Our house is about twenty years old with a deck that serves as a roof over the basement den and a storage room. The original decking was a fiberglass coating over plywood but it leaked into the den about four years ago. So we removed the fiberglass and replaced it with rolled roofing and a 1-1/2 inch concrete topping. Now the decking is leaking again. What would you suggest?

Lots of folks simply think that a deck over a living area is just a deck. However, you have correctly identified your deck as a roof. This is an important point because the material you choose to give you assured water protection should be an appropriate roofing material.  Unfortunately, you have just mentioned two commonly mis-used materials that will not perform well as roofing materials when installed as a roof on a deck. Here are the reasons. Even though fiberglass can be waterproof, it is not a reliable roofing material. Here’s why.

The installation of fiberglass requires mixing components that cure and harden over time. If the ratios of components vary from batch to batch or from one installer to another, the ability of the fiberglass to resist water penetration will also vary. As well, the surface gel coat is the real sealer. This is a micro-thin film on the surface of the fiberglass. If this veneer cracks due to deck movement or it is damaged by chairs or foot traffic, your waterproof seal is lost. This is the most likely reason your original deck leaked.

Roll roofing, your second choice, is a recognized roofing material, but it does not work well in flat or low slope applications because the narrow, tarred seams are vulnerable to water penetration. On moderate or steep pitched roofs these seams are well drained. But, in the long term, they will not resist standing water in a flat or low pitch application.

For a short term repair to help seal your existing deck leaks, you may try applying a concrete sealer to the surface of the concrete. But long term solutions will be costly. You may consider installing 60 mil vinyl decking over a new plywood layer installed over the existing concrete. If you like the appearance and feel of your existing concrete topping, you may be faced with removing this concrete and roll roofing under it. Then install either a torch-on or EDPM roofing membrane. Before you cast the new concrete topping, check with the manufacturer of the roofing product you have chosen. Be sure the manufacturer warrants their product for use under a concrete topping. You may find that the manufacturer will want their product to be accessible for future repairs. This means that you may have to install removable pre-cast concrete pavers over the roofing material you choose. Research these options thoroughly with installers and product manufacturers before making a final choice. Whichever of these product you choose, try to incorporate a slope to the outside edge of your deck. All roofing materials last years longer if they are not subjected to the freeze / thaw cycles of ponded water.