Chloe Zhang
Laying plastic under concrete is a common practice in the construction industry. The primary purpose of this practice is to create a vapour barrier that prevents moisture from the soil from penetrating the concrete and causing damage. Concrete is porous, and without a plastic barrier, moisture can seep through and cause structural damage to the concrete slab and the building on top of it. Plastic sheeting, typically made of polyethylene, is placed under the concrete slab to act as a protective layer. This practice not only enhances moisture control but also improves the overall energy efficiency, structural integrity, and resistance to pests of the building. Proper installation and sealing of the plastic sheeting are crucial to ensure its effectiveness in blocking moisture.
| Characteristics | Values |
|---|---|
| Purpose | Prevent moisture from the soil from moving up through the concrete and causing damage |
| Plastic type | Polyethylene sheeting |
| Plastic thickness | 10-15 millimetres |
| Vapour barrier thickness | 0.2 millimetres |
| Vapour barrier permeance | 0.1 Perms IMP or less |
| Seams overlap | 6 inches |
| Seams sealed with | Tape, sealing tape, or heat welding |
| Plastic placement | Underneath the slab, covering the entire area under the slab and extending through the edges |
| Plastic installation | Before pouring the concrete |
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What You'll Learn
Preventing moisture damage
Understanding Moisture Migration
Before constructing a concrete slab, it is essential to comprehend the porous nature of concrete. Without a proper barrier, moisture from the underlying soil can easily penetrate and migrate upwards through the concrete slab. This moisture migration can result in various issues, including structural damage, mould, and flooring damage. Therefore, addressing this potential problem beforehand is crucial.
Plastic Sheeting as a Vapour Barrier
Plastic sheeting, typically made of polyethylene, serves as a vapour barrier to block moisture from seeping into the concrete slab. This protective layer acts as a crucial defence against moisture-related damage. It is recommended to use plastic sheeting with a thickness between 10 and 15 millimetres, ensuring durability and near impermeability to water. Proper installation is vital, with overlapping seams sealed to prevent any gaps through which moisture could infiltrate.
Preserving Structural Integrity
The primary purpose of installing plastic sheeting under concrete slabs is to preserve the structural integrity of the foundation and the overall structure. By preventing moisture from the ground from penetrating the concrete, plastic sheathing helps maintain the quality and durability of the slab. This moisture barrier is especially crucial for indoor slabs, as moisture can cause damage to the flooring and other materials above the slab.
Additional Benefits of Plastic Sheathing
In addition to preventing moisture damage, the use of plastic sheathing offers several other advantages. It enhances the overall energy efficiency of the structure and provides resistance to pests. Properly installed plastic sheathing ensures that the concrete slab dries correctly, preventing premature drying and allowing adequate water for the hydration process. This results in a stronger concrete mix and a more durable slab.
Choosing the Right Plastic
Not all plastics are created equal when it comes to vapour barriers. It is essential to select a material that is durable, puncture-resistant, and impermeable to moisture. The alkali in concrete can disintegrate certain types of plastic over time, so choosing a suitable plastic, such as polyethylene, is vital. Polyethylene sheeting is favoured for its effectiveness in blocking moisture, affordability, and widespread availability.
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The curing process
Curing is an essential process that gives concrete strength and durability. It involves protecting the concrete from sun and wind to prevent premature drying and ensure adequate water for the chemical process of hydration to complete. Here is a step-by-step guide to the curing process:
Step 1: Initial Curing
During the initial set, if the bleed water evaporates faster than it rises out of the concrete, initial curing is necessary to prevent plastic shrinkage cracks. This can be done by spraying water on the slab to restrict evaporation and maintain moisture.
Step 2: Intermediate Curing
Between the initial and final sets, intermediate curing is required if the finishing or stamping is complete before the final set. This can be achieved by using wet coverings, such as burlap or cotton mats, which should be constantly wet to maintain a film of water on the surface.
Step 3: Final Curing
After the final set, the concrete needs to undergo final curing. This involves keeping the concrete surface wet for at least seven days, commonly through ponding for flat surfaces or spraying for horizontal surfaces.
Step 4: Temperature Control
Temperature control is crucial during curing. Hot concrete can lead to temperature differentials and rapid crystallization, resulting in weaker concrete. Cold temperatures can also affect the curing process, so maintaining optimal temperatures is essential.
Step 5: Moisture Retention
Proper curing techniques aim to retain moisture in the concrete. This can be achieved by using moisture-retaining fabrics, wooden forms, or spraying water to prevent premature drying.
By following these steps and maintaining adequate moisture and temperature conditions, the curing process ensures that the concrete develops the desired strength and durability.
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Vapour barriers
It is important to note that a vapour barrier is not always necessary. 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 plan to seal the surface with an impermeable membrane or layer, it is recommended to use a vapour barrier under the outdoor slab. This is because the ground moisture moving upwards through the slab will not be able to leave the concrete surface.
In addition to the vapour barrier underneath the slab, some sources also recommend installing a vapour barrier above the slab. This protects against water inside the concrete coming into contact with your building materials. Every single material that comes into contact with concrete should have a rot-proof barrier.
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Installation and sealing
When installing plastic sheeting under a concrete slab, it is important to ensure that the plastic covers the entire area under the slab, extending through the edges. The plastic should be a durable, puncture-resistant, and impermeable material, such as polyethylene sheeting, with a thickness of at least 10-15 millimetres.
Before laying the plastic, it is crucial to prepare the ground properly. The soil should be compacted and levelled to create a smooth and even surface. It is also recommended to lay a layer of sand or gravel over the soil to create a stable base and improve drainage.
Once the ground is prepared, the plastic sheeting can be installed. Overlap the seams of the plastic sheeting by 6 inches and seal them with tape or heat welding. Ensure that the plastic layer is intact and free of any punctures or tears, as this can compromise its ability to block moisture.
After the plastic is properly installed, the concrete can be poured. It is important to take care not to damage the plastic during the pouring process or by workers walking on it.
To further protect the concrete and ensure proper curing, it is recommended to spray water on the slab after pouring. This helps to prevent premature drying and ensures that there is adequate water for the concrete to cure correctly.
Additionally, it is important to consider the climate and weather conditions during installation. In cold weather, the concrete may need to be protected from freezing temperatures, and in hot weather, it may require additional measures to prevent premature drying.
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Structural integrity
The structural integrity of a building is paramount, and one way to ensure this is by placing plastic sheathing, or a vapour barrier, under concrete slabs. This layer of protection is crucial in preserving the quality and durability of the foundation and, by extension, the entire structure.
Concrete is porous and permeable, and 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. Over time, this moisture migration can lead to structural damage, as the moisture in the concrete can lead to expansion and contraction, which may cause cracking.
To prevent this, a layer of plastic sheeting is placed under the concrete slab to act as a vapour and moisture barrier. This prevents moisture from the soil from permeating through the slab and reduces the amount of water that wet concrete sucks up from the ground when it is first poured. By blocking moisture, the plastic sheeting also helps to deter pests from inhabiting and damaging the property.
The most common type of plastic sheeting used in construction is polyethylene sheeting, which comes in various thicknesses, typically between 10 and 15 millimetres. It is favoured for its durability, resistance to punctures, and ability to block moisture effectively. To ensure its effectiveness, the plastic sheeting should cover the entire area under the slab and extend through the edges, with seams overlapped and sealed to prevent any gaps where moisture could enter.
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Frequently asked questions
Yes, you should put plastic under concrete to prevent moisture from the soil from moving up through the concrete and causing damage.
Concrete is porous and can absorb moisture from the soil underneath it. Plastic sheathing acts as a vapour barrier, preventing moisture from seeping into the concrete and causing structural damage.
The most common type of plastic sheeting used in construction is polyethylene sheeting due to its durability, resistance to punctures, and ability to block moisture. The recommended thickness for polyethylene sheeting under concrete is between 10 and 15 millimetres.
The plastic sheeting should cover the entire area under the slab, extending through the edges. Overlap the seams by at least 6 inches and seal them with tape to prevent any gaps through which moisture could enter. Ensure the sheeting is not damaged during the concrete pour.
