Temporary greenhouse roof repair

A solar pool cover helps insulate the roof of my greenhouse.

My greenhouse was built in 2007, so the twin-wall polycarbonate is well beyond its rated lifespan. The walls, which receive oblique sunlight, seem still to be in good condition, but the roof is clearly deteriorating after sixteen North Carolina summers. Over the past couple of years, I have noticed that the outer layer is becoming brittle, and small holes are appearing. Thus far, the inner layer has remained intact, but loss of the dead air space between two layers of polycarbonate has surely reduced the insulation rating. It doesn’t help that the large hickory tree behind the greenhouse shed two enormous limbs in the summer of 2022. The first and largest miraculously hit the ground before it hit the roof, but the second punched several holes through both layers of polycarbonate and dented the roof ridge. I patched the holes with spray foam, but that is a temporary fix; UV exposure will soon cause the foam to deteriorate.

Fallen hickory limb on the roof of the greenhouse

Obviously, a new roof is required, but for a variety of reasons I did not get the repair completed this year. Instead, I have gone with a quick, temporary fix: a solar swimming pool cover. I got this idea from the old GardenWeb forums, probably about 20 years ago, but have never before had cause to try it. A solar pool cover is basically a big piece of tough, reinforced bubblewrap, and it comes with clips and nylon straps that can be attached to the frame of the greenhouse to hold it in place. The bubbles create dead air for insulation, rather like the twin-wall polycabonate or the standard bubblewrap that is sometimes installed on the inside of a greenhouse. As a single sheet of plastic, the cover will also help to prevent heat loss through any cracks in the roof panels or gaps along the ridge. In addition, the smooth surface should shed snow quite nicely, helping to protect the weakened roof in the event that El Niño brings us winter weather.

The clear cover reduces light a little in the greenhouse, but less than a shade cloth. It’s UV stabilized, so it should last for at least a few years. Even I get a new roof installed next summer, I may continue to use the pool cover.