Chloe Zhang
When constructing a concrete slab, it is essential to consider the thickness of the plastic vapour barrier placed underneath. This barrier plays a crucial role in preserving the slab's integrity and durability by preventing moisture from seeping into the concrete and causing damage. The recommended thickness of the plastic sheeting varies, with commonly suggested measurements being 0.2mm, 10 mils (0.0254 mm), 15mm, and 10-20 mils. Factors such as the type of structure, local regulations, and soil conditions may influence the specific thickness required. The use of polyethylene plastic is popular due to its durability, resistance to punctures, and ability to effectively block moisture migration. Proper installation techniques, including overlapping seams and sealing with tape, are also critical to ensuring an effective vapour barrier.
| Characteristics | Values |
|---|---|
| Purpose | Prevent moisture from seeping into the concrete, deter pests, and preserve the quality and durability of the foundation |
| Plastic Type | Polyethylene sheeting, Poly |
| Thickness | 10-15mm, 10-mil, 15-mil, 20-mil |
| Vapour Barrier Thickness | 0.2mm |
| Installation | Over tamped earth, sand, and/or aggregate base. Overlap seams by 6-12 inches and seal with tape or heat welding |
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What You'll Learn
Plastic under concrete slabs is a vapour barrier
Concrete is a widely used construction material, especially for structural bases. However, concrete slabs are susceptible to moisture damage, which can cause cracking and structural issues over time. To prevent this, a vapour barrier is placed underneath the slab during construction.
A vapour barrier is a layer of plastic that helps to protect concrete slabs from moisture damage. It is placed directly under the slab, on top of a compacted sand subbase, to prevent moisture from permeating up through the slab and into the structure. This is known as rising damp. Vapour barriers also help to prevent moisture from escaping into the ground below, creating a stronger concrete slab.
The thickness of plastic used for a vapour barrier depends on the specific requirements of the project. The minimum thickness recommended for an effective vapour barrier is 10 mils (0.25 mm), as thinner plastic may not be durable enough for construction. For Class 1 buildings, which include dwellings, the recommended thickness is at least 0.2 mm. To ensure proper sealing, the thickness should not exceed 15 mm.
Vapour barriers are commonly made of polyethylene plastic sheeting, which is durable and nearly impermeable to water. This material can also prevent contaminants and gases from penetrating the concrete slab and reaching the flooring and walls. When installing a vapour barrier, it is important to completely cover the area and overlap seams by 6 inches, sealing them with tape or heat welding.
In summary, plastic under concrete slabs is a vapour barrier that helps to protect against moisture damage, contaminants, and gases. The thickness of the plastic can range from 0.2 mm to 15 mm, depending on the specific requirements of the project and the local construction codes. Proper installation and sealing of the vapour barrier are crucial to ensure its effectiveness.
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It prevents moisture damage
Concrete is a porous material, and as such, it can absorb moisture from the soil underneath it. This can lead to several issues, including structural damage, flooring damage, and the corrosion of internal reinforcements. Structural damage can manifest as cracking, heaving, or sinking of the concrete slab. Flooring damage can occur in buildings with sensitive flooring materials like hardwood or laminate, where moisture rising from the soil can cause warping or buckling. Furthermore, steel reinforcements within the concrete are susceptible to rust when exposed to moisture, compromising the structural integrity of the slab.
To prevent these issues, plastic sheeting is placed underneath concrete slabs to act as a vapour barrier, impeding the migration of moisture from the soil into the concrete. This protective layer is particularly crucial since concrete serves as the structural base for most buildings. The recommended thickness of this plastic sheeting is typically 10 to 15 millimetres, ensuring sufficient coverage without being too thick to seal properly under the concrete. Thicker options generally offer more robust barriers and longer lifespans.
The type of plastic used for this purpose is crucial. Polyethylene sheeting, for instance, is favoured for its durability, resistance to punctures, and ability to effectively block moisture. Proper installation is also essential to ensure the effectiveness of the vapour barrier. The sheeting should cover the entire area under the slab, extending through the edges, with seams overlapped and sealed to prevent any gaps through which moisture could infiltrate.
In addition to preventing moisture damage, the use of plastic sheeting under concrete slabs offers other benefits, such as enhancing energy efficiency and pest resistance. By impeding moisture migration, the plastic sheathing helps to deter pests like termites and carpenter ants, which are attracted to moist environments and can cause damage to wooden structures and other parts of the building.
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Recommended thickness is 10-15mm
When preparing to cast a concrete driveway or other construction, it is essential to consider the use of a vapour barrier underneath the concrete slab. The primary purpose of a vapour barrier is to prevent moisture from the ground from seeping into and damaging the concrete. This is especially important as concrete is porous and can absorb moisture, leading to potential structural issues over time.
The recommended thickness of plastic sheeting for a vapour barrier under a concrete slab is 10 to 15 millimetres. This thickness provides an effective barrier against moisture without being too thick to seal properly under the concrete. Thicker options, such as 15-mil and 20-mil, are also available and offer more robust barriers and longer lifespans. However, it is important to note that thinner plastic sheeting, such as 6-mil, is often ineffective and can be damaged during construction, creating holes that allow moisture to penetrate the slab.
Polyethylene plastic sheeting is the most common material used for vapour barriers under concrete slabs due to its durability, resistance to punctures, and ability to block moisture effectively. When installing polyethylene sheeting, it is important to ensure proper sealing by overlapping the seams by 6 to 12 inches and using sealing tape or heat welding to create a continuous barrier.
By choosing the appropriate thickness and material for the plastic sheeting and following best practices during installation, you can effectively protect the concrete slab from moisture damage and ensure the durability and integrity of the structure. This is particularly crucial for concrete slabs used in heated structures, as specified by the 2018 IRC, to prevent issues such as expansion, contraction, and structural damage caused by moisture migration.
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Overlap seams by 6-12 inches
When preparing to lay a concrete slab, it is important to consider the use of a vapour barrier to protect the slab from moisture damage. Vapour barriers are commonly made of plastic and placed under the concrete to prevent moisture from permeating up through the slab and into a house. This is also known as rising damp.
When installing a vapour barrier, it is important to ensure that the seams overlap by 6-12 inches. This will create an effective seal and help to prevent moisture infiltration. To achieve this, start by unrolling the polyethylene plastic sheeting and covering the area completely. Overlap the seams by at least 6 inches and use sealing tape or heat welding to secure the overlap. For added reinforcement, Viper or Stego tape can be used on the overlap.
It is important to use quality cutting tools to ensure straight and precise cuts. This will make it easier to properly seal the protective layer. Once the seams are overlapped and sealed, you can proceed with pouring the concrete, ensuring that it fully covers the vapour barrier.
By following these steps and ensuring proper overlap of the seams, you will create a strong and durable concrete slab that is protected from the damaging effects of moisture. This will help to prevent cracks and structural damage caused by moisture infiltration.
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Polyethylene is the most common material
Polyethylene sheeting comes in various thicknesses, typically measured in mils (thousandths of an inch). The recommended thickness for polyethylene sheeting under concrete slabs is between 10 and 15 millimetres. This thickness provides sufficient protection without being too thick to seal properly under the concrete. It is important to note that thinner polyethylene sheeting may not be durable enough and may be prone to punctures, compromising its effectiveness as a moisture barrier.
When installing polyethylene sheeting as a vapour barrier, it is crucial to follow certain best practices. Firstly, ensure that the seams between sheets overlap by at least 6 to 12 inches, and use sealing tape or heat welding to securely seal the seams. Secondly, extend the vapour barrier beyond the edges of the slab area and slightly up the sides of the foundation forms if possible. This helps to ensure comprehensive protection against moisture infiltration. Additionally, take extra care during the installation of rebar and wire mesh to avoid puncturing the polyethylene barrier.
In some cases, reinforced polyethylene may be used, especially in rough terrain or where the sheeting is expected to be subjected to heavy stress during construction. Reinforced polyethylene includes a layer of reinforcing scrim between layers of standard polyethylene, increasing its tear resistance and overall strength. Polyethylene vapour barriers can be easily placed over tamped earth, sand, and/or aggregate base, providing a flexible and effective solution for protecting concrete slabs from moisture damage.
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Frequently asked questions
Plastic is placed under a concrete slab to protect it from moisture damage. Moisture from the soil can penetrate the concrete, causing it to crack over time and potentially risking structural damage.
The recommended thickness of plastic under a concrete slab is 15mm. However, the minimum thickness of an effective vapour barrier is 10mm, as thinner plastic cannot withstand the abuse of construction.
A vapour barrier is a layer of plastic that prevents or slows the entry of moisture vapour into the concrete slab. It is placed between the concrete and the prepared ground to stop moisture from rising into the slab.
A vapour barrier helps to preserve the quality and durability of the concrete slab by preventing moisture-related issues such as structural damage, mould, and discolouration. It also protects the concrete from pests that are attracted to moisture.
Yes, instead of using plastic, a true vapour barrier can be used. Vapour barriers conform to the requirements of ASTM E-1745 and provide a more effective solution by completely stopping vapour transmission rather than just slowing it down.
