Can someone explain why wood in contact with concrete causes rot?

[Nick Gromicko]

I need an explanation because often a member will call it out and then the homeowner asks why.

[Christian F. Mettel]

Originally Posted by gromicko

I need an explanation because often a member will call it out and then the homeowner asks why.

When wood is kept in direct contact with concrete, the moisture in the concrete will be drawn up into the wood, and after a period of time the wood will rot. A sill gasket made from a soft plastic material is designed to prevent moisture to seep from the foundation to the lumber frame. Sometimes just a simple plastic sheet is placed under the bottom plate if a wall is built on a basement cement floor which provides the same barrier.

I hope this helps,

Chris

[afrost]

Originally Posted by gromicko

I need an explanation because often a member will call it out and then the homeowner asks why.

most succinctly, as new concrete dries it uses the wood like a towel.

as the concrete gets older and drier, there is less and less water to transfer, but it is there. when concrete is freshly placed it begins its drying process. when wood is in contact with the concrete, the concrete leaches out its water into the wood. concrete cures this way for many years, it almost continually is drying out.

but it is especially a concern for fresh concrete.
on top of a foundation wall, there is a barrier (in cold climates, that's a thin foam called "sill seal") that separates the concrete from the wood sill. the wood sill should be pressure treated to resist decay.



andy frost
advanced inspections

[Nick Gromicko]

What about when a seller complains that his basement is perfectly dry, the concrete is dry and the wood is dry and there is no rot.

Where is the moisture you speak of coming from, air?

[Nick Gromicko]

So it is in the moisture in the concrete left over since the pour? Is this the moisture that causes the rot?

[Gerry Beaumont]

Originally Posted by gromicko

What about when a seller complains that his basement is perfectly dry, the concrete is dry and the wood is dry and there is no rot.

Where is the moisture you speak of coming from, air?

Nick, there are 2 issues, firstly that concrete is semi-permiable so if it is in a wet enviroment it will continualy take on water, secondly even concrete in the dryest environment will continue to release its initial moisture for very many years (concrete actually shrinks quite a bit due to its moisture loss)

The best bet for wood incontact with concrete is either a plastic foam gasket (as would be used under a sill plate) or a foot that leaves an air gap between the post and its footer as in the case of a deck post (see below)

Regards

Gerry

[Nick Gromicko]

There has to be something more to it than just the water from the initial pour. It might have to do with its ability to act like a desiccant, adsorbing water out of the air like a sponge and then keeping the wood in contact wet. It just can't be from the initial pour alone IMHO.

[Gerry Beaumont]

Originally Posted by gromicko

There has to be something more to it than just the water from the initial pour. It might have to do with its ability to act like a desiccant, adsorbing water out of the air like a sponge and then keeping the wood in contact wet. It just can't be from the initial pour alone IMHO.

Nick, there can be a problem with some "tight" homes where due to poor ventilation and a possible lack of sufficient combustion air for appliances, that the home operates under negative pressure. If this is the case then potentially damp air will be drawn into the building at any point (a poorly gasketed sill would be a good one) This tends to be a bigger problem in the more humid climates and is exacerbated by air conditioning.

Regards

Gerry

[Dale Duffy]

Originally Posted by gromicko

There has to be something more to it than just the water from the initial pour. It might have to do with its ability to act like a desiccant, adsorbing water out of the air like a sponge and then keeping the wood in contact wet. It just can't be from the initial pour alone IMHO.

Nick

Anytime concrete has something sitting on it, moisture collects.

Like a mat at your front door, move the mat and see the difference in the color of the cement, moisture condensates there, same as a piece of wood sitting against concrete.

Concrete draws moisture from the soil below continually, regardless of the length of time it was poured like a sponge, very much so without a moisture barrier.

[gbrasseur]

Originally Posted by gbeaumont

Nick, there are 2 issues, firstly that concrete is semi-permiable so if it is in a wet enviroment it will continualy take on water, secondly even concrete in the dryest environment will continue to release its initial moisture for very many years (concrete actually shrinks quite a bit due to its moisture loss)

The best bet for wood incontact with concrete is either a plastic foam gasket (as would be used under a sill plate) or a foot that leaves an air gap between the post and its footer as in the case of a deck post (see below)

Regards

Gerry

Yep! The post base in this picture Gerry provided is the best to use. It is a Simpson EPB44 (Elevated Post Base for a 4x4) or a EPB66 (6x6). They keep the wood free from the concrete.

[dcook1]

Originally Posted by gromicko

There has to be something more to it than just the water from the initial pour. It might have to do with its ability to act like a desiccant, adsorbing water out of the air like a sponge and then keeping the wood in contact wet. It just can't be from the initial pour alone IMHO.

I have worked with concrete and masonry over 25 years and have seen this millions of times.
One of the biggest areas that concrete is poured, is a basement. Concrete, even on gravel, will wick a small amount of moisture out of the soil it is placed on over the years. Most times if you see a concrete floor painted, the paint will eventually lift off in some places. It is a continuous action of moisture.
Even if a plastic barrier is placed under the concrete before a pour, it will over years, wick some small amounts of moisture.
That is why anyone finishing a basement, no matter what age, should always place a plastic barrier under the wood.
If an inspector sees framing in a basement that has no plastic barrier under it, they should inform the purchaser of this situation. Even if the wood looks good after some time, the client should be aware that any future wood to concrete contact should be protected by a plastic barrier.

[John McKenna]

Treated lumber helps to stop decay...

[afrost]

most commercial specs for building forbid non-treated wood in direct contact with concrete. it is better to require some sort of phsyical barrier between concrete and wood (treated or not treated).

after concrete's initial cure when it is expelling it's water content (and shrinking), it then acts like a sponge and absorbs moisture. steel reinforcing inside concrete will often rust due to this moisture penetration and then spall off the concrete. i'm sure you've seen it, esp under bridges. typicall specification is to have rebar covered with a minimum of 1 1/2" of concrete to help prevent this from occuring. contemporary exterior concrete, especially roads & bridges, now use epoxy coated rebar to prevent or block the rusting action.

i find it bothersome that alot of residential construction of slabs on grade do not use a vapor barrier to save about ten cents a sq ft. - it creates an irreversable (practically) moisture situation.

andy frost
advanced inspections

[cwillick1]

not only does the concrete wick moisture from the ground the moisture in the air is attracted to it. (just like the toilet tank being wet but every thing else is dry). with enough air movement the toilet is dry same with the concrete, the exposed area appears dry. if you busted a piece of concrete off the floor you would find moisture. with no air movement between the floor and wood there will be moisture perhaps very Minuit. with wood wanting to wick water faster than concrete, its like a sponge just wanting to soak it up. and over time it will soak up enough to cause damage.

[Richard Wiley]

Moisture in concrete is a given, wood in contact with concrete absorbs the moisture, moist wood is an ideal condition for WDO groth. How am I doing?