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DaveBell
Moderator
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# Posted: 2 May 2016 05:54pm
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I'm reading IRC Section R404 and I can't seem to find rebar requirements for the slab. I found rebar for footers. Look like two #4's (1/2 inch) in the bottom of the footers and one at the top to tie in the mesh? Is there a requirement for rebar in the slab? Or is it just a best practice? Either way, I was thinking #4 in the slab spaced 4'. This is for a single story 16x40.
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MtnDon
Member
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# Posted: 2 May 2016 06:16pm
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I didn't try to find a reference in the IRC, but around here (NM) we use remesh... the welded wire comes in rolls and panels.
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bldginsp
Member
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# Posted: 2 May 2016 08:43pm - Edited by: bldginsp
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There is no requirement for rebar in a non-structural slab, only in the load bearing footings. There is a requirement for a vapor barrier over gravel, called a capillary break, which prevents moisture in the earth from wicking upward.
The only thing you need to accomplish with the slab itself is to prevent it offsetting after the inevitable cracks occur. Welded wire mesh is probably adequate. I'd use #3 rebar at 16" centers. #4 rebar is way overkill. The rebar won't be in much tension, just shear as the two pieces of concrete on either side of the crack try to move up or down. The mesh or rebar just inhibits that.
Compact the H*** out of the dirt under the slab, 4" layer of clean gravel on that, 10 mil plastic on that, 1" sand on top of the plastic, rebar or mesh, make the slab 5 inches thick. 4 is all that's required, but that's skimpy. Get a good concrete finisher, it's not as easy as it looks.
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MtnDon
Member
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# Posted: 2 May 2016 09:00pm
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If it's for a habitable building I believe foam insulation under the slab is code though, depending on climate zone.
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bldginsp
Member
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# Posted: 2 May 2016 09:24pm
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I think you're right Don. But I have always thought that insulation under a slab is kind of a joke. You can't really put any serious thickness of foam, so you don't get much R value. And installing it is a nightmare, people stepping on it and breaking it. For a slab in cold areas hydronic tubing hooked up to a boiler is the best way to go to keep your toesies warm in January.
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DaveBell
Moderator
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# Posted: 2 May 2016 10:13pm
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Thank you MtnDon and especially Bldginsp for your insights and knowledge.
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MtnDon
Member
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# Posted: 2 May 2016 10:23pm - Edited by: MtnDon
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Quoting: bldginsp But I have always thought that insulation under a slab is kind of a joke. You can't really put any serious thickness of foam, so you don't get much R value.
Have to disagree. We have friends with a home built near us two years ago. Slab on grade, single level just like ours built in 1985. Our slab had some token white foam installed vertically around the perimeter. Nothing under the slab.
Our friends home has XPS foam under the entire slab as well as vertical around the perimeter. I seem to recall it is R10, 2 inches, but more around the perimeter floor area. Maybe 4 feet in from the wall is twice as thick?? Not certain. . Can't remember the vertical foam thickness. It was dirt, compacted gravel, foam, polyethylene, concrete.
We both have ceramic floor tile over the concrete. There is a world of difference in winter floor temperature, especially within a few feet of any exterior wall. I really wish we had the same. It may have been a bother to install, he certainly paid more for his slab, but the temperature difference is outstanding. The balance of the house is outstanding too energywise with walls, ceiling, windows all better than code.
It all adds up. Heating and cooling costs are minimal. Many days only the air handler runs with the air to air heat exchanger. Smaller than normal furnace and A/C.
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bldginsp
Member
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# Posted: 2 May 2016 11:06pm
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I see what you mean Don. I've never seen that much insulation in a slab, but I live in sunny California where cold winter days are an occasional inconvenience.
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MtnDon
Member
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# Posted: 3 May 2016 12:24am
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Like much with building the best home, it has to be best for conditions experienced. We may be in the SW but we also have altitude. The latter half of December through the month of march we freeze virtually every night here at 5500. Yesterday we were in Los Alamos and it snowed most of the day and dropped to 28 overnight; 7500 feet.
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bldginsp
Member
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# Posted: 3 May 2016 09:43am
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Yesterday morning it actually got below 50. I had to put on a jacket.
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DaveBell
Moderator
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# Posted: 3 May 2016 06:07pm - Edited by: DaveBell
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I can only pour 4" slab. The slab and the footings will take 16 CY which is two truck loads.
Here is a process I have come up with from your advice.
Rough in Plumbing and Electrical
Compact Dirt
Add 4" gravel (crusher run)
Compact gravel
Dig 24"x12"Footers
Two #4 rebar at bottom of footers on 4" chairs
10 mil plastic capillary break
2" foam insulation
#3 Rebar in 4 foot squares on 1.5" chairs
Wire Mesh wired to rebar
Install 16 J-Bolts
Pour concrete
Sounds like a plan.
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bldginsp
Member
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# Posted: 3 May 2016 11:44pm
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Couple changes- do all necessary excavation first, before anything else.
Slab rebar #3 at 24" maximum, not 4 feet.
At least one # 4 rebar bottom AND top in footings.
Don't forget to wrap the pipe with foam where exposed above gravel. Elec conduit can be in contact with concrete but not ABS drain line or copper supply.
Set a Ufer ground near where your elec panel will be.
Post pics!
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DaveBell
Moderator
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# Posted: 4 May 2016 08:00am - Edited by: DaveBell
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Rough in Plumbing and Electrical Pipe
3" Black ABS w/two 45 degree bends for Water (1" HDPE supply)
3" Schedule 40 for Power
1" Schedule 40 for Phone
3" PVC for Toilet (add Flange)
2" PVC for Shower (add Trap)
1.5" PVC for Sink
4" PVC Trunk line (1/8 - 1/4 Slope)
Wrap Foam around rough-in plumbing from gravel up
Compact Dirt
Add 4" gravel (crusher run)
Compact gravel
Dig 24"x12"Footers
Two #4 rebar at bottom of footers on 4" chairs and one #4 rebar at top
10 mil plastic capillary break
2" foam insulation
#3 Rebar in 2 foot squares on 1.5" chairs
Ufer ground: Tie in a rebar from Footer rebar up to Elec Panel location
Wire Mesh wired to rebar
Install 16 J-Bolts
Pour concrete
Thanks bldginsp, MtnDon, Toy_Tech, old243, 1tentman, and Bevis.
Excavation to start maybe next month.
Nice to have a detailed process; for me to follow, for the concrete finishers, and for all on the forum.
Will post pics.
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bldginsp
Member
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# Posted: 4 May 2016 08:52am
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We haven't talked at all about your wall design, but if you will need shear walls or alternate braced walls some hold downs will be needed. 16 J bolts for a 16 x 40 seems light.
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Steve_S
Member
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# Posted: 4 May 2016 09:21am
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Dave: On the Foam for under the slab, stick to high density XPS which provides R5 per inch. In my region (North East Ontario Canada) code requires 2" for a Frost Protected Slab, I was fortunate to get plenty of 4" XPS. Most other foams (especially under 2" thickness) will crush, compact or break apart.
Be sure to cover the foam with plastic before laying out your rebar & wire mesh... seal & tape the plastic sheeting with a minimum of 6" overlap. That's to prevent moisture wicking through from surface to concrete.
Lesson Learned Dept:
If your going full FPSF and want the foam on the sides of your concrete pad, you can install after pour & set (major PITA) and what I am stuck doing now.... so hear me when I say you don't wanna do this after....
Add 4" to your form's making it 16'4" X 40'4" instead of 16x40.
Cut your 2" foam sides to the depth of your pad, so they make contact with the foam under & reach the top of the pad. The outside of my pad is 8" from the foam under to the top surface of the concrete.
Insert your anchors through the foam and lay up the foam along the perimeter of your slab, so when the concrete is poured in the foam is where it should be & anchors there to hold it all in. ! Glue (with construction glue) the foam edges and to the bottom sheet.
I used these hilti.com insulation-anchors/r217 as provided by my concrete guy.
Good article using one of the methods for an FPSF Foundation here Kalamazoo Zero Energy Ready Home foundation NOTE how they setup their foam sheeting, gluing the sheets together and taping them before covering with plastic... The devil is always in the details !
Sealing the concrete.... if you followed my build article you'll know why I mention it... You want to seal it but you want to make sure it's done right the 1st time and accounts for how you are going top finish your floors. This sub-topic still vexes me, so I won't get further into it as it raises my grumble quotient.
Final thought.... regardless of what your average temps are for your region / area, plan for the worst / most extreme case scenario so you won't get caught off guard... If your typical winter averages at say 25-30F but sometimes can have colder winters or periods of 0F, then consider that as part of your planning strategy.... Better to take into all considerations, especially on things that cannot be changed after being built.
Considered Radiant Heat ? In case your pondering, check out radiantcompany.com I used one of their kits and the materials & workmanship are 100% Top Notch!
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Don_P
Member
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# Posted: 4 May 2016 10:00pm
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Use 57's rather than crusher run, you are shooting for creating a drainage layer there, stone dust is a wick.
The foam can be regular density, check the load capacity and think about it... unless you do own a Sherman tank. The allowable load for typical foam is as high as the allowable load for typical soil.
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creeky
Member
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# Posted: 5 May 2016 09:03am
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xps has advantages beyond the ability to handle pressure. it is "closed cell" or waterproof. in my tests it has proven more bug resistant. and it offers a higher r value per inch.
it is also easy to find "recycled" which can reduce costs.
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DaveBell
Moderator
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# Posted: 5 May 2016 10:52am
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Thanks DonP, will change to 57 gravel.
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Don_P
Member
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# Posted: 5 May 2016 11:07am
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I do agree with XPS, the high density is unnecessary for normal work.
In the same vein, we typically do not compact either the undisturbed soil or the gravel. Which to be technical, you consolidate gravel cause you sure can't compact it. We spread the 57's, rough screed them level, trench in the drains and then rework the gravel to a final flat surface to fully support the foam. Walking and working on the 57's, by the time of the pour we have consolidated them under foot. The final grading gives full support for the sheet foam. If it is breaking underfoot you didn't get the prep flat enough, air has zero bearing capacity, there's the problem area, bed it well. That info does all come from above my pay grade.
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bldginsp
Member
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# Posted: 5 May 2016 09:01pm - Edited by: bldginsp
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Quoting: Don_P air has zero bearing capacity
I have to remember this one. Today I saw a 5x16 20 foot ceiling beam with one post over nothing at all in the subfloor (where there was supposed to be a pier). I wish I had told the contractor that "air has no bearing capacity".
But, you know, Alladin's magic carpet was bearing on the air. And, a basic airfoil is, in effect, bearing on air. All of which my contractor friend should take into consideration as he tries to figure out the easiest way to support that beam.
Now you know why we need building inspectors.
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DaveBell
Moderator
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# Posted: 7 May 2016 04:11pm - Edited by: DaveBell
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Quoting: bldginsp 16 J bolts for a 16 x 40 seems light.
One J-Bolt every 6 feet, right?
Concrete Math
I have increased my footprint by 8 feet based on two truck loads of concrete, 8.5 CY each, total 17 CY and reducing my footers from 24" to 18". 12" minimum is code according to IRC R404.
So for a 16x48 concrete slab 4" thick...
(L' x W' x D') / 27 = CY
(L x W x 0.333) / 27 = CY
(47 x 15 x 0.333) / 27 = CY
(705 x 0.333) / 27 = CY
235 / 27 = CY
8.70 CY
Footers 12"Wide x 18"Depth...
(L' x W' x D') / 27 = CY
(L x W x 1.5) / 27 = CY
Linear Length = 48 + 48 + 15 + 15 = 126'
(126 x 1 x 1.5) / 27 = CY
189 / 27 = CY
7.0 = CY
8.7 + 7.0 = 15.7 CY
15.7 x 10% = 1.57
15.7 + 1.57 = 17.22 CY
So I'm 0.22 CY short of the 10% margin.
The rebar in the slab may make that up.
Approx $2400 delivered for two truck loads.
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DaveBell
Moderator
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# Posted: 7 May 2016 04:54pm - Edited by: DaveBell
Reply
Gravel, 57 type
4" gravel, 46' x 14' =
(L' x W' x D') / 27 = CY
(46 x 14 x 0.333) /27 = CY
220 / 27 = CY
8.2 CY Gravel
8.2 x 1.4 = 11.5 tons gravel
11.5 tons x $20 = $230.00
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bldginsp
Member
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# Posted: 7 May 2016 04:55pm - Edited by: bldginsp
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Here we put in J bolts every 4 feet, and more in shear walls if called for. But maybe you are not in a seismic zone.
I didn't double check your math but I think you are cutting it too close. The footings won't all be exactly 18", there will be cave ins, etc.- you always use a bit more than you think. If you are short half a yard you got a problem. Leave yourself some slack- better to buy a whole yard extra than come up 1/4 yard short. You gotta fill the forms.
Got another project that needs concrete like steps, shed slab, etc? Get another 1/2 truck and make use of it there.
12" wide footings are minimum for single story, if two story you need more width. Are you doing any structural calcs on this project? Have you designed shear or braced walls? This all needs to get figured before the concrete is designed.
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Don_P
Member
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# Posted: 7 May 2016 10:51pm
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Bldginsp, I think it probably came from Bryan Readling, check the pic from his powerpoint. He's an APA engineer, well worth it if he comes to your area on training.
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bldginsp
Member
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# Posted: 8 May 2016 12:03am
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Yeah, I see a lot of those air nails. They're really popular.
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1tentman
Member
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# Posted: 8 May 2016 12:22am
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Davebell
I have been following your topic and find it interesting because you are building a mirror image of my cabin. Mine is a 20x40 on a floating slab also. You mentioned you was increasing the length to 48' is that set in stone, if you go wider you can gain more sq. footage and use the same amount of concrete. I would also make the footing only 12" deep instead of 18. The foot print of the footing is still 12".
20x40 800 sq ft with 12" footing will take 14.2 cy
16x48 768 sq ft with 12" footing will take 14.1cy
This allows you to order a extra yard like bldginsp said and get it on 2 trucks.
The rebar- I would go with all 1/2" on 24" spacing and not use any wire mesh the mesh is not needed just a extra expense ,also no stands needed although they look nice on the job they are not really needed. Bend the end of the rebar at 90 degree and drive the end into the bottom of footing trench and extend up into floor, tie the bottom bar about 2" off the bottom of trench 2nd bar at about 10 o'clock in the bend and the first row for the floor at about 2 o'clock. The first two rows will be held up by the bend,find some old bricks and break them up and put under joints in the floor area and your ready to pour. When you bend the bars and tie the rebar in the bend area it ties the footing into the floor and makes for a strong joining area.
The foam is a good idea I recommend it.What you use for fill is your choice, Don P recommended gravel it creates a drainage area which sounds good ,but it also allows water to go under floor easier, myself I like using lime or sand it is easier to level and it compacts well. I dont think you need 4" though ,you need just enough to make it level. Peal off sod and any loose soil set your forms to obtain your floor height your foam will account for 2" you might need just a couple of inches in one end and maybe some more somewhere else ,I just never could figure out why you need to dig up good solid soil just to replace it with fill ,the virgin soil will be as solid as 4" of packed fill why dig deeper than you need to.
You have been given alot of good advice on this forum and I think it will help with your project, now you just have to figure out your plan and I think your well on your way to a good quality project.
I had a gentleman that poured alot of concrete in his life tell me there was two things to remember when pouring concrete. 1st the bottom of the floor needes to be as level as the top, he said to screed your fill just like you was screeding the top, because any variance on the bottom will not let the floor expand and contract at the same rate thus causing it to stress more thus causing more cracks,makes sense to me and he said to get a real good finish on the top, the floor will sweat less. I have tried hard to follow these two tips and my concrete has turned out pretty good in my opinion. Good luck and have a good time building your cabin
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Don_P
Member
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# Posted: 8 May 2016 09:17am
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The footings are the structure under load and should be reinforced with steel that can keep them from displacing if they crack, rebar.
The slab itself is a subfloor, its the plywood (I've had it described as the carpet), supported by the ground. We have been talking in numbers that put any framed floor to shame, in the thousand pounds per square foot capacity range vs 50 psf for a wood floor. Structurally the reinforcement there is optional, I think it is a good idea although fiber is more than up to the task and what I normally use, I think it does better with fine cracking. If the slab will see heavier traffic then 10 gauge welded wire is the next step up. I prefer mats to rolls because it is flatter, nice to tie to for radiant tubing. If it is a heavier use slab on grade then rebar is warranted. Nothing wrong with rebar in any slab though.
Rebar should always have concrete around it, do not poke it into the soil. This will fail inspection. Rust is many times the size of the steel and that expansion of rusting steel is just as effective as ice at breaking the concrete. Use chairs or similar to raise it off the fill. The bricks and rocks are frowned on because they create a thickness variation and a stress point. When placing the mud make sure the finishers hook and pick up the reinforcement as they go... another reason I like fibers, they are uniformly whipped in the mix.
Drainage, keeps things dry. Small grain size... dirt, sand, lime, crusher run...is easier to work with but capillaries water up to the slab, larger stone helps water drop. I like pea gravel, 9's, but it can capillary water under the right conditions. Clean larger fill is specified for this reason. I prefer to create a slight slope on the subgrade that drains to daylight or sump, then place the gravel level. You aren't going to exclude water, so give it an easy way out. Some people prefer a thin layer of sand on top of the plastic to separate the bottom of the concrete from the plastic to help with curling, I've never had a problem there.
Keeping the thickness uniform helps, try not to create inside corners whenever possible, the shrinkage will concentrate on those and generate a crack. (transition from footing to slab as a taper not a sharp corner, 1tentman's rebar over that corner is a good idea.) Saw the slab into roughly 12'x12' areas to help control cracking.
The best way to help prevent sweating is to insulate the slab, if it is warmer than ground temp it is less likely to sweat. If there is radiant and it does sweat, drive a few degrees into it and you'll be above dew point... ground temp is below dew point fairly often.
I'm assuming this is a FPSF from the minimal footing depth.
ACI, American Concrete Institute (which doesn't give anything away) is the main reference, this is one of their manuals; https://www.concrete.org/store/productdetail.aspx?ItemID=CCS110&Format=HARD_COPY
Have more help than you need lined up, nothing causes family emergencies or foul weather like having a truck leaving the plant.
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Littlecooner
Member
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# Posted: 8 May 2016 09:21am
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1tentman
You are "spot on" in your recommendations. I hope the OP follows your advice. He has received a lot of input on this topic and I hope he observes the best of the advice. Agree with the thickness of the slab having to be uniform for the uniform in expansion, contraction as it will crack if not uniform in thickness. I have observed people digging up great virgin soil to replace with expensive products. That does depend on exactly what soil is under your house site. Just a few minutes ago, went to "websoilsurvey" and printed the soil type and recommendations for a client who is building a high dollar residence for his primary residence. Question was " how much topsoil to remove and what kind of footer". The website is tough to learn just how to navigate but it there for the entire lower 48 ( I think). I have only used it in the SE for properties in TN, AL and GA. It is the answer to your foundation questions for your exact site as these soil surveys have been completed by the PHD's of the world and after 40+ years of using them, I have never saw anything that looked incorrect to me in reading what they said was there before I even looked at the soil.
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Littlecooner
Member
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# Posted: 8 May 2016 09:28am
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Don_P
And Don P is "spot on". Rebar is placed in concrete because concrete is super in "compression" and terr able in "tension" (bending). Like he says, the footer is the structural component for the building and the pad is just a subfloor. Ever since the industry started using fiberglass, I have recommend hightly to people to add the fiberglass to any pad, like a floor. It is a little more aggravating to finish, but you have such a superior product in these continuous pads that do not receive saw joints for expansion and contraction.
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