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

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,


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

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?

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

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)





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.




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.

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.

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.

Treated lumber helps to stop decay…

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

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.

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?

This might help.

Wood Rot


William F. Lyon

**Many homeowners are familiar with wood damage caused by rot. They see it in structural lumber, log homes, eaves, garage doors, exterior door trim, window casings and other wood used in construction. Current estimates show that replacement materials, needed to repair damage caused by rot alone, account for nearly 10 percent of U.S. annual wood production. **
**Blame for this destruction is sometimes incorrectly placed on termites or other wood-destroying insects. However, there are no mud tunnels or mines in the wood such as seen with termite and other wood-destroying insects, nor is there any sawdust, which would be evident in the case of carpenter ant damage. **
**The key to preventing rot is to control the wood’s exposure to moisture and to employ an effective prevention and treatment program. Most wood decay fungi grow only on wood with a high moisture content, usually 20 percent or above. Green (unseasoned) lumber is a prime target for decay fungi. **

**There are two main classes of wood rot. In one type, the decayed area has a brown discoloration and a crumbly appearance. It usually breaks up into variously-sized cubes, giving rise to the name “brown cubical rot.” Another type of rot results in a white or yellow discoloration, with the decayed wood being “stringy” or “spongy.” **

**Although many decay fungi may grow for long periods without producing any external evidence of their presence, others produce “fruiting bodies” on the surface of decaying wood. Fruiting bodies are usually “crusts” or shelflike “brackets” which are a few inches or so in diameter. The fruiting body of Serpula lacrimans, e.g., is a rust-brown, crust-like structure on the wood surface. It has a waxy appearance, with shallow, net-like folds or “wrinkles.” The fruiting body of Poria incrassata is also crust-like. It is white to light buff when initially formed, but becomes brown as it ages and dries out. Small pores can be seen in the crust when it is examined with a hand lens. Gleophyllum trabeum forms bracket-like fruiting bodies. The upper surface of the fruiting body is dull gray-brown and smooth. The lower surface has elongate openings (pores) or split-like openings (gills). These fruiting bodies produce millions of tiny spores which may, in some cases, serve to spread the decay fungus to other areas. **
**Also, surface molds, “mildews,” and stain fungi are often found growing on the surface of damp wood and can be confused with decay fungi. Although these organisms may discolor the wood, they do not break down wood fibers and thus do not weaken its structure. However, these organisms indicate that moisture is present and that decay will likely proceed if a wood-rotting fungus becomes established in the wood. **
Life Cycle & Habits

**Decay fungi are living organisms which send minute threads called “hyphae” through damp wood, taking their food from the wood as they grow. Gradually, the wood is decomposed and its strength is lost. Such damage is often inconspicuous until its final stages, and in a few instances homeowners have suddenly found floors breaking through or doors falling from their hinges due to wood rot. When previously dry wood is placed in contact with moist soil, or in a location where it is subject to condensation (such as unventilated crawl space), it is likely that wood decay problems will occur. Rain leaks, faulty plumbing and leaky downspouts also are common sources of moisture. In some instances, water can be transported to the site of decay through strands or “rhizomorphs” of the decay fungi. Water-transporting strands may extend for thirty or more feet across brick, concrete or similar materials. The wood decay fungus, Serpula lacrimans, has been known to transport water up three stories to an area where decay is occurring. Poria incrassata is also capable of transporting water long distances. However, these fungi are exceptions to the rule. Most wood-rotting fungi must have a direct supply of water at the site of decay. Thus the term “dry-rot,” sometimes applied to decay in wood structures, is erroneous. **

**Wood Rot **

Sometimes called house cancer, wood rot is something most home owners will have to deal with at some time. Wood rot is caused by several different fungi that are usually present, but dormant, in most lumber. When paint fails, or wood is in contact with soil or concrete, it can begin to absorb moisture. When the moisture level is sufficient, the wood rot fungi start digesting the proteins and sugars contained in the wood fiber. There are two kinds of wood rot: wet rot and dry rot. Both require wood, moisture, oxygen and warmth to grow. They do not grow well in water logged wood because of a lack of oxygen. When the moisture level reaches 19% in lumber and temperature is between about 40º F and 105º F the fungi begin to colonize.
Wet rot uses the moisture readily available in wood to digest it. Dry rot may extend tentacles up to 25’ through masonry to tap into moisture sources to bring enough water to the wood to digest it. These tentacles can grow as big around as fingers.
Both types of rot eat wood. It is their function in nature. Wood rot can grow quickly, destroying the strength of lumber. As these fungi digest the nutrients in the wood all strength is lost. The wood may powder, splinter or come apart in sheets. It will spread until it runs out of moisture or wood, or the temperature becomes too hot or too cold.
Before repairing damage caused by wood rot the conditions that caused it must be corrected or it will return. This means drying up the source of water. The required moisture can come from inside or outside. The most common sources of water that feeds wood rot come from plumbing leaks, roof/siding/trim leaks, poor drainage or condensation.
It’s may be obvious when you have a plumbing, or roof, leak but many people over look drainage, and condensation, as sources of potentially damaging moisture. When the earth is sloped toward your house water is brought to your foundation. It can penetrate foundation walls, possibly causing a variety of damage and health hazards.
This seems basic, but the true drainage around a house may be hidden by poor landscaping, or beneath a deck. When landscaping, correct the drainage before you begin. A hard, clay type soil is best for draining water away from your foundation. Ideally, the ground should slope away from the house a minimum of 1/2’”/foot and continue 10 feet from the foundation.
One of the most common, damaging defects I have found in homes in the Denver area is water intrusion through the foundation from ground sloped toward the house and hidden beneath stones or bark. If the water drains toward the house, covering it with a porous medium like stone, will not fix the problem. It only makes it harder to identify the problem.
Homes are being built more energy efficient and air tight. This increases the amount of moisture in the air that is unable to escape to the outside. Bathing, cooking and indoor plants all increase indoor humidity. You may notice small black specks on the ceiling above your bath or shower, or condensation on the inside of your windows on a cold day. These are indication that you need to evaluate condensation and air ventilation.
A solution that can help control moisture, and other indoor air quality issues, is a fresh air heat exchanger. Air from inside the house is forced through a radiator like structure, venting to the outside. Outside air is simultaneously drawn through the heat exchanger collecting heat from the exiting air. These units reclaim 85% of the heat and keep the indoor air cleaner and healthier.
When the source of moisture, allowing wood rot to grow. has been eliminated all affected wood must be removed and replaced. Painting will do no good. The wood will be still be spongy and hold moisture infecting the wood around it.
Examine all exposed wood inside and out, spring and fall, for signs of water stains, cracked or pealing paint, or contact between wood and earth. Under the right conditions, wood rot can spread quickly and cause a great deal of damage. Prevention and early repair often require very little time or money. Repairing damage from moisture left to do it’s worst can be very expensive.

Courtesy of Carl Brahe - Inspection Perfection, Inc.
Your Denver Real Estate Resource

Hope this helps.


Concrete cures from the inside out. Concrete doesnt completely cure for at least 28 days after set.

Check out the facts about it here

Concrete has to cure for 28 days and be tested under the ACI guidelines to show that the compressive strength has reach the design requirements.
Curing means moisture in order to inhibit the necessary ingrediant to promote the required hydration for compressive stength.

Concrete will continue to cure for many years and increase in strength until it reaches it’t optimum performance or peak and then decline. This takes many years to occurr.

The traditional age for measuring concrete properties is 28 days. The ACI Building Code (1) clearly states, “Unless otherwise specified, f’c shall be based on 28-day tests. If other than 28 days, test age for f’c shall be as indicated in design drawings or specifications.” The term “f’c” is defined as the specified compressive strength of concrete.

Understanding the fact that concrete survives on moisture for curing and compressive strengths, does this not tell one that wood in contact with it, will make it absorb more than it’s original 15-19 % content. A moisture meter on concrete will tell the story.

Concrete moisture does not mix with bare steel, and wood without some sort of protection.

Hope this helps. :slight_smile: :slight_smile:

The concrete wicks the moisture up from the soil. Ever notice a house built on a slab, when it rains the carpet will wrinkle, if the slab has not been sealed properly. Lay a paper towel(slab) down flat on top of your kitchen counter, and place a sponge(2x4) on one side, then pour a small amount of water at the farthest corner from the sponge. Then watch the capillary action of the water traveling thru the PT until it becomes saturated, then the sponge will fill up. When the moisture level in wood reaches the magic number of 19%, then decay starts. Ken

Then according to the posts here (that the moisture that causes the rot at the point where the wood contacts the concrete comes from the moisture released in the curing process)… wood in contact with concrete blocks (likely made and cured months before being used in construction) isn’t a problem.

True or false?