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It sounds like later this summer I will be building a cabin with a Permanent Wood Foundation. I generally would steer away from a wood foundation, but due to the site location and redimix availability it’s seeming like the best option. <!—-><!—->
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The foundation will just be a dirt crawlspace with a poly barrier.. Foundation Panels will be 4’ tall.. I’ve read the manuals and have a couple of questions…<!—-> <!—->
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– The manuals show the crawlspace partially backfilled on the inside, but the manual also says not to backfill until after the floor structure is built and sheathed.. I understand the reasoning behind this, but it seems to present a logistical problem. Any suggestions or experience on how this should be done? At first blush this is what I’m thinking…<!—-> <!—->
1) Excavate area for foundation. Only fully excavate to depth where the foundation needs to go to frost depth.. <!—-><!—->
2) Build walls as suggested in manual, stake bottoms frequently with heavy rebar to avoid lateral movement. Brace tops frequently on both inside and out. <!—-> <!—->
3) Gently backfill with loose fill (rock) in small lifts (4-6 inches), compact between lifts. Only backfill approx 18 inches…<!—-> <!—->
4) Re-string walls and adjust tops as necessary… Build floor framing and finish back filling<!—-><!—->
My concern is keeping the bottom of the foundation from kicking in when backfilled or over time. Is there a better way than what I suggested?<!—-> <!—->
– Any suggestions on how to finish the above grade portion of the walls? Green treat is not the most eye-appealing and a don’t really care to run the siding all the way to the dirt..… I’m thinking maybe stucco over Durock.<!—-> <!—->
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Thanks,<!—-> <!—->
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Erk <!—-> <!—->
Replies
I think I would back fill both inside and outside partialy at the same time and in equal lifts.
A neighbor down the road from me has a log cabin on a wood foundation. His is a full basement with a poured floor. He ued a synthetic stone to cover the wood down to within a few inches of grade.
Dave,
I didn't mention that in my message, but my intention was to fill both sides in small lifts and compact. Glad we think alike...
Synthetic stone might be a good option. If I can keep the coverage to a fairly small area the material cost probably would not be that much..
Thanks,
Erk
Since readymix is not an option, can you get crushed stone?
My thinking is that a dense grade aggregate would be ideal to use as fill for the lower portion of the wall. Even sand would work if it is contained within a trench that the wall is built in. Both dga and sand are easier to get a decent compaction on than most native soils.
Maybe Catskinner will come along and chime in on this. He's the dirt guru.
What kind of crawl space height will have beneath your joist ? It could be a real head banger trying to compact anything in a low space with joist over your head.
I will have to get crushed stone on site as it is required for under the treated foundation. Loads will be fairly small, but I'm confident it is doable.. <!----><!----><!---->
I'll only have about 2.5-3' of clearance under the joists. <!----><!---->
I'd like to put a few of the 4-6" lifts inside/outside before the joists with the walls just staked and braced. Straighten everything out, add the joists/sheeting, then backfill the outside..
Am I crazy? I can't imagine trying to backfill and compact under the joists. I think I'd hand mix crete footers and stack block first..
>>Straighten everything out, add the joists/sheeting, then backfill the outside..
That is going to be the really hard part is if it moves very much durring the partial backfill stage. I think I would be checking my string lines or laser after every lift and compaction run. It won't be easy even making small adjustments as you move up in the fill. Remember your are not just trying to keep the walls straight but plum too.
It is a lot of work, but my old tight #### would have to give it a shot.
Good luck
Hey, I don't have much time right now, so I'll need to get back to this, but one option to keep the base of the walls from pushing inward may simply be to run what are essentially joists from one wall to the opposite wall at the base of the foundation, maybe every third or fourth stud. you could even lay them flat against the sole plate of the foundation. It may sound nuts to have them lying in the dirt, but the foundation will be already. Otherwise, could you utilize some shear walls perpendicular to the foundation? If these were placed directly under your floor joists, that should lock them into place and keep them from kicking up at the inside.
I had thought about running joists across the width at slightly above footer height. I'm pretty sure that I can't get 28' material, so I would have to have a real good connection in the center to avoid bowing.
As you said, as sheer wall or kicker could be attached to the joists to help as well...
Thanks,
Erk
I built a few 4 and 5 unit apartment buildings 20 years ago using PWF's with good success.
It sounds like you are planning to use the crawl space detail shown in Figure R 403.1(3) in IRC 2006 where the inside of the wall is backfilled partially leaving a minimum of 18 inches space between the poly sheet and the underside of the floor joists and backfilling the outside higher up.
There's no specification for the inside depth of the backfill to resist the exterior backfill force. How deep are you planning the interior backfill height to be?
We took a different approach - instead of creating a crawl space,
we backfilled both sides of the walls equally and poured a concrete slab within the wood foundation walls. This eliminated the need to deal with any of the problems with backfill pressure differences, crawl space venting, .... Unless you have a reason to make a crawl space, this may work for you as well.
We cut a 2 ft wide trench and laid the crushed stone pretty level then dropped in prefabricated 4 ft tall x 8 ft long foundation panels atop 2x10s. A local foam expander cut 43 inch x 14 3/8 inch x 2 inch EPS which we inserted into each stud bay before backfilling. We didn't fuss too much with bracing - one interior brace every 8 ft. to a stakes and rebar pins on both sides of the 2x10 every 8 ft.
We ran a string along the continuous top plate and kept an eye on it as the excavator dumped buckets of fill smack on top of the wall so each side got about half of each bucket drops on each side of the wall. We compacted every foot or so on the way up.
The buildings were around 60 ft long and 40 ft wide and we had no problem keeping the walls straight using this method.
Mike,
I was hoping to get 18" on the inside. I'm thinking that if it was well compacted this would probably be enough. Do you agree? Having the walls kick in over time is still my biggest concern.
We really need the crawl space to run HVAC with the design. With 18" of fill on the inside, we'd still have close to 30".
The staking and backfilling method that you describe was almost exactly what I was thinking.
Thanks,
Eric
I think 18 inches inside is more than enough.
<<Having the walls kick in over time is still my biggest concern.>>Are you talking about the foundation walls bowing after you've backfilled and set your floor joists? I have 4' of backfill on my 2x6 walls 16" o.c., 8' tall, with a slab basement floor holding the plate in place and 5/8" plywood. I've had no bowing on the wall over the 8' height in the last 9 years.
Edited 4/13/2009 12:22 am by splintergroupie
The only change I would make to the design is to put some washed pea stone on top of the poly in the crawl to protect it in the future. It's
not too bad to work and keeps the poly from getting torn up.
I have done a few crawls and building the floor actually worked well. We just added enough sheathing to keep it from racking (skip the first row) and poured the fill on the rim board so it filled in and out at the same time. Shovel and a broom clean-up(a leafblower if you have one) and your good to go.
Garett