• Welcome to the new and improved Building Code Forum. We appreciate you being here and hope that you are getting the information that you need concerning all codes of the building trades. This is a free forum to the public due to the generosity of the Sawhorses, Corporate Supporters and Supporters who have upgraded their accounts. If you would like to have improved access to the forum please upgrade to Sawhorse by first logging in then clicking here: Upgrades

Floor joist to 'rat slab' clearance

R

Rio

Silver Member
Joined
Mar 31, 2010
Messages
224
The clearance from grade to the bottom of floor joists in enclosed areas, such as crawl spaces under a house, is per the CRC/IRC 18"

R317.1 Location required. Protection of wood and wood

based products from decay shall be provided in the following

locations by the use of naturally durable wood or wood that is

preservative-treated in accordance with AWPA Ul for the species,

product, preservative and end use. Preservatives shall be

listed in Section 4 of AWPA Ul.

1. Wood joists or the bottom of a wood structural floor

when closer than 18 inches (457 mm) or wood girders

when closer than 12 inches (305 mm) to the exposed

ground in crawl spaces or unexcavated area located

within the periphery of the building foundation.

We have a situation where there is a concrete slab where the grade is and from the top of that slab it's about 14" to the bottom of the floor joists. We've had this situation before in doing remodel additions and there's never been an issue but now there is. The inspector wants the slab removed or pressure treated joists used. I'm trying to find a code section that addresses this issue and haven't found anything except for the one above which does in a roundabout way. Does anyone know of a more specific citation?

Any suggestions would be appreciated,

Rio
 
Rio said:
The clearance from grade to the bottom of floor joists in enclosed areas, such as crawl spaces under a house, is per the CRC/IRC 18"R317.1 Location required. Protection of wood and wood

based products from decay shall be provided in the following

locations by the use of naturally durable wood or wood that is

preservative-treated in accordance with AWPA Ul for the species,

product, preservative and end use. Preservatives shall be

listed in Section 4 of AWPA Ul.

1. Wood joists or the bottom of a wood structural floor

when closer than 18 inches (457 mm) or wood girders

when closer than 12 inches (305 mm) to the exposed

ground in crawl spaces or unexcavated area located

within the periphery of the building foundation.

We have a situation where there is a concrete slab where the grade is and from the top of that slab it's about 14" to the bottom of the floor joists. We've had this situation before in doing remodel additions and there's never been an issue but now there is. The inspector wants the slab removed or pressure treated joists used. I'm trying to find a code section that addresses this issue and haven't found anything except for the one above which does in a roundabout way. Does anyone know of a more specific citation?

Any suggestions would be appreciated,

Rio
Click to expand...
to the exposed

ground in crawl spaces or unexcavated area located

within the periphery of the building foundation.
 
Gregg gave you the answer.....here's a little more ammo

R317.1 Location required. Protection of wood and wood based products from decay shall be provided in the following

locations by the use of naturally durable wood or wood that is preservative-treated in accordance with AWPA U1 for the species,

product, preservative and end use. Preservatives shall be listed in Section 4 of AWPA U1.

1. Wood joists or the bottom of a wood structural floor when closer than 18 inches (457 mm) or wood girders

when closer than 12 inches (305 mm) to the exposed ground in crawl spaces or unexcavated area located

within the periphery of the building foundation.

2. All wood framing members that rest on concrete or masonry exterior foundation walls and are less than 8

inches (203 mm) from the exposed ground.

3. Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.
 
Last edited by a moderator:
Jobsaver said:
Is exposed ground still exposed ground when covered by an impervious moisture barrier? What say you code geeks of ole?
Click to expand...
Well no....it's ground covered by an impervious moisture barrier. Exposed ground isn't shovel resistant like impervious ground.
 
Thanks for all the replies. I would say from reading and thinking on it the question is 'Is a slab considered the same as exposed ground?' I read the interesting attachment that Mark posted and according to that, it is considered the same even though it is obviously not. What's funny about that is when I was involved directly in construction, it was in San Diego and this issue came up several times and was signed off on no problem. That is why I was taken aback when it came up this time.

Thinking on the intent of this code section I would guess that the intent is to keep moisture a good distance from the wood. This does bring up the question of why is it okay for a girder to be 12" and a floor joist has to be 18"? From reading the attachment from the city of San Diego it appears that moisture is the concern also as if the slab has a moisture barrier under it then it's okay for the slab to be less than 18" to the bottom of the joists.

I posted this also on Contractor talk and someone suggested a solution might be to lay 6 mil plastic over the top of the slab, lapped 6" and sealed. Any feedback on that idea? I'm also now thinking that a possible solution if moisture is the issue might be to seal the top of the slab with a water sealer or a low permeable paint.

Thanks again for all the comments and any suggestions,

Rio
 
Rio said:
I posted this also on Contractor talk and someone suggested a solution might be to lay 6 mil plastic over the top of the slab, lapped 6" and sealed. Any feedback on that idea?
Click to expand...
3. Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.

That is what the code allows. I have approved that several times.

If the wood can be 6 mils away from the concrete, 14" shouldn't be a problem. I would use 10 mil because it stands up better while construction takes place.

As a side note I included the ventilation requirement to show how effective the IMB is held to be.

IBC

2. The total area of ventilation openings is permitted to be

reduced to 1/1,500 of the under-floor area where the

ground surface is covered with a Class I vapor retarder

material and the required openings are placed so as to

provide cross ventilation of the space. The installation

of operable louvers shall not be prohibited.

IRC

R408.1 Ventilation. The under-floor space between the bottom

of the floor joists and the earth under any building (except

space occupied by a basement) shall have ventilation openings

through foundation walls or exterior walls. The minimum net

area of ventilation openings shall not be less than 1 square foot

(0.0929 m2) for each 150 square feet (14 m2) of under-floor

space area, unless the ground surface is covered by a Class 1

vapor retarder material. When a Class 1 vapor retarder material

is used, the minimum net area of ventilation openings shall not

be less than 1 square foot (0.0929 m2) for each 1,500 square

feet (140 m2) of under-floor space area. One such ventilating

opening shall be within 3 feet (914 mm) of each corner of the

building.

http://www.finehomebuilding.com/item/5090/whats-the-difference-vapor-barriers-and-vapor-retarders
 
Last edited by a moderator:
Here's the update on the concrete slab being less than 18" to the floor joists. I just got off the phone with the plan checker and he said the concern they had is not with vapor coming up from below the slab but a plumbing leak getting the slab wet and then the concrete letting the slab hold the water. He said they discussed it with the B.O. and the contractor and agreed to jackhammer the slab enough to allow any water that might get on top of the slab to work its way down to the earth and be soaked up by the ground. Thanks again to all for their feedback and a special thanks to Mark and ICE.
 
Rio said:
Here's the update on the concrete slab being less than 18" to the floor joists. I just got off the phone with the plan checker and he said the concern they had is not with vapor coming up from below the slab but a plumbing leak getting the slab wet and then the concrete letting the slab hold the water. He said they discussed it with the B.O. and the contractor and agreed to jackhammer the slab enough to allow any water that might get on top of the slab to work its way down to the earth and be soaked up by the ground. Thanks again to all for their feedback and a special thanks to Mark and ICE.
Click to expand...
That's the most ridiculous thing I've ever heard.
 
WOW.

From the 2006 IRC Q&A:

Q "If the wood joists were 17 " from the exposed ground in the crawl space within the periphery of the foundation and an approved poly vapor barrier was laid on top of the exposed ground, would those joists still need to be treated...."

A "NO"

Of course the Q&A doesn't cover spills!
 
Rio said:
Here's the update on the concrete slab being less than 18" to the floor joists. I just got off the phone with the plan checker and he said the concern they had is not with vapor coming up from below the slab but a plumbing leak getting the slab wet and then the concrete letting the slab hold the water. He said they discussed it with the B.O. and the contractor and agreed to jackhammer the slab enough to allow any water that might get on top of the slab to work its way down to the earth and be soaked up by the ground. Thanks again to all for their feedback and a special thanks to Mark and ICE.
Click to expand...
By this logic - we should jackhammer holes in all basement floors. Right?
 
Darn, now I've gotta crawl back under there and fork the plastic
 
Darren Emery said:
By this logic - we should jackhammer holes in all basement floors. Right?
Click to expand...
No. All buildings with concrete floors!

Actually jackhammering holes in the slab is only going to increase the possibility that moisture will now come from those silly holes.
 
You must log in or register to reply here.
Top