Should Wood Be Put In Contact With Concrete?

wood and concrete

It’s a common sight: wood in contact with concrete. You see it everywhere, especially in construction. Wood is often used as a formwork to hold concrete while it sets, and the two materials can be in direct contact with each other.

But what happens when they are? What is the effect of that contact? In this article, we’ll take a look at some of the most commonly asked questions about wood and concrete in contact.

Should you contact wood with concrete?

The answer to this question depends on the specific conditions and the type of wood being used, but in general, it is not a good idea to place wood and concrete together, especially in environments that can be very humid.

Concrete can generally cause wood to rot due to its high alkalinity.

However, pressure-treated wood or wood with protective coatings can withstand contact with concrete without deteriorating.

Additionally, proper maintenance and care, such as sealing and regularly checking for signs of damage, can help prolong the lifespan of the wood in this situation.

Overall, while there may be cases where it is safe to have wood in contact with concrete, it is important to carefully consider the specific circumstances before making a decision.

My bad experience using wood and concrete together

To give you an idea of how bad an idea it can be to clad wood with concrete or vice versa, one day on a construction site, it occurred to me to clad some 6 in. x 6 in. treated wood pillars with concrete.

It wasn’t really my idea, but the owners.

The thing is, after plastering the wood with concrete and finishing it, a few days later cracks started to appear all over the surface, giving a terrible appearance of structural failure.

This happened because the wood can absorb moisture through the porosity of the concrete, or directly from the ground, The wood expands when it gets wet, and this expansion tends to cause cracks in the concrete when in direct contact.

This problem could have been avoided by using an anti-moisture material or a material that dampens the natural expansion of the wood, such as polystyrene boards.

These moisture barriers are available on Amazon at a very affordable price.

Of course, in my specific case, it was concrete over wood, you won’t have a similar problem putting 2×4 lumber over a concrete floor for example.

But anyway, using a moisture barrier can be a good idea to prolong the life of your structure and avoid moisture problems in general.

The fundamental problem in using wood and concrete together lies in the properties of both materials, they are not compatible, both are porous and absorb moisture, but concrete does not alter its volume when wet, wood tends to expand and then shrink to its natural volume when it dries.

This expansion/contraction movement of wood in the presence of moisture considerably weakens the rigid structure of concrete, so they are not compatible materials to be used in direct contact.

What do you put between concrete and wood?

It is important to use a moisture barrier, such as a sheet of plastic or rubber, to prevent damage to the wood from moisture seeping through the concrete.

Additionally, shims to level the wood and ensure even weight distribution can help prevent warping or cracking. Using wood properly treated for outdoor or damp environments is also important.

You can also use thin sheets of polystyrene to insulate the wood from the concrete. Polystyrene can serve as a flexible material that absorbs the deformations that the wood tends to suffer when it gets wet, thus avoiding cracks in the concrete.

These are a bit like putting a raincoat between the wood and the concrete. They’re great for keeping out moisture but are usually a little more labor-intensive to install. You’ve got to make sure they’re laid out flat and that there are no punctures or tears.

You may also consider using metal brackets or fasteners to securely attach the wood to the concrete rather than relying on adhesives. This can provide added stability and support.

In construction, they also have these nifty things called “sill gaskets” that go between the wood and the concrete to help stop moisture from making the leap. There are also various sealants and treatments you can apply to make the wood more resistant to moisture.

There are also bituminous coatings, that’s a fancy way of saying it’s a thick, tar-like substance you can paint on, Imagine putting a layer of super-water-resistant paint between the wood and the concrete.

It’s a bit messier and takes some time to dry, but some people swear by it, especially for projects where you need that extra layer of protection.

Now, if you’re thinking, “I’ve got a concrete basement, and I want to frame out some walls,” that’s a classic scenario where you really don’t want the wood studs sitting directly on the concrete floor.

Use a pressure-treated bottom plate or add a barrier to keep that wood nice and dry. Your future self, not to mention your home’s structural integrity, will thank you.

Use a sill sealer to protect the wood from damage

So, a sill sealer is basically like the cushion in a shoe for your home’s structure,  imagine you’re laying down a wooden frame on a concrete foundation, right? The sill sealer goes between the bottom wooden plate (sill plate) and the concrete.

It acts as a buffer, a little protective layer that has a couple of important jobs.

sill sealer

Firstly, it’s there to stop moisture from wicking up from the concrete into the wood, like I said before, wood and moisture are not BFFs. Over time, if that wood starts sucking up water, it’s going to deteriorate, and nobody wants a rotting foundation.

Secondly, sill sealers help to smooth out any little imperfections or irregularities between the wood and the concrete, you know how even a tiny pebble in your shoe can become super annoying?

Well, even small bumps or gaps between the wood and the concrete can create problems, like making the frame unstable or letting in drafts. The sill sealer squishes down to fill in those gaps, making everything snug and secure.

Most sill sealers are made from flexible foam or similar materials, so they’re pretty easy to install, you just roll them out along the concrete, put your sill plate on top, and then secure everything down. It’s like giving your house a comfy insole before you start building up.

You can find Styrofoam sill seals on Amazon, or at any local hardware store.

Should you put a barrier between wood and concrete?

It depends on the specific situation, in some cases, a barrier may be necessary to prevent moisture from seeping into the wood and causing damage. In other cases, a barrier may not be required.

It is important to consider the environment and conditions in which the wood and concrete will be in contact, as well as any treatments or coatings applied to the materials.

Can untreated wood touch concrete?

It is not recommended for untreated wood to have direct contact with concrete, as the moisture and alkalinity in the concrete can cause the wood to rot and deteriorate over time.

Using a protective barrier such as plastic or a waterproof sealant between the wood and concrete is best. Alternatively, using pressure-treated wood chemically resistant to moisture and rot can also be a solution.

Most code enforcement requires pressure-treated lumber when in contact with concrete in addition to the moisture barrier under the slab. Non-treated floor plates can be used when acceptable moisture protection is provided under the plate or the plate is naturally decay-resistant.

It is important to regularly check for any signs of deterioration or decay in the wood and address it promptly to prevent further damage. Proper maintenance can help extend the lifespan of the wood in contact with concrete.

Will concrete rot pressure-treated wood?

The short answer is no; pressure-treated wood will not rot when in contact with concrete. This is because the process of pressure treatment injects the wood with chemicals that make it resistant to rot and decay.

However, it is important to note that concrete can still cause corrosion in metal fasteners and connectors that join the wood and concrete together.

To prevent this, it is recommended to use hot-dipped galvanized or stainless steel fasteners. Additionally, it is important to ensure proper drainage and limit prolonged contact with moisture to prevent any potential issues.

Pressure-treated wood should be a suitable choice for use in conjunction with concrete.

How do you seal a gap between concrete and wood?

One option is to use a waterproof sealant such as silicone or urethane. Another option is to use pressure-treated wood that has been specially treated to resist moisture and rot.

Keeping the area clean and dry before sealing is important, as any trapped moisture or debris can affect the sealant’s effectiveness. Additionally, it is important to regularly check and re-seal as needed to keep the wood protected.

Can you screw wood into concrete?

Yes, it is possible to screw wood into concrete. However, it is important to use screws and fasteners suitable for the specific material and environment and the weight of the wood attached.

Steel anchors and lag bolts are the best alternatives for fastening wood to concrete. The fastening capacity of these fasteners is greater than other available options.

Pre-drilling holes in the concrete may also be necessary before screwing in the fasteners to ensure a secure hold. Consulting with a professional or doing further research can help determine the best approach for your specific situation.

Proper installation and suitable materials can help ensure a sturdy and long-lasting connection between wood and concrete.

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