I’m still kicking this little vacation house around, design-wise. Thanks go to those that have looked and commented so far.
Here’s the deal. Pifin has convinced me to not do it atop an alaskan slab, a.k.a. frost-protected shallow foundation. So I am going to propose a crawlspace foundation, a simple 8″ poured concrete wall sitting on footings down below frostline, a rat-slab floor, internal piers where needed for the floorframe and post loads, and a fireplace footing in the middle if we will do a real stone fireplace.
Here is a view of the place.
I’m focused now on the two unheated but under-roof wings of the place, the one on the nearside of this view being the entry porch, and the one on the opposite side being a screened-in porch.
I can choose to do each of these unheated section pieces with a wood-framed floor, using sonotube-formed round piers where needed, ACQ treated lumber rims and beams and joists, but that has me concerned.
First of all, there is all the hanger expense and the questions about longevity associated with using P.T. lumber, and then, there is the question of how well a deck frame will do sitting just about atop finished grade, with no air circulation under it at all.
So I ask, would you do with wood-framed floor decks for these wings? And if so, what kinds of details would be important?
There is an alternative, that being to make the crawlspace foundation bigger and more complex, by doing walls and footings under the ELLs of the outer walls under these unheated sections. There would still need to be foundation walls under the gable walls that are common to inside and outside space, the function of those walls being to close off the “warm” crawlspace from the “cold,” and to provide bearing for the separate floor frames, inside and out.
Another option is to forget about wood framed floors for the two porches, and do slabs instead, but in that case I’ll still want some piers under the slab at the load points.
What do you think?
Replies
This may be overkill, but the two images here show my first-cut idea for doing the wing-room floors as slabs.
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The entry porch side is different from the screened porch side, in that its outboard corner has a footing-and-block pier for the stone-faced piers with wood columns that dress and support that corner.
But the scheme for slab support is the same for both sides. First, raise the foundation walls adjoining these slabs so the inside floor frame is built within at those walls, rather than sitting atop mudsills with its rims butting to exterior fill and slab concrete.
Widen the footings under those walls as shown, so we can fix some sonotubes in place after we strip the forms, and pour the tubes that hug the walls. We'll pour the outside tubes at the same time.
The loads from the roofs above these rooms is entirely borne by the outer corner piers. The rest of the tube piers are there just to help keep the slabs from sinking.
I thought that if we did a good job with the right kind of material and using compaction, we can get adequate base for the slabs. Bent dowel #4 rebar coming out of the piers and into the slab, plus #4 bar at 12x12 and up on chairs when we pour, in the slabs, should do some good here, too. We would like an exposed aggregate finish for these.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
Gene,
Yes, I immediately thought slabs as well.
Regarding your slab drawings, why not tie the inboard side of each slab to the footed foundation wall alonside, without the need for sonotubes and piers to the footing? And what is the reason for the large left-side footing? - your right side doesn't show this. Maybe I missed something - greater loads on the left?
Take what I say with a grain - around here the frost depth is about 9".
Regards,
Jim x 3
I had thought about the slab-to-wall joint, and how maybe, in lieu of doing the sonotube piers below, just sparing the expense of the extra footing width and the three tubes, and just . . .
. . . thickening the slabs where they meet the walls, drilling the walls and using epoxy to fix #6 dowels in place on, say, 12" centers, then pouring slabs. Here's pic, and in the pic you can see how the concrete walls of the crawlspace foundation are raised at this interface, with the floorframe hung to its inside face. This detail has been used a few times around town here, when a slab is tight adjacent a foundation, and its top needs to be up close to the elevation of the floor inside the house.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
I don't think you want to tie those slabe to the foundation. Let them sit on a ledge.
The slabs need to float independently of any of the load bearing points on the "ells".
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
Thanks for the input, Eric.
This sketch, below, shows a detail we used on that job you visited, a few years back. Bent #5s in the tube reach into the thickend slab. No adverse consequences yet, but we still have our fingers crossed.
A guy that is building something I drew up for him is using this same thing now.
I've slabs tied to my foundation in my house here, built nine winters ago, using the dowel-no-ledge detail, and no movement yet.
The piers hugging the wall seemed like a good solid way to give the slab bearing and prevent sinking. So did the idea of thickening the wall and doing a ledge. Forming a ledge could be done, too, but that is beyond the ken of our local formbuilders.
Doweling seemed more straightforward, and likely will save money.
Nothing is cast in stone yet, as they say. It is all in the fantasy land of 3D CAD, and we can do what we want. Let's keep thinking.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
Edited 9/23/2009 10:22 pm ET by Gene_Davis
Instead of those sonotubes on a ledge, keep the ledge and add pilasters to the wall?
Here in Ohio, we'd pour a wall on a footer, add the rigid foam insulation boards, back-fill to cover the foam and pour a floor on the back-fill. For a crawl space type of thingy, we'd keep the wall below the bottom of the floor, and poured the floor over the wall. Wall would have bent rebars running from footer up to the floor, with one leg (footer end) going to the outside, and the other leg turned inside for the floor. Rebar for the floor was then tied to the bent ends sticking out of the wall, giving a continius run from footer, through the floor slab, and back down to the other footer.
Pilasters would have either a wider footer ( with a rebar mat to tie into) or just a simple "pier footer" if they were to stand alone ( as in a frost wall/grade beam). They make a special rebar "insert" for these. One "half" gets form up in the wall pour at the pilaster's center line. The other "half" will get screwed into the forst as the pilaster is formed up. You can then add either anchor bolts or, bent rebar to the top of the pilaster as needed. Forms for pilasters can be from forms stripped off of the wall pour. No sonotubes to buy.
" Although I have the right to remain stupid, I try not to abuse that right"
One other item. On that big sonotube pier all by it's lonesome, why not go the caison route? Just dig a hole to the depth you need, make up a cage of rebar to drop in the hole, wrap the cage in a plastic "bag" , drop this assembly down in the hole and pour a pier. No tube to buy again. You can then epoxy "dowels into the finished caison to lock in whatever you want on the pier's top. Hole does not have to be perfectly round." Although I have the right to remain stupid, I try not to abuse that right"
Why not Block for foundation? If you did block, just dig out where the porches sit and us footer, block wall to support the slab and point loads from the roof. Probably less expensive than the poured wall!
No one ever considers block here. The work would be done by stonemasons, all who have their noses way up in the air, having had way too cushy a ride for the past dozen years or so doing fieldstone fireplaces for the wealthy.
Even when the do a foundation for a doublewide, it goes poured 'crete.
So it must be due to not just the lack of masons, but just plain old "that's the way we do it." We lived in western PA for quite a while, where all foundations are done in block. Out where we lived in the midwest, everything got poured, and it is the same here.
A lot of projects done by smaller builders and owner/builders are getting foundations done with ICF.
This house will get occasional use, maybe a week or two, each winter. Primary use will be for summer vacations. That is why I am considering the use of a simple poured 'crete crawlspace foundation. Our state energy code (if we were building in a jurisdiction where we had to follow it) has a prescription for insulated foundations for houses built in this zone. To meet that code, we would have to put rigid foam under the rat slab, and enough insulation on the walls to achieve about an R-13.
Someone else asked, why not slab, and someone also asked, what is your frost depth, or where are you building.
Depending on soil type, drainage, and siting, frost can go as deep as six feet. We are a zone 3 climate with 9000 heating degree days, and our record lows from December through February are in the 45 below range.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
Depending on soil type, drainage, and siting, frost can go as deep as six feet. We are a zone 3 climate with 9000 heating degree days, and our record lows from December through February are in the 45 below>>>>>>>>>>>>>>>
I am aware.
Block masons are used around here, because the poured concrete contractos, to put it bluntly, suck. Can't square a foundation to save them!
Gene,
Nice looking 'cabin.'
Just some thoughts...
You're already excavating to footing grade under 3/4 of the entry slab area and 1/2 of the porch slab area.
Since you're placing a slab over these spots, backfilling will be $low and tediou$ to get proper compaction to support the slabs.
I just don't like mixing footing types under a slab on backfill.
For very little more cash, %-wise, you can use T-footings under all edges of the slabs.
In that vien, excavating another 2' deep would give a usable basement and two under-slab rooms by putting stairs behind, say, the porch.
They make precast concrete "spans" you can use if you don't want to pour your own. You've probably seen those, they look like slabs with large holes drilled thru lengthways.
IIRC, they're 8" thick. Adding 2" insulation under and 2" finish over will put them about the same thickness as your interior sub floors.
If you stick with piers, I would thicken the footings they bear on to compensate for the uneven loading they'll put on the footings. I would also bring in a soils engineer to verify the compaction uder the slabs. He'll have to be there for their entire backfill time.SamTA Pragmatic Classical Liberal, aka Libertarian.
I'm always right! Except when I'm not.
Nothing wrong with a slab. Where will you be building?