Bottom wood sill on CMU

[Alan CA]

Hi,

Can you base the bottom wood plate of a stud wall directly on CMU (concrete masonry unit) foundation? Is such a foundation system acceptable? If yes, how will you fasten the plate with the blocks?

 

[jayrod12]

You would want to separate the wood from the masonry to prevent rotting of the wood, but otherwise yes you could.

Usually it is connected via anchor bolts.

 

[phamENG]

Pressure preservative treatment of commodity lumber is quite common, too.

 

[masonrygeek]

The top of the concrete masonry wall is usually solid or filled solid with grout. The anchors that jayrod mentions are embedded in the grout. See NCMA TEK 5-3A:

http://www.ncma-br.org/pdfs/5/TEK 05-03A.pdf.

 

[Mike Mike]

Alan CA, Yes, wood walls can be set on CMU walls.

jayrod12, I typically set my walls directly on my CMU. Are you suggesting a plastic sheet barrier? What products do you specify to separate the two?

phamENG, If the wood is more than a foot or so from soil, no protection against decay is required. For a more in depth discussion refer to IBC 2015 2304.12.

masonrygeek, In my experience contractors typically prefer post-installed over cast-in-place. Simpson Titen 2 screws or Titen HD screws or EQ are my standard.

 

[jayrod12]

We have used 15 lb felt, or poly, or sill gasket. Also depends on the proximity to grade.

Most typically we just specify that any wood in direct contact with concrete to be pressure treated.

 

[phamENG]

Mike Mike - the 12" and 18" rules are for treating of girders and joists (respectively) that span over the soil. Sills sitting on a masonry foundation wall should always be treated. I'll grant you that the code does say "and 8" above soil," but that strikes me as a terrible idea. There may be some wiggle room for local conditions, but where I am any masonry or concrete that is exposed to weather or humidity (which is everything not inside the building envelope and climate controlled areas) will absorb water. So around here, the standard is for EVERYTHING in contact with masonry or concrete to be preservative treated. But I also live in a swamp.

 

[Mike Mike]

phamENG, Our office seems to be divided. Some of us agree with you that all exterior masonry tends to draw water from the elements into the wood wall. Some of us say why spend the money if code doesn't require it for walls that are further from soil. Personally I might take the middle ground and only treat wood within say a couple feet of soil. Here's 2304.12.1.2 for everyone's convenience:

"Wood supported by exterior foundation walls. Wood framing members, including wood sheathing, that are in contact with exterior foundation walls and are less than 8 inches from exposed earth shall be of naturally durable or preservative-treated wood. Experience has shown that foundation walls can absorb moisture from the ground and convey moisture to wood framing in contact with the foundation. This section requires a clearance that minimizes the chances of moisture being drawn to the framing. The clearance also provides an exposed foundation surface that allows for a visual inspection for termite tubes."