Foundation ideas for timbered shed

catvet · July 06, 2007, 08:23:29 PM

I'm thinking that this is my first post to the Timber Framing forum though I've been on the Forestry forum for a while. Starting to cut timbers for my proposed 18 x 16 "cabin". I'll be the first to admit I have little (read that as none) experience building. This will be my first timber frame project.

A nagging question is what to do foundation wise. My site is out in the woods, can get my tractor to it but not concrete truck. Also looking to do minimum site disturbance. There are a couple of trees which the cabin will be nestled between which I would like to preserve I figure that means not messing with their root system. The site is on a reasonable slope. Our frost line is 4-5 feet down.

Here is my current idea:
1. Place large stones on the downhill side of the site for "retaining wall".
2. Bring in crushed stone (front loader of tractor) and level the area uphill of the stones after placing some drain pipes Plan on having at least 6" of crushed stone at highest point of site. Compress stone as much as possible.
3. Place large flat stones on top of crushed stone to act to spread out weight of building.
4. build slip-form walls approx. 2 feet high on top of flat rocks to act as perimeter wall to set timberframe on using local stone and hand mixed concrete.

So how many things can you find wrong with those ideas ???. How would you do it?
Any suggestions appreciated.

Left Coast Chris · July 06, 2007, 10:41:10 PM

Suggest checking with your building official. The UBC (uniform building code) requires foundations too be a minimum of 1'-0" below the surface and more is needed in freeze areas. Treated wood can be in contact with the ground but that is not recommended for a permanant structure. You may have more options than we do out west (unrienforced stone wall or footing).

The issue with the freezing of the ground is expansion. The degree of expansion is dependant on soil type, how much water is in the soil, how cold it gets and how long the cold lasts.

If soil with moisture is closer to the surface under one part of the structure than another you could get differential movemet ---- in other words one part of the structure gets heaved up more than another.

If there is a chance of a problem, suggest getting a local engineer or the building inspector to take a look at the site.

Tim L · July 07, 2007, 06:15:19 AM

Here in NH I saw a guy lay down crushed stone as you suggest ,then used pressure treated timbers lap jointed together. He then built conventional stick built on top 16" on center with strapping and board and batts siding.(horse barn) I thought he was nuts, but he said as long as the water drained away it would work. It's been 6 or 7 years and the ridge is still straight and everything looks OK . I don't know how deep the stone is and he's not around to ask anymore.

sbishop · July 07, 2007, 06:16:05 AM

not sure if this is ok for you since you have a slope but i went with the "pappy system" check out these threads.

Pappy's cabin

My cabin

I got the idea from Pappy. he used 24" dia, 4" thick cement blocks laid on the ground. Then I put pieces of treated telephone poles to my sills.

Got luck and keep us updated!

Sbishop

Don P · July 07, 2007, 06:44:06 AM

The old cabins around here used rock piers dug down to frost depth and stacked up to the sill. They often spat a rock and things settled, but mortar and good technique can improve upon that. Between these piers, later on as the family got time and money, they often built curtain walls of brick underpenning... pier and curtain wall. A pier under each post above, minimum.

I think you're retaining wall will prove too active for a solid foundation, I am working off a gravel footing now, they do work if done right but the gravel cannot move. We have about 3' of gravel in trenches 3' wide if I remember right.

maineframer · July 07, 2007, 08:03:25 AM

I think what Tim is describing is what I know as a rubble trench foundation. It consists of digging a trench 16-20" wide down to just below the frost line. The bottom of the trench must be pitched with a 4" drain tile in it to remove water. The trench is then filled with crushed stone . A Mudsill system of 8x8 pressure treated timbers or otherwise rot resistant material should be used. In my area (which is similar to yours geographicly) I would plan on going 41/2 - 5 feet deep if possible with the stone. Another method would be to excavate the top organic soils and replace with a pad of well compacted gravel, lay down 18" round concrete pads level them and build of them.This method is requires some maintainence through the years and is similar to the old barns just built on rocks placed on the ground.

TW · July 07, 2007, 09:09:13 AM

A thought from a place far away.
A dry stacked retaining wall tends to fall outwards, pushed one or two millimeters per year by frost. Sometimes they last well and sometimes not. Our frostline is 2 metres (6'4") and the hills are usually a mixture of silt and gravel and a lot of stones, dumped by the ice age.

I do not know the geology over there and is unable to give reliable advice in this case.

Tim L · July 07, 2007, 09:11:55 AM

I am going to build a barn (30x50) and have thought about this myself. I think what Maineframer described is probably what was done .Forum members: I'm asking, do you all think this feasable in New England weather. It sure would help me to stay within the budget. I just don't want to be kicking myself later on.

catvet · July 07, 2007, 10:34:58 AM

My concept on this foundation was that I would treat it like they do the "floating" slabs around here. With them they level off an area and then put down a bunch of stone to make sure water doesn't accumulate under the slab. They then form up the slab and pour. They seem to work fine. From what I gather the fact that the gravel keeps water from accumulating, and freezing, under the slab keeps everything from heaving.

My thought was the lower retaining wall would hold the stone in place. I would lay perforated drain pipe on the ground and run the ends out through the retaining wall. I would then place gravel (cheaper) up to a certain point to level out the site behind the wall and then add the crushed stone. I figure with the underlying slope, drain pipe, and gravel, things should drain out pretty well.

I was thinking large flat stones for distributing the weight out over a larger area and then the stone wall on top to build on.

I should point out that there is no zoning in the town so no code which has to be met. Not an excuse for shoddy work but also don't have to meet set requirements.

Don P · July 07, 2007, 11:37:54 AM

As long as the retaining wall cannot move, its rocks need to be massive. I think you're talking about a lake of gravel dammed by the retaining wall. Pushing down on that lake is the weight of the building. If the gravel moves, so does the building. The rubble trench works, building gravel above grade seems risky to me. Thats where I'd start thinking of piers or a wall on the rubble trench.

TW · July 07, 2007, 03:47:49 PM

I agree with Don P
Weight on the surface of ground immidiately behind a retaining wall adds greatly to the horizontal load on the retaining wall.
If the building is a cabin you do not need flat and level ground under the wooden floor. Concrete piers make sence to me. You can cast them one at a time. If the ground is clay or silt then piers are not so good.

I am no expert though.

Jim_Rogers · July 07, 2007, 05:18:23 PM

The idea about the drainage pipes and having the water run off is great, until it's below freezing then this water will freeze and not run off.....

catvet · July 07, 2007, 06:23:09 PM

I appreciate the feedback. The comments all make sense. Again I'm trying to protect the roots of the trees which will be close to the cabin which makes me want to try to stay away from digging trenches for footings.

The comments would make me lean towards maybe putting the struction on posts to minimize the amount of root disturbance. I was hoping to use a stone foundation but not if I'm digging 5 foot deep trenches to make it work. Don't want the cabin frost heaving.

maineframer · July 07, 2007, 07:45:26 PM

Catvet, Check out ------ technometalpost.com -------- a customer of mine has used these with success.

catvet · July 07, 2007, 08:32:11 PM

David,

Thanks for the link. Look interesting. Don't seem to have an installer in Vermont and I suspect I couldn't afford them for the project I'm doing.

ARKANSAWYER · July 07, 2007, 08:47:23 PM

My office just sits on stones on the ground and we have not had the ground freeze more then a few inches deep in many many years. (Just got to love the South)
I would auger holes and pour in concrete for a bottom then sit post in on top with crushed stone or gravel around the post. It is a simple way to to do it. You can then align the post and saw them off level and lay down you sill plates on top and bolt down into them.

catvet · July 08, 2007, 09:04:34 AM

Arkensawyer,

I expect you are right that I will end up planting posts to support the structure. The feedback I'm getting both on the forum and in talking with people in the area is making me lean in that direction. I'm thinking of using Post Protectors to help preserve the posts longer and prevent uplift.

My next question is what to do as far as putting the cabin on top of the posts. My plans call for an 8 x 8 sill but probably the best I'll do with posts is 6 x 6.

Don P · July 08, 2007, 09:44:21 AM

Got any friends at the power co?

Norwiscutter · July 08, 2007, 11:35:20 AM

Hire out someone with a skidloader and 24" auger. You can special order at most lumber yards 24" forming tubes up to 8 foot in length. Rent a mixer and bag mix to fill the tubes, or use a front end loader to transport concrete from the truck to your site. If going with a slab, foam insulation around the perimeter can be used to negate frost penitration.

TW · July 08, 2007, 02:55:48 PM

It is no problem to cast concrete pilings or pillars or whatever they are called. They will last much longer than wood. Wood below ground or directly on ground is only temporary.
Rent a concrete mixer, or buy a used one and bring there some aggregate and dry cement. You nail together some molds from ordinary sawn timber and put in some reinforcement bars and pour.
Pours up to 2 cubic metres are no problem if you get some friends to help.

catvet · July 08, 2007, 09:55:15 PM

TW how nice of you to assume I have friends ;).

Jim_Rogers · July 09, 2007, 08:03:51 AM

Another type of below grade foundation system using wood is called the short pole system.
In this system only the part of the post below grade is pressure treated wood. Above grade you use regular wood and you sister a piece of pressure treated over the seam.
Like this:

And if you get the pressure treated stuff that lasts 60 years you won't have to worry about it.

If it should rot out before then you just jack up or support the building and then remove and replace only the rotted lower section of the short pole system.....

Jim Rogers

catvet · July 09, 2007, 08:24:24 AM

Jim,

Interesting foundation system. Hadn't seen anything quite like that before, thanks.

I assume the sistering boards are on all four sides, not the two shown for illustrative purposes. I would also think you want to somehow seal the top of the connection (between non PT and sisters)

Jerry

Jim_Rogers · July 09, 2007, 09:45:35 AM

Yes, well it depends on how your building will sit down over the short pole system...

ARKANSAWYER · July 09, 2007, 07:07:20 PM

Had a customer come in today that is building a pole barn way out in the country. He had bought some 1x8's and 2x4's a week or so ago. Seems he made open troughs that screwed together and had 2x4's around the middle and ends. He then cut re-bar to length and but his forms on a two axle trailer and went to the concrete yard and boutght 1 yard of concrete and had them mix and pour into his forms. Said that he moved over to the side in the shade after the pour and put in the re-bar and "J" bolts in the end where he drilled holes and took a nap. After the concrete sit up for a few hours he drove home with his piers. Said that he had to adjust the height by adding gravel to the bottom of the holes.