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Common Ground News

Do you put plastic under concrete?

Author

Chloe Ramirez

Updated on March 16, 2026

Do you put plastic under concrete?

Almost everywhere (except “California”) that slabs are placed on the ground—a plastic sheet—a vapor barrier—is located directly under the concrete slab and on top of the ground (Photograph 1). This plastic sheet is in direct contact with the concrete—as it should be in a rational world.

In respect to this, do you need vapor barrier under concrete slab?

Water vapor will always move from a high relative humidity environment below the building into an area of low relative humidity – even through concrete. That's why nearly every expert in the concrete industry recommends a below-slab vapor barrier to stop the vapor drive of water upward and into your building envelope.

One may also ask, how do you prepare the ground for a concrete slab? Just scrape off the sod and topsoil and add gravel fill if needed. If you have clay or loam soil, you should remove enough to allow a 6- to 8-in. layer of compacted gravel under the new concrete. If you have to remove more than a few inches of dirt, consider renting a skid loader or hiring an excavator.

Keeping this in view, what is the purpose of plastic under concrete?

A concrete vapor barrier is any material that prevents moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete is poured wet, it's not supposed to stay that way. It needs to dry and then stay dry to avoid flooring problems.

What is the best base for concrete?

Most concrete contractors want a mix of coarse and fine aggregate to create a compactable base that is going to be safe for settlement and drainage. Crusher run (a mix of crushed stone and stone dust) is one of the best base materials for concrete flatwork.

Is gravel needed under concrete?

Whether you pour concrete for a walkway or patio, a strong gravel base is required to prevent the concrete from cracking and shifting. Gravel is especially important in clay soil because it doesn't drain well, which results in water pooling under the concrete slab and slowly eroding the soil as it finally drains.

Do I need a membrane under concrete?

For something like this, you just need to put a membrane underneath the slab really. The proper stuff is expensive but any bit of polythene that fits in the space underneath the slab will do or you might find a cheaper membrane from B&Q or somewhere.

Why do you put Visqueen under concrete?

A concrete vapor barrier is any material that prevents moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete is poured wet, it's not supposed to stay that way. It needs to dry and then stay dry to avoid flooring problems.

What do you put under concrete slab?

Whether you pour concrete for a walkway or patio, a strong gravel base is required to prevent the concrete from cracking and shifting. Gravel is especially important in clay soil because it doesn't drain well, which results in water pooling under the concrete slab and slowly eroding the soil as it finally drains.

Do you need Visqueen under concrete?

A concrete vapor barrier is any material that prevents moisture from entering a concrete slab. Vapor barriers are used because while fresh concrete is poured wet, it's not supposed to stay that way. It needs to dry and then stay dry to avoid flooring problems. This is why a vapor barrier under concrete is essential.

Can water come up through a concrete slab?

Concrete is a porous material so it will naturally absorb water. You can prevent this by sealing it. It's also possible that there are cracks along the basement or garage floor. Hire a professional concrete contractor to spot the cracks and to seal them to prevent water from coming up through.

How do I keep my concrete floor from sweating?

How to stop concrete sweating
  1. Dry out the room by air movement: a high volume low speed fan works well.
  2. Turn down the air conditioning/turn up the heat to keep the concrete floor temperature and the air temperature at similiar levels.
  3. Use a dehumidifier to remove moisture from the air and reduce the condensation.

What can I use for vapor barrier under concrete?

  1. Use a low permeance vapor retarder to protect floor slabs that will be covered with moisture-sensitive floor coverings, adhesives, and coatings.
  2. Include a capillary break of granular material below the slab.
  3. Consider using concrete having a w/c not greater than 0.45 to hasten slab drying time.

Can concrete be poured on dirt?

The clay can compress under the weight of the concrete, causing the slab to shift or sink over time. It also can leach moisture from the concrete itself, resulting in uneven curing and a brittle base. With proper preparation, however, it's possible to pour a slab on clay soil without encountering these problems.

How do you install a moisture barrier on a concrete floor?

  1. Roll the moisture barrier onto the floor lengthwise.
  2. Overlap the next row by 3 to 8 inches, depending on the manufacturer's instructions.
  3. Seal the seams with a moisture-resistant tape all the way down the length of the row.
  4. Repeat these steps until the whole floor is covered.

How do you prep a concrete pad?

Just scrape off the sod and topsoil and add gravel fill if needed. If you have clay or loam soil, you should remove enough to allow a 6- to 8-in. layer of compacted gravel under the new concrete. If you have to remove more than a few inches of dirt, consider renting a skid loader or hiring an excavator.

Should I insulate under my concrete slab?

Creating an insulation layer and a vapor barrier between the concrete slab and the ground beneath it is the best practice in insulating basement floors. The insulation barrier will help keep warm air in, and the vapor barrier will keep unwanted moisture out.

What flooring is best for concrete slab?

The best options for flooring over a concrete slab foundation are wood/laminate, carpet, or tile for your whole house. Many concrete floors have a tendency to get damp, so when you are installing wood floors over a concrete slab foundation there are some important things that you need to do.

Why does my concrete floor sweat?

Concrete sweating usually occurs when warm air comes into contact with a cooler concrete slab and it causes the concrete floor to sweat. After a humid night, water vapor in the air will come into contact with the cooler concrete floor and will condense into morning dew.

What mil plastic should be used under concrete?

According to the Guide to Concrete Floor and Slab Construction published by the American Concrete Institute, a vapor retarder should not be less than 10 mils thick. (A mil is one-thousandth of an inch.) You might need an even thicker barrier though if you're covering material with sharp angles.

Can I use plastic sheeting as a vapor barrier?

In simple terms, a vapor barrier is a material that won't allow moisture to pass through it, such as plastic sheeting. It's designed to stop the moisture before it can enter the wall cavities. There are two basic types of vapor barriers used with exterior wall insulation. The most common is paper-faced insulation.

What kind of plastic goes under a house?

Crawlspace Vapor Barrier Material
Reinforced polyethylene plastic sheeting (poly) of at least 6 mil thickness is the material most commonly used as a vapor barrier.

Is wire mesh necessary in concrete driveway?

Use wire mesh for driveways that are 4 to 5 inches thick, and rebar for those that are 5 inches or more. Place rebar in a grid pattern with a spacing between bars of approximately 12 inches. In either case, blocks should be used under the reinforcement to keep centered within the concrete.

Is sand a good base for concrete?

Pour concrete over a solid, well-drained base
Because concrete slabs “float” on the soil, soft ground or voids underneath may cause unsupported areas to crack under heavy weight like vehicles. of sand or gravel over clay and other poorly draining soils to provide even support.

Do you need rebar for 4 inch slab?

Do you need rebar for a 4-inch slab? - Quora. It all depends on the size of the slab and whether the slab is on grade. A concrete slab reinforced with rebar or welded wire fabric should have a minimum 1 1/2 inches of clear cover between the reinforcing and the top of the slab.

Do I need plastic under my concrete slab?

Vapor retarders have been used since the 1950s. Recently, though, research has shown that the old traditional layer of 6-mil Visqueen (polyethylene plastic) under the slab is seldom effective for two main reasons: Although it may seem water-tight, this grade of material allows a lot of water vapor to pass through.

Do I need rebar in a small concrete pad?

Most thing slabs, like driveways or sidewalks have a wire mesh reinforcement in them not rebar. 6x6x10/10 mesh is typical. Rebar should have a minimum 2″ embedment in the concrete. 2″ of concrete from the outside of the rebar to the surface.

Can I use pea gravel under concrete?

Backfill beneath concrete flat-work along foundation walls should be accomplished with a granular material that is uniform in size. Some fill materials, such as round stones like pea gravel, tend to be self compacting. “I recommend open-graded granular material be used,” says Tull.

What is a good base for a concrete slab?

Subgrade and subbase are the foundation of a concrete slab and play critical role in its performance. According to the ACI Code, the subgrade is a compacted and improved natural soil or brought infill whereas subbase is a layer of gravel placed on the top of the subgrade.

What type of gravel goes under concrete?

Backfill beneath concrete flat-work along foundation walls should be accomplished with a granular material that is uniform in size. Some fill materials, such as round stones like pea gravel, tend to be self compacting. “I recommend open-graded granular material be used,” says Tull.

What kind of stone goes under concrete?

Because concrete is a very porous material, it will absorb any moisture that it contacts. This can cause pooling. Without crushed stone, pooling water will settle under it and erode your slab. Adding a layer of crushed stone will add proper drainage, as well as create a barrier between your slab and the ground.

How much does it cost to pour a 12x12 concrete slab?

Average Cost To Pour Concrete
To pour a typical 10x10 concrete slab will cost $670 to $930, a 12x12 slab for a patio will cost $796 to $1,476, a 20x24 driveway slab runs between $1,440 and $3,360, and a 24x24 slab for a garage will cost $3,058 to $5,944.

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