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-   -   Wood on concrete (http://www.houserepairtalk.com/f17/wood-concrete-13223/)

angierios 02-12-2012 08:07 AM

Wood on concrete
 
I have an 8x8 concrete slab that I want to build and enclosed porch on top of and I've been doing some research on the best way to add 2x4 up the slab to make the frame. I'm not sure if glue would suffice or if I have to drill into it. Any advice would be great. This is my first home project and it me and my husband trying to do it on our own.

nealtw 02-12-2012 10:12 AM

A slab that has not got a foundation and footing down to the frost depth can move up and down when it freezes outside. The dirt around your house should be at least 8" away from the wood. So I am telling you that this is not the best idea.

angierios 02-12-2012 02:11 PM

It does have a foundation it was just put in to support the structure - I just want to know the best way to put the studs on to it.

nealtw 02-12-2012 03:11 PM

The bottom plate should be bolted down every 3 or 4 ft. The tool rentle store will have a hammer drill and ancher bolts. Foam sill gasket between plate and concrete. Did you put drainage around this?

drunkenDIY 02-16-2012 08:03 PM

you could also use a powder actuated nailer. Might cost more to rent, but will likely save you some time. As nealtw says, though - don't forget the foam barrier, or your sill plates will be rotten before you finish the project.

joecaption 02-16-2012 08:10 PM

Use pressure treated for the bottom plate, Tap Con can also be used to hold the bottom plates in place.
The walls need to be built so there at the edge of the slab so the sheathing can be run past it so water does not get under the walls.
The sheathing should be no closer then 12" from the ground.

ConcreteTreat 02-18-2012 06:39 AM

Quote:

Originally Posted by angierios (Post 67873)
I have an 8x8 concrete slab that I want to build and enclosed porch on top of and I've been doing some research on the best way to add 2x4 up the slab to make the frame. I'm not sure if glue would suffice or if I have to drill into it. Any advice would be great. This is my first home project and it me and my husband trying to do it on our own.

Glue and other adhesives are rarely a good idea for concrete for many reasons:

1. Concrete will expand and contract with moisture and temperature. If the adhesives cannot "flex" along with it, then they will eventually be worked loose.

2. Concrete has a high acidity level when new and still curing, and can also have high acidity levels as moisture works its way through the pores. Again, this can begin to work loose adhesives.

3. Concrete is porous, and moisture will continually make its way through these pores from the dirt around the slab. As it does, a portion of that moisture can build up underneath the wood, contributing to wood rot.

Your best bet is to mechanically attach the wood to the concrete.

----------------------
Jacques Bouchard
Concrete Treat: Concrete Sealer

mudmixer 02-18-2012 08:35 AM

Concrete is NOT acidic. It is an alkali material because of the materials.

Concrete is not as porous as the wood and does not absorb as much moisture.

Mechanical attachment is always best when you attach a temporary material(wood) to a more permanent material(concrete). Some sort of barrier is always good, but codes are just the minimums.

Dick

ConcreteTreat 02-18-2012 09:50 AM

Quote:

Originally Posted by mudmixer (Post 68200)
Concrete is NOT acidic. It is an alkali material because of the materials.

Concrete is not as porous as the wood and does not absorb as much moisture.

Mechanical attachment is always best when you attach a temporary material(wood) to a more permanent material(concrete). Some sort of barrier is always good, but codes are just the minimums.

Dick

Dick -- I can cite sources that should prove my point about the acidity, particularly in new concrete that is still curing. I have them at my office, and will follow up on Monday, instead of researching this all over again.

Concrete is not as porous as wood, definitely not. But it will wick moisture that the wood does not normally have access to, and bring that to the wood substance, or really anything else, for that matter. Also, as a porous substance, it is liable to transfer moisture from elsewhere towards the wood, like any material that can sponge water would.

ETA -- Dick, I've done some quick research, and I'm clearly drastically misunderstanding whatever I was reading about this topic -- NASA agrees with you on the pH numbers, as does the books I have on hand in my house. I'll come back Monday with a better explanation -- I KNOW I read something about the acidity level, and not the alkaline level, interfering with adhesion in new concrete. Until then, this is going to drive me nuts!

mudmixer 02-18-2012 12:55 PM

I welcome your investigation. Acid and alkalies are two totally incompatible compounds and come from different materials and processes.

Obviously, all materials expand and contract depending on the temperature and moisture to stabilize.

If there is moisture and a substance (wood), moisture can transmit the moisture to any material, be it wood, fiberglass or other materials that have an affinity for moisture since this is a common factor for all materials.

The article from "NASA" is very narrow and not very complete. Since I worked on many NASA projects for rocket test facilities at Edwards AFB, Canoga Park/Santa Susana, CA and Huntsville there was never a mention of the corrosion or acidic effects of concrete because of the basic materials (cement, calcium based based limestone/dolomite or igneous aggregates), The minute snippet of an article on a NASA letterhead may have been a folly or blown out of proportion. In all of the designs for the launch facilities at Kennedy/Cocoa Beach there was not a concern about acid, but salt water and salt contaminated aggregate was minor concern as it is in all of construction Florida, but not universal.

Dick


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