I tried the reused billboard tarps last year. They did “OK” but certainly were NOT waterproof, and with time became even less protective. They are USED when you get them, seams may be the weakest spots. Lasted thru winter, but was getting issues and was very stiff, hard to deal with when we got new loads of sawdust in and had to move it.
Husband had an inspiration, and purchased me a tarp like the semi truck folks use for covering loads on flatbed trailers. It has nylon strips, with D-rings under the strips, for tying it down. You can get the SIZE YOU WANT, unlike the billboard tarp company. They sell you what they have, which really wasn’t the size I NEEDED for covering my sawdust pile.
We got the semi tarp, which is pretty heavy material. Grommets on the corners and TONS of rings to keep it where you want it. I padded the tops of bunker posts, anywhere the tarp might get rubbed, with pieces of carpet. Our tarp is 14ftx24ft for our sawdust bunker, which covers it well, with overlaps on the wall sides for good water runoff. It looks like the one shown as a Steel Hauler tarp.
http://www.tarpsplus.com/trucktarps.html
This is just an example, we didn’t buy from them. Can’t actually remember where we DID get ours! It is black, both sides, pretty heavy. Truck tarps are not cheap, but you should get good quality. If we don’t have to buy a tarp for a couple years, maybe longer, it will have been worth the cost.
I am very happy with the performance of the semi tarp since last spring. It doesn’t leak, though it may have condensation from dampened sawdust delivery. I want my sawdust a bit damp, which keeps it from being dusty in the stalls. We used Combi stock panels of welded heavy wire as a curved cover on the sawdust bunker, laid the tarp over the panels. The panels keep the tarp in the air, lets it drain off rain and snow, lets me drive tractor with loader underneath to get bedding. No having to move, clean off or lift the tarp to get to the sawdust underneath! We call it the “redneck shed” but love how it works for us. Tarp is easy to move back, remove the front roof panels, for a new load of sawdust. Husband pushes the sawdust back and high up inside, so truck dumping it doesn’t leave it spread way outside the bunker. Then we clip the wire panels back inside the bunker walls, pull the tarp forward and anchor it back down.
So with my previous experience, I would now recommend getting a semi truck tarp for the best protection when you need a tarp. Not cheap, but doing an EXCELLENT job for us.