Most tarp questions come down to one decision: what the tarp is made of. Size matters, but the material decides how long the tarp lasts, whether it keeps water out completely, and how it treats whatever is underneath. There are three main families.
Vinyl (PVC-coated polyester)
A vinyl tarp is a woven polyester base fabric coated on both sides with PVC. The polyester gives it strength and tear resistance; the PVC makes it fully waterproof and lets the seams and hems be heat-welded instead of sewn. Weights are given in ounces per square yard: 13 to 14 oz for medium duty, 18 oz for heavy duty, 22 oz and up for the heaviest work.
- Use it when the cover stays out for months or years, faces highway wind, or covers something that can't get wet: flatbed loads, equipment, long-term roof patches, boats.
- Watch for weight (an 18 oz 20 x 30 ft tarp weighs about 75 lb) and condensation. Vinyl doesn't breathe, so a damp load can sweat underneath.
Poly (woven polyethylene)
Poly tarps are made from flat polyethylene tapes woven into a sheet and laminated with a thin polyethylene coating. Thickness is usually given in mils (thousandths of an inch): roughly 5 to 6 mil for light duty, 10 to 12 mil for "heavy duty" poly.
- Use it when the job lasts days or weeks: a roof until the roofer arrives, a woodpile through a wet spell, a job-site cover.
- Watch for sunlight. Ultraviolet light breaks polyethylene down, and an outdoor poly tarp is usually done within a season or two. The grommets also tear out more easily than on vinyl, so use stretchy tie-downs.
Canvas (treated cotton duck)
Canvas tarps are woven cotton, usually treated with wax or silicone to shed water. The weight on a canvas tarp is usually the finished weight after treatment, so a "16 oz" canvas may start as a lighter cloth.
- Use it when the load needs to breathe: hay, firewood, green lumber, machinery stored for months, or as a heavy drop cloth.
- Watch for the fact that canvas is water resistant, not waterproof. In a long, hard rain it can wick water, and untreated cotton can mildew if it is folded away wet.
Side by side
| Vinyl | Poly | Canvas | |
|---|---|---|---|
| Keeps water out | Yes, fully | Yes, when new | Sheds it; can wick in long rain |
| Breathes | No | No | Yes |
| Life outdoors | Years | Weeks to a season or two | Years with care |
| Weight | Heavy | Light | Heavy |
| Price | Highest | Lowest | Middle |
A simple rule
If the cover is coming off within a few weeks, poly is fine. If it has to stay on through a season, through storms or at highway speed, buy vinyl once. If what's underneath needs air, use canvas. When in doubt, tell us the job and we'll tell you which one we'd use.
What actually wears a tarp out
Each material ages by a different mechanism, and knowing which one explains most of what you see in the field.
Poly: sunlight breaks the chains
Polyethylene is a long chain of carbon atoms. Ultraviolet light in the 290 to 400 nm range carries enough energy to start photo-oxidation: it creates free radicals that react with oxygen in a chain reaction, breaking the polymer into shorter chains. Shorter chains mean a weaker, more brittle sheet. That's why an old poly tarp crackles, sheds flakes of coating and tears at the grommets. Manufacturers add UV stabilizers, usually hindered amine light stabilizers (HALS), which mop up the radicals and slow the process; they don't stop it.
Vinyl: plasticizer loss
Rigid PVC is the material of drain pipe. What makes a vinyl tarp flexible is a plasticizer blended into the PVC. Over years of heat and sun, plasticizer slowly evaporates or migrates out of the surface, and the coating stiffens. Heat and UV speed this up, and a stiff, cracked vinyl tarp is the end result. Good coated fabrics include UV inhibitors and often a protective topcoat, which is a large part of why they last years instead of seasons.
Canvas: water swells the fiber
Cotton is cellulose, and cellulose fibers swell when they absorb water. In a tightly woven, treated canvas, that swelling closes the gaps in the weave, which is how canvas sheds rain while still letting water vapor out when it's dry. The weakness is biological: cotton that stays damp can grow mildew and rot, so canvas needs to dry out between rains and before storage.
Sources
- Tarps Now, 18 oz heavy duty vinyl tarp specifications
- Tarps Now, 14 oz vinyl coated polyester tarps
- Wikipedia, "Tarpaulin" (materials)
- University of Wisconsin Extension, big bale storage losses
- Safic-Alcan, UV stabilizers for plastics (HALS and UV absorbers)
- Q-Lab, accelerating plasticizer migration testing in PVC
Written by the TrueTarps team. Questions about your job? Contact us or request a custom tarp.