Do I Need a Vapor Barrier Under My Shed?


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Before building your shed, it’s the time to prepare the ground, removing any grass and roots before you begin with the type of foundation you have chosen to use.

If you live in a climate that gets little rain, you probably don’t need a vapor barrier underneath, especially if your shed will be off the ground a few inches. For most people, however, a vapor barrier is a good idea.

Do I Need a Vapor Barrier Under My Shed?

If you plan to build a shed with a concrete slab, for instance, a vapor barrier underneath the concrete will keep ground moisture from seeping into the concrete and eventually into your shed. This will keep mildew and mold out of your shed and keep it from smelling musty.

When it rains, the parts of your building that get rained on are not the only portions that will be affected by dampness. Moisture evaporates and can collect on any solid surface, especially horizontal ones. If that surface is at all permeable, the moisture seeps inside the material, bringing with it all those unpleasant surprises such as rot, mildew, mold and fungus. Moisture also can attract insects such as termites.

What most people don’t realize is that insects need water too, and termites are attracted to moist wood. The moisture also makes the wood easier to invade. They may not invade treated wood, but they can build tunnels to protect them while they travel to untreated wood. Another option may be black plastic, you can read my article Can You Use Black Plastic for Vapor Barrier? for more information.

What’s the Best Vapor Barrier for Under a Shed?

Clear polyethylene plastic has been the standard for many decades. However, the usual 6-mil thickness has been shown to be susceptible to tearing and can actually be permeable to moisture. A 10-20 mil thickness will do a much better job.

This is important when pouring a concrete slab, as the pouring process can tear thinner plastic, even if a rebar grid is used. A 20-mil barrier is preferred for a concrete slab. The thicker plastic will also keep the concrete from curing too quickly. If your foundation is gravel, a vapor barrier is advised underneath the gravel.

How to Protect the Underside of the Shed Floor?

The best things to protect the underside of a shed floor are a vapor barrier and airflow. While the vapor barrier is important, if you live in a humid climate, it takes longer for the moisture in the soil to evaporate and your flooring can eventually be affected. If you have wooden skids or joists touching the ground, this is especially true.

While you would use treated lumber for any wood sitting on the ground, evaporating moisture can affect the flooring. Having the shed floor raised a few inches to allow air to flow dries the moisture more quickly and keeps it from settling and collecting where you don’t want it.

If your shed rests on piers or skids, a vapor barrier between the joists and shed floor keeps dampness from rising and collecting on the underside of your shed floor. Also consider if your shed is too close to the ground, you can read my article How High Should a Shed Be Off the Ground? for more information. Also you may find my article How Do I Keep My Shed from Sinking? helpful.

If you buy a shed rather than build, your shed will probably come with a floor but not joists or supports underneath, and the floor will probably not be built using treated lumber. The shed will still need a foundation and moisture barrier.

Does a Shed Floor Need to be Treated?

Any wood that will sit on the ground or touch your foundation material should be treated. Most Considering the nightmare of having a rotting floor, it just makes sense to use treated lumber for the floor plate.

Treated plywood is available if your flooring system will use it. While home improvement centers don’t often stock this product, it should be readily available at a traditional lumber company. Treated lumber only costs a little more, but the problems it can save will pay you back well over the extra costs.

How to Prevent Shed Floor From Rotting?

The most common kind of rot is often called dry rot. This term may have come about because of the darkened appearance of the rotting wood and that it is visible even when the wood is dry. This is a mistaken term, as rot occurs only when moisture is present. Any moisture can allow fungi to grow in the wood and cause rot. Fungi and certain bacteria feed on wood. Sufficient moisture, food and the right temperature conditions allow for reproduction. Once wood starts to rot, replacement is the only solution.

Basically, the more protection you can give your floor, the better. Airflow is more important than people may think, as it naturally keeps condensation from collecting in the first place. Keeping something dry just makes more sense than trying to keep moisture from seeping from one object to another.

Of course, when it rains, the foundation material will naturally become damp or even wet, but with sufficient airflow, it should not remain damp. Airflow under your shed is especially important in humid climates that don’t allow quick evaporation. Areas that receive a good amount of snow also keep moisture around as the snow melts. Use of a vapor barrier, using treated lumber for skids if your design calls for them, joists and bottom flooring all help. Each level of moisture barrier protects the floor.

A newer product many are not aware of is plastic shed bases. They are made of interlocking sections made of recycled heavy-duty plastic. The sections are in the form of a grid, so pea gravel can be filled in to help drainage and form a solid base. Even though the sections are light enough to handle easily for assembly, they are surprisingly rigid and won’t ever rot.

They are said to be so strong they can support the weight of a vehicle. Since they sit on level ground, these bases provide airflow and shouldn’t need an additional vapor barrier. They are touted as being as supportive as a concrete base and will support any type of shed. Some even have used them for summer houses and garages. This may be an option for you to investigate.

Conclusion

A shed is intended to last a very long time. Giving attention to protecting the bottom is as important, if not more so, than protecting the roof. You want your shed to last as long as your home, so why not give the same consideration to protecting it as you would your home?

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