground.
Most composite decking will come a part if in direct contact with the ground. It is mostly wood (not treated), mixed with plastic. Works good in the weather above the ground but not in contact with it.
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bldginsp
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# Posted: 4 May 2016 09:07am
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Quoting: NorthRick There are a about a bazillion utility poles that are made from wood and set in the ground. Many are 30, 40, 50, or more, years old. Properly treated, wood will last a lifetime in the ground.
Average life expectancy for utility poles is 60 years, so I hear, but that's here in the dry west.
Your point is valid but what you are talking about is wood that has been treated differently than the PT you get at the lumberyard. I wouldn't want that stuff within 30 yards of where I'm living. I don't know if they still use creosote, but that is some of the nastiest stuff on the planet. Kills everything that comes near it. No wonder the poles last so long.
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