Should I put plastic down before I pour concrete?

Author: Shanelle Lockman  |  Last update: Wednesday, July 2, 2025

Why Do Builders Need an Underslab Vapor Barrier Plastic? An underslab vapor barrier protects the subfloor and, subsequently, the main floor from moisture by adding a protective layer underneath the concrete foundation before it is poured on wet. There's water in the ground underneath every building.

What material do you put down before concrete?

Choosing the right material for your sub-base is crucial in the overall performance of your concrete base. The most commonly used materials for a sub-base include gravel, crushed stone, and sand. Gravel: It's a popular choice due to its excellent drainage properties and ability to distribute the load evenly.

What kind of plastic do you put under concrete?

The American Concrete Institute (ACI) also recommends a minimum of 10 mil polyethylene or thicker for vapor retarders under concrete. According to its Guide to Concrete Floor and Slab Construction ACI 302.1R-15, thicker poly provides a better barrier and is also more puncture resistant.

Can I concrete over plastic?

Concrete can stick to high-density polyethylene (HDPE) plastic, but other forms of plastic, such as siliconized or rubberized plastic, do not allow adhesion.

Will concrete cure under plastic?

The critical early curing phase involves the hydration process, where the concrete absorbs moisture from the mix to gain strength and durability. The plastic sheeting helps maintain a controlled and consistent moisture environment, ensuring proper curing and preventing the surface from drying out too quickly.

Why a Plastic Vapor Barrier Under Concrete Slab - Mondays with Mike

Will Quikrete stick to plastic?

A: QUIKRETE® Advanced Polymer Construction Adhesive adheres to most plastics.

Can you lay a concrete slab straight onto soil?

Can I pour concrete straight onto soil? It's essential to have a base under your concrete slab, and it's recommended to use a sub base underneath from compacted gravel, rather than sand or soil alone, as this can lead to movement and moisture damage.

When to put a vapor barrier under concrete?

Generally, you'll want to use a low-permeance vapor barrier when protecting a slab that moisture-sensitive materials like adhesives and floor coverings will cover.

What plastic is best for concrete?

The most commonly used vapor barrier under concrete slabs is polyethylene (poly) plastic sheeting with a thickness of 10 mil or 15 mil.

Can I pour concrete directly on dirt?

It is not advisable to pour concrete directly on dirt. Even compacted soil—known as a subgrade—is not strong enough to maintain the form of concrete. You can, however, pour concrete over concrete as long as the overlay has proper bonding agents and the slab below is clear of damage.

What needs to be done before pouring concrete?

Here are five of the most important things you can do before you start pouring concrete at a jobsite:
  • Conduct a pre-slab meeting to iron out plans. ...
  • Inspect formwork thoroughly. ...
  • Check reinforcing steel. ...
  • Make sure you have the right materials and equipment. ...
  • Double-check the materials and jobsite to ensure readiness.

What surface will concrete not stick to?

Concrete doesn't stick to very much at all. First of all, wet concrete will not bond to dry concrete, hence the need for a bonding agent. It also doesn't adhere to paint, as paint is another material that is often made without any bonding agents. It also won't adhere to mortar and glue has absolutely no effect on it.

What do you put down before pouring concrete?

A base of gravel will help prevent erosion and keep the slab from settling. Pour concrete sand to fill the gaps in the gravel base. Use a tamper or plate compactor to flatten the gravel base. If needed, add more gravel and compact until you have a 4-inch base.

Does plastic make concrete stronger?

The research revealed that exposing the plastic to gamma radiation actually made it stronger. The irradiated plastic was then ground into a power and mixed with cement. The subsequent concrete was up to 20 percent stronger than concrete made without the irradiated plastic.

Why do you wet the ground before pouring concrete?

Ideal soil moisture ensures that the soil has the right consistency to support the concrete and prevent future movement. Too Dry: Soil that is too dry can shrink, leading to gaps under the concrete footing. This lack of support can cause the concrete to settle unevenly, leading to cracks.

Is it good to put plastic under concrete?

Why Do Builders Need an Underslab Vapor Barrier Plastic? An underslab vapor barrier protects the subfloor and, subsequently, the main floor from moisture by adding a protective layer underneath the concrete foundation before it is poured on wet. There's water in the ground underneath every building.

When should you not use a vapor barrier?

Because a vapor barrier is a low permeable barrier, it keeps moisture from passing through. This is beneficial to keep moisture away from a dry surface but will trap moisture behind it if installed on a water-damaged concrete wall, causing the wall to fail further.

What is the code for vapor barrier under slab?

4.505. 2 Concrete Slab Foundations

A 4-inch-thick (101.6 mm) base of 1/2 inch (12.7 mm) or larger clean aggregate shall be provided with a vapor retarder in direct contact with concrete and a concrete mix design, which will address bleeding, shrinkage and curling, shall be used.

What happens if you don't put gravel under concrete?

Skipping the gravel layer under a concrete project might seem like a way to save time or money, but it can lead to significant problems in the long run, including: Cracks and Surface Damage: Due to poor drainage or uneven settling, cracks can develop, which weaken the structure and lead to costly repairs.

How many inches of gravel under a concrete slab?

The recommended depth of gravel under a concrete slab for a barndominium typically ranges from 4 to 6 inches, but this can vary based on local building codes and soil conditions.

Do you need crushed rock under concrete?

Adding crushed stone under your concrete slab will provide a level surface for your foundation. Simply pouring concrete onto the ground will expose it to elemental erosion that will cause cracking and sinking. Similarly, if there are roots or plants under your slab, they may rot and cause unevenness.

What do you put under quikrete?

Add a compactible gravel subbase to provide a level, stable foundation for the concrete. The compactible gravel also improves drainage—an important consideration if you are building on soil that is high in clay content.

Will concrete stick to a 5 gallon bucket?

Concrete can stick to most 5 gallon buckets made with High-Density Polyethylene (HPDE). Coating the bucket with an oil-based release agent before mixing wet concrete is an effective solution to preventing it from sticking.

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