Concrete footer and slab questions
Looks like we have finally narrowed down a set of plans for our next house and I have a few questions for someone who has worked with a ICF home built on a “frost protected shallow foundation.
Question 1, Can this be done as a monolithic pour? Trench my footers and lay in my rebar etc as well as have my slab, fairly large one at 2100 sq ft. with a 315 lf of footer. See houseplans.com plan #17-174. Would like to pour the entire slab all at one time if possible. 2ac flat lot with good access and good soil, no standing water etc. Part of the original homestead, appears to have been a hay field at one time.
Question 2, Planning on Amvic 6″ or 8″ thick x 10′ high walls (this is potentially a high seismic area, (zone 5 if memory serves). What I have read is footer should be 2x wide as the wall for stick frame but I have not found any reference to ICF. Looks like if I go 2x width of outside of form the footer has to be 22″-26 wide or do I go with the concrete core and end up with a footer 12″ – 16″ wide? My planned depth is 14″
Question 3, Would you go with a 4, 5 or 6″ slab? I will have in floor pex for heat.
I am planning on getting a soils test before I start but rule of thumb looks like 3K psf will work based on the tables from the concretenetwork.com “concrete footing dimensions” table for a single story 8″ solid wall. If I go mono I will plan on running the rebar out of the footer to the slab and tie it all together in the prescriptive manner.
I know my local Amvic dealer will probably have these answers but would like a second opinion from those that work with this type of construction as well.
Thanks in advance
Replies
Question 1, Can this be done as a monolithic pour?
Yes, but doing it in two pours makes installation of subslab plumbing, drains, radon vent, water line, etc simpler. And the slab can be poured on top of a gravel bed which will create a capillary break.
The vertical and wing insulation for the shallow foundation is meant to protect it from frost heaving, but for a radiant slab, additional insulation is necessary at the edge where most heat loss occurs.
Question 2, What I have read is footer should be 2x wide as the wall
A standard footing for an 8" concrete wall is 8" deep by 16" wide. Poor or questionable soils might require a wider footing. If you're only excavating 14", are you going to get below top soil horizons to stable subsoil?
Question 3, Would you go with a 4, 5 or 6" slab? I will have in floor pex for heat.
Standard slab is 4" thick. But for a slab that large and with PEX, I'd consider going thicker and using minimum 3500# concrete and fiber. Typically, the PEX is tied to welded wire mesh and the mesh is lifted an inch or so into the concrete to place it in proper position for tensile strength. The PEX ideally would be placed not so low as to create long lag time for heating, but low enough to prevent accidentally nailing through.
Solar & Super-Insulated Healthy Homes
The OP hasn't said where he is building. Our code mandates a minimum 18" wide footing regardless of the bearing capacity of the soil.
Riversong, thanks for the excellent reply. You are right and I will make it in two pours to simplify the process and give me time to properly prepare the sub-grade.
I spoke with my Amvic supplier today and he recommended minimum of a 24"x 12" thick footer, I will probably go for a 24x14 with extra rebar (earthquake zone 5 here). I think at this point I am going to go with 8" walls.
My supplier quoted me 17K and change for 8"- 10' walls. I will be shelling out 1k a week for two sets of bracing, considering having my son who is a long haul trucker pick up a couple of sets out of Canada or watch Craigs list when the time comes. What would you do?
If I have my own sets I can take my time and also have them around to build my kids house with later on. I have roughly 315 lineal feet of wall. I should be able to sell them after I finish without taking a big hit on the price if I take care of them.
Know anything about this company? http://www.wass2.com/ looks like a nice setup, are there others that are easily adjustable that you would recommend? I have no idea what WASS wants for a set. Think I will email and find out.
Thanks
Jim
Thanks guys for the input, I will go with a 5" slab here in Wasilla AK.
Jim
I don't use ICFs.
I build with natural materials with low embodied energy, low greenhouse gas contribution, low environmental impact, and locally-sourced as far as possible.
That is, rough-sawn lumber and cellulose insulation.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Riversong I applaud your convictions but getting a banker here to buy off on something like that would be quite difficult. Plus I like the idea of a solid wall when it comes to EQ country. I will live about 1/2 mile from a potentially big fault line. Also since our population density is so low we have few alternative building choices.
Guess I could go with the original native covered semi-buried lodge with walrus skin outer cover and a inner shell covered with caribou skins but they too are a little difficult to come by and I doubt the subdivision covenants would be met as well.
Although they use to be able to heat them with just seal oil candles. For some reason I don't think I would be still married either. Seal oil really stinks. Plus the Greenpeace wackos would have a cow if I (a white man) killed a walrus or a seal these days.
Jim
Riversong I applaud your convictions but getting a banker here to buy off on something like that would be quite difficult. Plus I like the idea of a solid wall when it comes to EQ country.
Something like what? A house framed with wood and insulated with the best performing insulation on the market (in terms of heat loss, hygroscopicity, fire-proofing and sound attentuation)? We're not talking foundation, since you're using a SFP foundation (which I do as well, to minimize site disruption and use of high-embodied energy concrete).
And earthquate resistance isn't achieved with rigid walls which can only crack under tensile stress, but with resilient walls that can flex without damage.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Riversong, I have to agree with what you said about the wood framed home. That idea is still not totally settled in my mind and I do like your "larson truss" design. I plan on keeping an open mind about the final material choices, it will ultimately boil down to cost per sq. foot and if I have to have a crew or not. Also the subdivision covenants require at least the outside of the home to be completely finished not more than a year after beginning construction.
Many homes here in rural Alaska seem to get to the point where the homeowner/builder gets the house wrap on and runs out of money before the siding is ever put on. They do not want any "tar paper shacks" in the subdivision, can't say as I blame them as my lot will be one of the last to be built on.
I have lived in the house I built since 78, its a 2x6 on 16"s ' three level home and main floor supports are honking big glue lams. With a AWWF foundation. 2x8 on 12" on the main earth bearing wall and 2x6 on 12 on the rest. We have had many eq's over the years none larger than a 4 that I can remember and only a few minor cracks in the SR. So yes depending on the cost of concrete down the road (4-5 years from now) I may well look at a super-insulated wood frame structure.
At this point I like ICF because of the following reasons, 1. I can do all but the pour by my self. That way there is now one else to blame if things are not right. 2. With enough rebar (prescriptive methodology) and the new and by then (should be) well accepted steel fibers it should be able to ride out anything but a major 6-7+ eq. 3. It will be sound wise far quieter than my present home (granted most noise comes through the windows). 3. Easy to heat and ventilate. I have a quote from my local Amvic supplier of $17k and change for 350 lf for 10' walls. Granted that is only a drop in the bucket of the total cost. I have not put a fine point on the cost of concrete yet but in todays dollars its going to be $100+ per cuyd. and it looks like somewhere around 163 cubic yards.
In this locale to build totally green (at this time anyway) comes with a price that does not make economical sense, ie freight is a bear here. Local timber is of a poor choice for dimensional lumber, Spruce is the predominant species and when it is milled green and then air dries you could use it as a big corkscrew. There are not any mills here and the logs around here are not big enough to make into house logs. Most log homes here are built from imported logs from Canada, big $$.
I plan on keeping an open mind and would like to attend one your seminars but due to work and the travel distance it is rather cost prohibitive. When/if you come out with a DVD of your lectures I would appreciate purchasing one.
Jim
Where are you building? Where I live, all frost protected slab on grades have to be signed off on by an engineer.
We just poured one today. We pour them monolithically. Can't really how you would do it another way. Or why you would. The rebar cage in the footing pretty much has to be up in the top 5 inches of concrete. Our footings are 18x18, and it's hard enough to get enough cover on the rebar as specced.
I'm not sure the rule of thumb of footing being twice wall width has much meaning on a slab on grade. But anyway, you would consider the concrete to be your wall thickness from an engineering point of view. Our footings on ICF basements are 3 times wall width anyway.
Again, if you have an engineer, he will spec this. Most of ours have 5" slab. Easier to get cover on rebar and heating pipes.
Talisker2,
Check out the site http://www.logixicf.com and follow links through technical specs, then look in engineering and product manual...
Different product, but same idea and great engineering tables for soil condditions, wall heights, widths etc....
Also check there pricing on bracing, usually pretty good.
Good luck
Bwhite, thanks for the info, unfortunately here in Anchorage AK we have to go with the local supplier. They (Advantix) has a block plant here. Anything else is shipped in. The saving are wiped out by the freight.
Thanks
Jim