One of the most important things to note about a tarp is whether it is waterproof, water resistant, or neither. The "neither" may not make sense at first because tarps are generally regarded as protection from rain and snow. But there are many reasons to use a mesh tarp, and it's certainly not waterproof. So let's dig into the details on this particular feature of tarps!
Waterproof
Tarps that are completely waterproof are those that provide a solid barrier to both water and air. Typically these are poly tarps or vinyl tarps. Both of these types of tarps have an inner woven scrim that's coated with a laminate. It is this outer laminate that makes them waterproof. What sorts of tarps are made this way? There are a wide variety of them:
lumber tarp
fire retardant tarp
insulated tarp
various other specialty tarps
Water Resistant
So mesh tarps aren't waterproof at all, and solid poly tarps and vinyl tarps are entirely waterproof. But what falls under the description of "water resistant," and why? The answer is surprisingly simple: canvas tarps. Specifically, treated canvas tarps. After all, canvas tarps are made of fabric.
Untreated canvas is not even water resistant. After an oil-based or wax-based treatment is applied to the canvas, water will bead up and roll off of the material. At first blush that may seem like it's waterproof, but the vulnerability of treated canvas is evident when water pools on the surface. It will not take long at all for water to begin seeping through the material. When using canvas tarps, be sure to keep the material angled and taut, so water cannot pool on its surface.