One of the common problems when tarping utility trailers is that water can collect inside the tarp. If the top of your trailer is open, no amount of tightening will prevent this from happening. This won't necessarily damage the tarp but if the water sits there for a while it will start to get rancid and slimy. The weight of the water will also put stress on the grommets and trailer fasteners. If the water freezes, it could also expand and cause structural problems with the trailer or be too heavy to remove.
There are a couple of common strategies for preventing water from pooling.
If your trailer is open at one end, or if you can remove the tailgate to open up one side you could loosen the tarp at the open end in order to form a channel to allow rain water to run off. If you are planning to use this technique, make sure you order your tarp a little bit wider than you would otherwise so that there is enough extra material to make a proper channel.
Another option is to use boards (planks, plywood, etc.) to prevent the tarp from sagging into the trailer opening. In the example shown here, the back of the trailer is a little taller than the sides so we angle the last board so that it lines up with the back.
You could use one large piece of plywood on top but you might find that this is kind of heavy and awkward. It may be more convenient to use several smaller planks or even 1" x 6" planks instead.
Obviously these boards will need to be wider than the sides of the trailer so they don't fall in. Nevertheless, you don't want them to be too wide so you might consider adding angle brackets or some other form of clips on the edge of the boards to prevent them from falling into the trailer.
Once you have a solid surface on top of your trailer you can add your tarp and tie it down to prevent the wind from blowing it off. The rainwater should nicely flow off the sides.
Another potential solution is to arc the top of the trailer by adding bows so that rain water runs off to the sides. If you go with this option you will likely need to have a bow at least every two feet to prevent water from collecting between the bows. The bows shown in this photo are actually too far apart to be fully effective. Bows can be made of thin wood slats, copper piping or even PVC piping.
If your trailer is not too large, you might be able to do something like one of our customers from Portland Oregon. James writes:
"I received my cover for my utility trailer. It fits great. Slightly large by design. I spent a lot of time trying to find the best way to keep rain from pooling and thought you might get a kick out of my simple solution. I bought a 32 gallon Rubbermaid waste can for $15 at Home Depot. I placed it right in the middle of my trailer. With the domed top it works perfectly."
As James noted, you won't be able to raise the center of a fitted tarp without drawing in the sides. Therefore you will want to make the length and width a little bigger than the sides of your trailer to allow for the pitch you are going for. You will also need to make sure the bottom of the sides is pulled tight so that the top of the tarp stays taunt. The trailer shown here is about 4 feet x 6 feet. It it was much longer than this you would likely need to go with two or more trash bins to prevent sagging and water pooling. Obviously the height of the bin is important as well since you will need it to be several inches taller than the sides of the trailer to create a proper slope. You can always raise the height by putting boards under it if necessary.
Here are the photos sent in by James. The photo on the right was taken during a good rain and yet there are no signs of water pooling.
One of our customers from Seattle Washington, who is a retired sheet metal worker, sent us these photos showing how he constructed a nice flat surface out of sheet metal to prevent water from pooling. First he formed six panels using 20 gauge galvanised sheet metal. Bending them in this manner is necessary to make the metal more load bearing. Notice the exposed lip at the end of the panel, this will rest on top of the side of the trailer once this panel is flipped over.
Here we see the six panels in place forming a nice flat surface that will prevent water from pooling.