Omar Khalid
Concrete slabs are prone to decay and cracking without a vapour barrier to protect them from moisture damage. Moisture from the soil can penetrate the concrete, causing structural damage over time. A vapour barrier, such as plastic sheeting, can be placed underneath the concrete slab to prevent moisture from rising through the concrete and causing damage. This protective layer also reduces the amount of water that wet concrete absorbs from the ground when it is initially poured, resulting in a stronger bond.
| Characteristics | Values |
|---|---|
| Preventing moisture from the soil | Plastic under a concrete slab prevents moisture from the soil from permeating through and damaging the slab |
| Reducing water absorption | Plastic reduces the amount of water absorbed by wet concrete when it is first poured |
| Preventing damage | Plastic prevents damage to the slab from water loss, moisture, pests, and extreme temperatures |
| Strengthening concrete | Plastic reduces moisture in the slab, leading to a stronger bond of concrete |
| Safety | Plastic prevents the growth of mold and mildew and protects against radon gas, the second leading cause of lung cancer in the US |
| Cost | The cost of plastic sheeting is low, and its benefits are high |
| Compliance | ASTM compliance standards require vapor barrier plastic underneath concrete slabs |
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What You'll Learn
Plastic sheets prevent moisture damage to concrete slabs
Concrete slabs are highly susceptible to moisture damage. This is due to the porous nature of concrete, which allows moisture to easily enter a concrete slab from the soil underneath it. Over time, this moisture migration can lead to several issues, such as structural damage, flooring damage, and corrosion of internal reinforcements.
Plastic sheets, also known as plastic sheathing, are placed under concrete slabs to act as a vapour barrier and protect them from moisture damage. This layer of protection is crucial in preserving the quality and durability of the slab. The plastic prevents moisture from the soil from permeating through the slab, reducing the moisture in the concrete and leading to a stronger bond.
The use of plastic sheathing under concrete slabs is a fundamental building practice that helps protect the foundation from moisture-related damage. It is especially important to use plastic sheathing when the concrete slab will be sealed with an impermeable membrane or layer. Without a vapour barrier, ground moisture moving upwards through the slab will be trapped inside due to the outer impermeable layer, causing the concrete to expand, cup, or deteriorate.
To be effective, the plastic sheeting must be properly sealed. Thicker plastic polyethylene sheeting between 10 and 15 millimetres should be used, as it is more durable and almost impermeable to water. The American Concrete Institute (ACI) recommends using polyethylene sheeting that is at least 10-mil thick, as thicker poly creates a better barrier and is more puncture-resistant. By selecting a durable, puncture-resistant, and impermeable material, builders can ensure the long-term stability, safety, and comfort of the structure.
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Plastic sheets help concrete cure
Concrete slabs are prone to damage from moisture rising from the ground. This can cause the concrete to crack, expand, cup, or degrade. To prevent this, a vapour barrier is often placed under the slab. Plastic sheeting is commonly used as a vapour barrier.
Plastic sheeting can also be placed over concrete slabs to protect them from the elements and prevent the surface from drying out. This is especially important in cold and/or wet weather conditions. However, it is important to ensure that the concrete is not too wet when the plastic sheeting is applied, as this can cause air pockets to form.
The thickness of the plastic sheeting is important. For use under a concrete slab, thicker plastic is better as it is more puncture-resistant and creates a better barrier. A minimum thickness of 6-mil (0.15 mm) is required by the 2018 IRC, while the American Concrete Institute (ACI) recommends a thickness of at least 10-mil (0.25 mm). For use over a concrete slab, thinner plastic sheeting can be used, but it should still be thick enough to provide protection from the elements.
When using plastic sheeting to protect concrete slabs, it is important to consider the potential for discoloration. Plastic sheeting can sometimes cause permanent discolouration of the concrete, especially if it is not applied properly. It is important to consult with a professional to ensure that the plastic sheeting is used correctly and to avoid any unwanted discolouration.
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Plastic sheets act as a vapour barrier
Vapour barriers are required under a concrete slab for a Class 1 building (a dwelling) and for Class 10 (e.g. a garage) where the slab is continuous with the dwelling. In New South Wales, Clause 3.2.2.6 of the National Construction Code requires the damp-proofing membrane to be 0.2mm polyethylene high-impact resistance. The NCC also states that a vapour barrier should lap up the outside edges of the slab. The IRC requires a minimum of at least 6-mil polyethylene vapour retarder to be installed between the concrete and prepped ground. The American Concrete Institute (ACI) recommends 10-mil polyethylene or thicker, while ASTM E-1745 suggests thicker poly creates a better barrier and is more puncture-resistant.
If you plan to seal the surface with an impermeable membrane or layer, it is recommended to use a vapour barrier under the outdoor slab. The impenetrable surface will retain ground moisture travelling through the slab, leading the concrete to expand, cup, or otherwise deteriorate, eventually failing. 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.
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Plastic sheets are cheap and easy to use
The primary cause of cracking in a concrete slab is premature water loss. Plastic sheets help to prevent this by reducing the amount of water that the wet concrete sucks up from the ground when it is first poured. This leads to a stronger bond of concrete. The thicker the plastic, the better the barrier and the more puncture-resistant it will be. Thicker plastic is also less likely to be damaged over time, providing long-term protection for your concrete slab.
You can find polyethylene sheeting at most building supply stores. The cost of plastic sheeting is very low, at around 5 to 7 cents per square foot for 8-mil polyethylene sheeting. Thicker 15-mil vapour barrier sheeting costs around 25 cents per square foot. The minimum thickness of an effective vapour barrier is 10-mils, although thicker plastic is recommended for greater protection.
Using plastic sheeting is a simple and cost-effective way to protect your concrete slab from moisture damage. It is an easy way to ensure the longevity of your concrete and prevent structural issues.
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Plastic sheets can prevent cracking in concrete slabs
Concrete slabs are prone to cracking, and one of the main reasons for this is moisture. Moisture from the soil can penetrate the concrete, causing it to crack over time, which can lead to structural damage. This is where plastic sheeting comes in.
Placing a vapour barrier, such as plastic sheets, under a concrete slab can protect it from moisture damage. The plastic will prevent moisture from the soil from permeating through the slab. It will also reduce the amount of water that wet concrete absorbs from the ground when it is first poured, leading to a stronger bond of concrete. This is especially important if you plan to seal the surface with an impermeable membrane or layer, as this will trap ground moisture inside the concrete, causing it to expand, cup, or deteriorate.
The type of plastic sheeting used is important. The most common type is polyethylene sheeting, favoured for its durability, resistance to punctures, and ability to block moisture effectively. Thicker plastic is generally better, as it creates a better barrier and is more puncture-resistant. For concrete slabs, the plastic should be between 10 and 15 millimetres thick. This thickness is more durable than most plastic sheeting and almost impermeable to water, while still being thin enough to seal properly under the concrete.
To ensure the effectiveness of the plastic sheeting, proper installation is crucial. The sheeting should cover the entire area under the slab and extend beyond the edges. Seams between sheets should overlap by at least 6 inches and be sealed with tape to prevent any gaps through which moisture could enter. During installation, care should be taken to avoid puncturing the vapour barrier, and any tears or punctures should be repaired with waterproof tape.
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Frequently asked questions
Yes, plastic should be placed under a concrete slab to protect it from moisture damage. A vapour barrier put under a concrete slab can protect it from moisture damage, which can cause cracking and deterioration of the slab.
A vapour barrier is a sheet of plastic that goes under a concrete slab to prevent moisture from seeping up through the slab and causing damage.
The minimum thickness of an effective vapour barrier is 10 mils (0.01 inches). However, thicker plastic is better at creating a more durable barrier and is more puncture-resistant.
