Should I Put Plastic Under A Concrete Slab?

Concrete is a great construction material, especially when it comes to supporting structures with concrete bases. But is the concrete all there is to it, or should you put plastic under a concrete slab?

Plastic should be placed under a concrete slab to protect it from moisture damage. Moisture from soil can penetrate the concrete causing it to crack with time and potentially risking lots of structural damage. A vapour barrier put under a concrete slab such as plastic sheets can protect it from this.

It’s important to protect a concrete slab because it is a structural base for most buildings. There are different plastics that you can use under concrete slabs for protection. You should always have some kind of vapour barrier under concrete slabs.

Does a concrete slab need plastic underneath?

To stop any moisture damage occurring to a concrete slab, you should always put plastic underneath the slab. Due to the permeable nature of concrete, moisture can easily enter a concrete slab from the soil underneath it. This can seep through and damage the flooring on top of the concrete slab, any frames, and the concrete itself. This is why it’s important to protect a concrete slab with a layer of plastic underneath it.

Dampness and moisture entering a concrete slab that doesn’t have this protection can also cause:

  • Aggregate damage
  • Ground vapour off-gassing
  • Sealer delamination
  • Efflorescence production on the surface
  • Condensation on floor
  • Overlay bond failure
  • Concrete slab curling

All of these impacts can cause serious damage to the concrete slab and the building on top of it. So, you need to put a vapour and moisture barrier underneath a concrete slab and plastic sheets are a great option. The plastic will prevent moisture from the soil from permeating through a slab. It will also reduce the amount of water wet concrete sucks up from the ground when it is first poured. Overall, it reduces the moisture in the concrete slab leading to a stronger bond of concrete. You can be sure to have a well-laid concrete slab that isn’t going to cause slippery concrete floors from moisture damage.

Why do you put plastic down under concrete?Does a concrete slab need plastic underneath?

You should put plastic down under concrete to prevent moisture from the soil from moving up through the concrete and causing damage. Water vapours move from areas with high humidity (below a building), into areas with lower humidity (into the building). With the porous nature of concrete, it is easy for moisture to work its way through a concrete slab. Moisture can move up through the soil and a concrete slab causing problems for the concrete and the materials above it. Moisture can end up cracking the concrete and ruining the floor. For indoor slabs especially, you should be putting plastic under concrete for a moisture protection layer.

What plastic do you use under a concrete slab?

The most common material used under concrete slabs is polyethylene plastic sheeting with a recommended thickness of 15 millimetres. A vapour barrier should be made of high-performance materials capable of stopping moisture from upward migration.

Using materials like polyethylene can also prevent contaminants and gases from penetrating the concrete slab and reaching the flooring and walls.

Polyethylene vapour barriers can be easily placed over tamped earth, sand, and/or aggregate base. When using the polyethylene vapour barrier, you just have to unroll the plastic and cover the area completely.

Make sure to overlap the seams by 6 inches and seal with sealing tape or by heat welding. It’s important to use quality cutting tools to get your polyethylene plastic sheeting to the right size for your concrete slab. Otherwise, it will be hard to properly seal this protective layer and moisture will still infiltrate the concrete slab.

While you can find polyethylene sheeting at most building supply stores, you should remember that not every poly sheeting is a vapour barrier.

What thickness of plastic do you need under concrete?

Thicker plastic polyethylene sheeting between 10 and 15 millimetres should be used under concrete. This sheeting is more durable than most plastic sheeting and almost impermeable to water. Class 1 buildings should use plastic and vapour barriers of at least 0.2 millimetres thickness. 10 to 15 millimetres covers this measurement without being too thick to seal properly under concrete. This thickness of plastic will also be less likely to damage over time, protecting concrete from moisture for much longer.What thickness of plastic do you put under concrete?

Do you need a vapour barrier under concrete?

You need a vapour barrier under concrete to stop moisture from getting into the concrete slab and causing damage. Once a concrete slab dries, the main way moisture will enter it is through the ground underneath. Adding plastic and vapour barriers underneath prevents the moisture from the soil from entering a concrete slab.

A vapour barrier used under concrete should have a permeance of 0.1 Perms IMP or less for the best protection against moisture.

Do you need a vapour barrier for the outdoor slab?

For outdoor slabs, a vapour barrier is not essential. There will be no problem if some moisture is on the surface of exterior concrete slabs or the slabs in unheated structures.

However, if you’re planning to seal the surface with an impermeable membrane or layer, it’s recommended to use a vapour barrier under the outdoor slab. The reason is that ground moisture moving upwards through the slab will not be able to leave the concrete surface. Instead, it will be trapped inside due to the outer impermeable layer. This can cause the concrete to swell up, expand, cup, and degrade, leading to damage and failing eventually. You could then end up having to remove outdoor concrete slabs.

Do you need plastic under a concrete driveway?Do you need plastic under the concrete driveway?

For concrete driveways, outbuildings, and unheated carports you don’t have to put plastic under the concrete. If you would like to protect your concrete driveway from moisture you can put plastic underneath it. This is a good idea to prevent cracks from moisture in the concrete. Plastic under a concrete driveway can extend the lifespan of the driveway, preventing moisture from expanding and deteriorating the concrete. However, it isn’t necessary to put plastic under a concrete driveway.

While you’re checking out information about concrete slabs, have you ever wondered if you can use an angle grinder to level concrete? The answer to this and many other construction questions can be found in our Paragon Tools resources! For quality cutting, grinding and bricklaying tools, visit our Paragon Tools shop today!


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