When using a heavy duty tarp as a covering for materials or equipment outdoors, moisture will play a role in which product will provide the best protection. Some, like a canvas material, are breathable so that moisture can pass through them. Others, like a poly material, are waterproof and will form a barrier that locks out water and traps moisture inside. Choosing the right heavy duty product depends on how water and moisture will affect stored materials and can be a crucial decision.

Let’s take a look at how different materials will handle water and which are best for certain applications.


Canvas is a natural material so it is breathable. While canvas is water resistant, it is not water proof. Moisture will slowly pass through a canvas tarp over time. For this reason, moisture underneath it will not stay trapped and will not form condensation on the underside of the covering. On the flip side, standing water on top of a canvas tarp will eventually drip through. If it stays constantly soaking wet, canvas will eventually begin to breed mold and mildew.

Canvas is a good material for a lot of outdoor applications. Since it will not trap moisture underneath, covers made from canvas will work well if used directly on the earth. The moisture from dirt and mud will escape, keeping the covered goods more or less dry. This same property will also allow heat to escape if temperature is an issue.

However, this material should not be used as a storm cover or in any extremely wet situations. A canvas tarp will let water slowly seep through if used as a shelter or roof cover and there is standing water on canvas. Canvas is treated with chemicals, wax and dyes. The dyes and waxes can rub off during use, making canvas tarps poor boat and car covers.


Polyethylene, or poly for short, is a water proof material. Moisture will not pass through the material so long as there are no tears or holes. This is a two way street, water will not seep through a poly tarp, but any moisture underneath will stay there. A poly product is an effective barrier against moisture and water seepage even in standing water or during a storm.

For this reason, they are very effective as shelters and temporary roof coverings. However, it is not a good idea to place a poly tarp directly on the ground if they materials underneath need to remain completely dry; the ground moisture will condense on the inside of the tarpaulin. For example, covering a lumber pile that is sitting directly on the ground with a poly cover may result in damp wood. Also, be careful when using poly material outdoors, it is possibly susceptible to damage from the sun’s rays and UV radiation.

When considering the effects of moisture and water, remember that heavy duty covers can either act as a barrier against water by using a poly material, or allow water to pass through if using a canvas material.