Calculating PSI load garage floor

[Gregg]

Happy Holidays everyone. I am installing radiant heat in the garage and trying to calculate the PSI of the foam required to be installed under the slab. The xps is rated at 25, 40, 60 & 100psi. Recommendations have run from 25 psi to 60 psi. Price as you can guess increases at approx 50% more for the 60 psi over the 25. Availability is also a factor.

Has anyone installed foam under their slab and if so, what was the psi rating and was it sufficient.

Concrete will be 4500 psi with wire mesh, probably fiberglass fibers as well, poured to 6 inches.

Any and all assistance is greatly appreciated. Thanks. Gregg

 

[Trevor Booth]

You need to consult a civil/structural Engineer. There are many factors to be taken into account-- The rating of the foam is related to the mass of the slab and the loading that will be applied plus the ground pressure rating beneath the foam. 6" is one hell of a slab for a garage - do you really need that thickness ?

The consulting fees you pay will ensure quality and a cost effective job.

 

[David Sturch]

Gregg,

This was the product we used on a friends garage 24'x24' and I think we used 40psi on a good bed of compacted gravel. We used wire mesh on about 4" of concrete. We also put down a 6 Mil vapor barrier first. The building code here does not requre it.
Dave

STYROFOAM? Brand HIGHLOAD 40 Insulation

 

[Dave Lindemann]

You need to consult a civil/structural Engineer. There are many factors to be taken into account-- The rating of the foam is related to the mass of the slab and the loading that will be applied plus the ground pressure rating beneath the foam. 6" is one hell of a slab for a garage - do you really need that thickness ?

The consulting fees you pay will ensure quality and a cost effective job.

For what it's worth I agree with Trevor - consult with an engineer. I had to reinforce the roof of my pole building when I decided to insulate/heat the structure. Just having an engineer's stamp on the drawings made the entire approval process a piece of cake - and it gave me peace of mind. The cost was minimal. Best of luck!
Dave L

 

[Cliff Beer]

Gregg, should be a very nice garage to work in. I have heated floors in my house and they're great. I thought about installing in my garage as well but didn't in the end (because I wasn't around to supervise the install).

There are floor heating products designed to go right in the concrete (so you don't have to worry too much about engineering pressure loads and such) such at the tough cable system here: Other Radiant Heat Products by Heatizon Systems

The floor heating systems are all pretty much the same, other than the type of element. They convert high voltage electricity into low voltage and use the resistence of the element to create and control the heat. You have to "tune" the voltage converter to the type and size of element but it's not that difficult. The "in concrete" elements are good as the element is stable and protected, however, the one concern I had was with cracking of the concrete (all concrete cracks eventually) and invasion of moisture from below in a garage setting.

In any case, hope the above is helpful.

 

[Trevor Booth]

(all concrete cracks eventually)

a properly designed concrete slab will not crack, you may get surface "checks" but that is not a crack.

yes he does need to consider engineering pressure loads, the foam underlay is the insular barrier and if it is not specified correctly the foam will collapse and the RC Slab WILL crack due to differential movement. A membrane will stop moisture penetration AND stop water loss during the curing process (28days) and is particularly important in an "in floor heating" RC slab.
Gregg, talk to an Engineer I have specced many In floor heated RC slabs and when done correctly it will not have issues in the future. Hot water circulated in 3/4 alkathene pipes is common in Oz

 

[David Sturch]

Trevor,
He won't be using water for the recirc system unless it's a commercial garage. I,m across the big pond(Lake Ontario) and it's -18 Deg C here. He's a bit vague on what he's building. If he's got a building permit he will have to summit his drawings to the local municipality and it will be done to code.
Dave

 

[Trevor Booth]

David,
understood, same scenario here, however, some structures just "appear" -

 

[Gregg]

Gents, thanks for all the replies and advice. Plans are on file and structure is up. I did consult a structural engineer who advised 25 psi was adequate under the slab, however it seemed light to me.

Plan was to pour 5 inch slab but contractor advised to pour 6 inch. I agree with you Trevor, 6" appears to be way overkill. I will revisit the issue again.

Thanks again for all the replies.

 

[Cliff Beer]

Good stuff guys.

I poured my garage slab to 10 inches which, of course, is major overkill, however, I have a 10,000lb two post auto lift in there (w/o bridge) which sometimes has to hoist up my Chevy Suburban so I wanted the slab to be very, very strong.

Trevor is right, a properly designed slab won't crack. Trouble is, ground is always shifting and eventually there are areas below the slab (or water invasion) which eventually causes the slab to crack - might be a couple years, might be 20+. The only concrete slabs I'm aware of which should be crack free into the future are the slabs which are poured with pre-tensioned re-bar (the pre-tensioning prevents cracks even with differential pressures) but that's pretty exotic and not for home garage usage.

If you're planning on having a two post lift in there Gregg then the 6 inch slab might be advisable.

Cheers and Happy Holidays!

 

[Roark_John]

Good stuff guys.

I poured my garage slab to 10 inches which, of course, is major overkill, however, I have a 10,000lb two post auto lift in there (w/o bridge) which sometimes has to hoist up my Chevy Suburban so I wanted the slab to be very, very strong.

Trevor is right, a properly designed slab won't crack. Trouble is, ground is always shifting and eventually there are areas below the slab (or water invasion) which eventually causes the slab to crack - might be a couple years, might be 20+. The only concrete slabs I'm aware of which should be crack free into the future are the slabs which are poured with pre-tensioned re-bar (the pre-tensioning prevents cracks even with differential pressures) but that's pretty exotic and not for home garage usage.

If you're planning on having a two post lift in there Gregg then the 6 inch slab might be advisable.

Cheers and Happy Holidays!

10" is beyond overkill for a 2 post, 4 post or even a lift for semis...it's a complete waste of time, money & resources. The inner 4" of your pad won't be dry for another 30 years...the very base of the Hoover Dam is still curing, it's 200 ft. thick and has been around for 80 years.

A 4" pad is sufficient to support a 15-20,000lb machine tool like a VMC or CNC lathe on 4 points of contact. A 6" pad is sufficient to support a 50-60,000lb machine tool like a HMC or VTL on 4 points of contact. An 8" pad is more than sufficient to support a transfer-line setup like is seen in an engine production plant. In these cases the thickness of concrete is about stability for the machine and avoiding transferance of excessive vibration to & from the machine...not about the loads the pad will actually see.

To put it another way, the autobahns have 4-6" of road-base, 4" of reinforced concrete and another 2-3" of asphalt on top...the asphalt is resurfaced every 4-5 years, but the concrete is unharmed after millions of road-miles and billions of tons of traffic from personal vehicles to OTR trucks.

If 4" of concrete is good enough to support the dynamic load of thousands of 80,000lb semis year after year, it's more than adequate for a static load of a 2-post lift with a 6,000lb vehicle on it.

 

[Dwight]

When I build my shop I ask four different concrete contractors about the fiber. They all said they would not put fiber in their slab.