Laying Floor Joist Directly On Conctrete

Larman

Hi,

I am a painter by trade but would like to add a room to the back of my garage. I would like the floor to match the same height as my exisiting floor inside my house. This would require me to raise the floor off the cement garage floor. I have to raise it about 10 inches. Can I just lay my floor joist directly on the concrete?

Forry

Think about moisture barrier and pt lumber and insulation and the unevenness of the existing concrete floor.

BuiltByMAC

4x4 pressure treated beams 5' o.c. or so, sitting on the concrete, leveled w/ plastic shims (3 1/2")
Definitely add vapor barrier and insulation
2x6 d. fir or syp 16" or 24" o.c. for your floor joists (5 1/2")
3/4" plywood subfloor brings you up to 9 3/4"
Floor covering (tile, carpet) brings you to just over 10"

Mac

TaitINC

I would def. try something along the lines of what mac said. I would lay 2x4 PT w/ 24" between them and glue and nail them inplace. Then install 24" x 1 1/2" foam between the 2x4's. Then lay 6 Mil poly over it and staple it down. Lay another 2x4 plate around the perimeter and like make said again at 4-5' O.C. going the opposite direction of the PT on the concrete. Then your F.J. over them going the direction of the PT on the concrete @ 16 O.C. 3/4" subfloor over them. The hights of everything need to be determined on sight. Just the way i would do it

spencer

Teetorbilt

Here, PT is required for direct contact with concrete.

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BuiltByMAC

Yup, I think it's that way pretty much nationwide. That's why all three suggestions included the use of PT lumber where wood touches concrete.

Mac

K2

That would not be allowed in my area and they use the minimum height in crawlspace to enforce it with the local requirement being 36 inches. I've done joist on concrete and seen it done in other parts of the country but I never liked it.

What if you get critters in those joist spaces or you get some drainage water down there you would never even know it until it was too late even with pt. yuk. I'd get some roadbase fill and pour a slab.

K2

Here the wood to concrete rule pertains to bottom plates,sill plates, and studs shot to the walls. Not to joist systems.

shed-n-deck

If you want the floor to be level, I would suggest that you use treated sleepers. Find your level along 2 parallel sides, run a 2x4 upright between those sides. Using a pencil laying flat on the floor, scribe a line on the 2x4. Cut along the scribe. Now, place the sleeper on the floor at the location that you scribed it, and snap a string line from one side to the other(along the level lines). Cut that, and then attach it with screws, hard nails, or pins. You don't need too many sleepers, it depends mostly on what you are using for joists, how far apart the joists are, and how much area you have to span. As far as the level lines go, assuming you use 2x8's with 3/4" sub-floor, you would want the level line to be 8" below your existing sub-floor. That would mean that your sleepers would be approximately 2" tall.

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BuiltByMAC

The only part of the joist system that would need pt for this idea would be the house side rim joist and the two outermost joists (the only wood in the joist system that would be near/touching concrete). This is assuming the garage has stemwalls on three sides.

Mac

K2

I'd still go for a slab but the best would be to see what the local building dept allows... I was just reading the ubc and they have a minimum space under a joist system of 18 inches. I think the IRC would be close to the same.

And if someone gave me a 18"crawler I'd probably have to shoot them. As I get older I find that stand up crawlers work the best.

shed-n-deck

I've heard that too, but I don't think that it applies in this situation...I've always wondered why they require 18" under foot for structural floors, yet you can't have any clearance under a slab.....But there are situations that do allow a wood joist floor system to be built without the minimum 18" CS clearance...I don't know what the CS requirements are, but I think they might have something to do with plumbing.

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Kaiser

I'm in Fl now but I lived in Co most of my life. I believe the 18" rule has something to do with moisture. But on the front Range the soil is the problem. (Expansion) I saw a garage pad in Ken Cayl that heave 2 ft.

pknyryan

Can you not eliminate "touching" the garage floor at all and simple secure a 2x8 to perimeter walls and use joist hangers to build a deck sitting approx. 1.5 to 2 inches above garage floor. This will eliminate the need to worry about shimming an uneven floor, vapor barriers, ect. I would apply a dry-lock type "paint" to the floor, install the floor, insulate it with polystyrene, cover it with 23/32" T&G flooring (glued and screwed) and apply desired fininsing (tile, carpet, ect).

I am assuming your exterior walls in you garage are dimensional lumber, altough a similar system would work with CBS type construction using PT lumber and "tapconning" it to exterior walls.

I would be hesitant to rely working off a poured concrete floor as they are rarely level, and inherently not designed to allow for a floor to be built on top of them. If the floor happens to selttle, crack, ect, what happens to your new floor? (imagining floor tiles cracking of poppoing up) At least when using the exterior walls to support the weight, you have the added assurance you are transferring the load to exterior walls, which theoretically are spported by a footing and designed to carry a load and resist settling.

K2

Appraiser, home inspectors, fireman, and buyers have problems with inaccessable spaces under floors. And how would you vent these closed spaces?

Even if you did pt, vapor barriers, and found a way to vent it, what do you tell the next buyer/ home inspector/ appraiser?? You can show him pic of what you did but what about the 3rd buyer 12 years from now? There would be no way to know what mold, bugs, and dead critters are down there. I'd go slab. It'd probably be cheaper anyway. In floor heat too.

neolitic

Have done quite a few floor systems
over slab, never had a complaint from
HO, inspector, or mechanicals.
How does slab on slab solve
any of the problems you raise?
Well except for "critters".
I usually wouldn't allow for
"coon access" anyway.
What I've found in regular crawl spaces
would be a pretty long list.
Many folks prefer wood floor systems
to slabs, especially if half the room
is already wood.

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NC-SC, GC

Simple answer is Yes, it's just a sleeper floor, I would suggest you glue the floor joists to the floor, also be sure to use a good vapor barrier. As the code is written a crawl space foundation must have a minimum of 18 inches although there is an exception that states "the use of pressure treated lumber is acceptable if the clearance is less than the minimum requirement" .

Suggestion is to have the entire floor treated with a generous Boracare treatment to prevent the infestation of termites.

K2

Ok, does it need to be vented and how do you vent it??

neolitic

In the OP's scenario,
an attached garage--no.
Start with a 6 mil vapor barrier
and there is nothing to vent.
I don't know what you are doing though.

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K2

I would think that R408.1 would apply.

I'm not doing anything I'm just questioning whether a "sleeper floor" is allowed under the IRC or the UBC..

I would say that i could not get a sleeper floor approved in my county for a number of different reasons including R408.1.

For my area with 36"min stem walls it would be easy to get a skid into a garage for a half a day and dig a proper accessable, (irc R408.3) crawl space in about 4 hours and then do a joist system. Or the slab over that i already mentioned.