Is plastic needed under concrete?

Concrete is one of the most versatile and widely used building materials. It’s strong, durable, and relatively inexpensive. However, concrete has some limitations that can be addressed by adding a layer of plastic underneath before pouring. Using plastic under concrete has become a common practice in many construction projects. But is it really necessary? Here we’ll examine the pros and cons of using plastic under concrete.

Table of Contents

What is plastic used for under concrete?

Plastic sheets, usually polyethylene, are commonly placed under concrete slabs for several reasons:

Moisture barrier – The plastic blocks moisture from seeping up from the ground into the concrete. This prevents damage from moisture.

Vapor barrier – Plastic blocks water vapor from moving through the concrete. This prevents moisture issues inside the building.
Tear resistance – Plastic makes the concrete more resistant to cracking if the ground settles or moves.
Friction reduction – The slick plastic allows the concrete to move independently of the ground in response to soil movement.

In summary, plastic under concrete helps protect against moisture damage, reduces cracks, and allows movement without breakage. It acts as both a moisture barrier and a vapor barrier.

The benefits of using plastic under concrete

Here are some of the main benefits of using plastic sheeting under concrete:

Moisture protection – Plastic creates a moisture barrier between the concrete and the earth. This prevents water from seeping upwards from the damp ground over time. Too much moisture can degrade, pit, and crack concrete.

Vapor barrier – Plastic blocks water vapor from moving through the concrete. This prevents moisture issues inside the building such as mold, mildew, and dampness.
Crack prevention – Plastic makes the concrete slab more able to move independently of the ground. This helps prevent cracks from ground settlement. The plastic acts as a slip sheet.
Durability – By protecting against moisture damage, plastic helps concrete last longer. Typical life expectancy is increased.

Smoothing out ground – Plastic can compensate for slight variations in the ground level creating a smooth pour surface.
Faster curing – Plastic holds moisture in the concrete longer leading to more ideal curing conditions with less cracking.

Using plastic sheeting under exterior slabs like driveways, patios, and foundation footings provides long-term protection and durability. The relatively low cost of the plastic pays dividends over the life of the concrete.

The downsides of using plastic

Despite the benefits, there are some potential downsides to using plastic under concrete:

Additional cost – The plastic sheeting adds more expense to the project, increasing total cost.
Labor time – Correctly installing the plastic takes more time and labor.

Tears – The plastic can tear during installation or as rebar is laid.
Improper installation – If installed incorrectly, the plastic is much less effective.
Voids under plastic – Gaps under the plastic can defeat the moisture barrier goal.

Earth contact lost – Concrete laid on plastic has no direct contact with the earth underneath.
Recycling difficulty – Concrete over plastic is very hard to break up and recycle later on.

Proper installation is key to gain the moisture protection benefits and avoid problems. Using a thick polyethylene plastic minimizes tears. Care must be taken not to create voids between the ground and the plastic sheeting.

When is plastic absolutely necessary under concrete?

Here are the situations when using plastic sheeting under concrete is recommended:

Underdrive and basement areas – Plastic should always be used under interior slabs on grade in basements or lower levels where moisture migration would be very detrimental.
Flooring with no vapor barrier – Under floor coverings like tile, stone, or hardwood that have no vapor barrier, use plastic to prevent moisture issues.

Cold climates – Areas with cold winters and freeze-thaw cycles benefit greatly from a moisture barrier to prevent damage.
Wet ground conditions – Plastic helps protect concrete when the ground is damp or retains excessive moisture.
Cracking concerns – For slabs where cracking would be very undesirable, plastic reduces movement cracks.

Steel reinforcement – Using rebar or wire mesh in concrete makes plastic almost mandatory.

Plastic sheeting is inexpensive insurance against moisture and vapor related issues over the life of the concrete. It also adds long term durability. For these critical applications, the benefits of using plastic far outweigh the small additional cost in materials and labor.

When is plastic unnecessary under concrete?

There are some instances when using plastic may provide negligible benefit or even be counterproductive:

Above grade exterior slabs – For driveways, patios, and sidewalks with proper drainage, plastic is optional.
Well drained soil – With non-expansive, granular soil that drains readily, moisture is less of a concern.
Low humidity climates – In dry environments, moisture migration though concrete is minimal making plastic less beneficial.

Plain unreinforced concrete – Where cracking is not a major concern, plastic has fewer advantages.
Short term structures – For temporary slabs or foundations, the long term benefits of plastic are less relevant.
Insulated slabs – With rigid foam insulation under the slab, a vapor barrier is already created.

For above grade exterior slabs and non-critical applications, skipping the plastic sheeting can save on labor costs without sacrificing significant performance or longevity of the concrete.

How thick should the plastic be under concrete?

For moisture barrier applications under concrete slabs, polyethylene plastic sheeting should be at least 10-15 mil thick. Some recommendations are:

Residential slab on grade: 6 mil

Commercial slab on grade: 10 mil
Underdrive and basement: 15 mil

Thicker plastic provides better puncture and tear resistance during installation and long term performance. It also allows proper placement and consolidation of the concrete without damage. Too thin of a plastic layer can easily get ripped by sharp objects.

Many construction standards call for at least 10 mil plastic for slab on grade applications. Heavier 15 mil plastic is better suited to basements and lower levels that will experience more hydrostatic pressure and moisture migration stresses over the life of the structure. The thicker material also compensates for imperfections in the subsurface when concrete is poured directly over it.

What mistakes should be avoided when using plastic under concrete?

Some common mistakes to avoid when installing plastic sheeting under concrete include:

Insufficient overlap at seams – Seams should overlap 6-12 inches and be taped to prevent moisture intrusion.

Leaving plastic loose – The plastic must be tightly fitted with no gaps or voids under the slab area.
Tears during rebar installation – Care should be taken when placing rebar or wire mesh to avoid ripping the plastic.
Failure to seal penetrations – Any pipes or conduits passing through the slab should be sealed to maintain the moisture barrier.

Incorrect placement – Plastic should extend under the entire slab area including the perimeter edge.
Rushing installation – Poor installation practices when workers hurry can compromise the plastic layer.
Not checking for damage – The plastic membrane should be checked for rips, holes, or dislodging just before pouring concrete.

Proper installation instructions, training, and inspection ensures the plastic goes down correctly. Workers need to take care not to compromise the critical moisture barrier during construction.

How does plastic under concrete work?

Here’s a close look at how plastic functions under concrete:

Forms a moisture barrier – The impermeable plastic blocks liquid water from seeping up from the ground over time. No water = no moisture damage.

Prevents vapor transmission – Water vapor cannot pass through the solid plastic layer, only through the concrete above. The plastic is a vapor retarder.
Allows slab movement – With minimal friction between the slick plastic and concrete, the slab can shift independently of the ground if soil settles.
Compensates for ground variations – Small rocks and dirt unevenness get smoothed out by the flexible plastic sheet underneath.

Protects bottom steel reinforcement – Plastic keeps moisture from corroding any rebar, wire mesh or fibers on the bottom of the slab.
Holds moisture in concrete – The non-porous plastic does not absorb water from the curing concrete, leading to proper strength gain.

The plastic sheeting acts as both a fluid moisture barrier and a water vapor retarder when installed properly under concrete. This provides long-lasting protection against moisture damage.

Plastic sheet alternatives

Instead of polyethylene plastic, builders sometimes use alternative under slab membranes:

Rigid foam insulation – Foam boards like expanded polystyrene (EPS) provide insulation as well as moisture control underneath the concrete slab.
Sprayed polyurethane foam – Foam can be spray applied directly onto the ground before pouring concrete. It expands and hardens into a seamless protective layer.

Rubberized asphalt – Asphalt sheets contain a rubber waterproof membrane that blocks moisture from passing through to the concrete.
Geotextiles – Special non-woven geotextile fabrics can be used as a moisture barrier but are less common.

Each material has its own advantages and disadvantages in terms of cost, performance, and ease of installation. The most popular under slab membrane choice is flexible polyethylene plastic due to its low cost and high effectiveness.

Steps for properly installing plastic under concrete

Here is a summary of best practices for installing plastic sheeting under concrete:

Grade and compact the soil subgrade completely before installing the plastic moisture barrier.
Unroll the 6-15 mil polyethylene plastic sheets over the slab area with several inches overlapping the footing edges.

Overlap sheets by 12 inches minimum and seal seams with plastic tape to create a continuous layer.
Seal the plastic thoroughly to the footing perimeter with adhesive or tape to prevent moisture intrusion.
Weigh down the layered plastic as needed until ready to place steel reinforcement and pour concrete.

Avoid tears, holes, and gaps by taking care when installing rebar, mesh, or fibers over the plastic.
Inspect plastic membrane just prior to the concrete pour and repair any damage.
Keep the plastic clear of debris during concrete placement for proper curing conditions.

Proper installation techniques, overlapping, sealing, and inspection helps the plastic moisture barrier work as intended. This protects the concrete slab over its entire service life.

Conclusion

Installing plastic sheeting under concrete slabs provides important moisture protection in many cases. Polyethylene plastic acts as both a fluid moisture barrier and a water vapor retarder when properly incorporated. This prevents damage from ground moisture over the life of the concrete.

Plastic under concrete is recommended for interior slabs like basements and underdrives. It also benefits exposed exterior slabs in wet climates, those with steel reinforcement, and where floor coverings lack a vapor barrier. Taking the time to install plastic sheeting correctly avoids future moisture issues.

However, plastic may provide negligible advantage for above grade slabs in dry conditions and normal soil. Consider the climate, soil type, reinforcement use, and moisture risks before deciding if plastic beneath concrete is warranted. For many critical structures and locations, the long term durability and damage prevention plastic provides makes it a wise investment.