Yestermorrow Practicum Project - Post 8

A double-shell roof. Latest ideas and a request for help.

Coming up with a method for venting an organically-curved roof was a challenge, but thanks to the help of John Unger Murphy and Bill Hulstrunk I believe I have a solution. In essence the roof consists of a double shell. The inner shell is composed of plywood triangles, which are attached to the framing, and covered on the outside with a vapor permeable "paintable" membrane like Tremco ExoAir 220. The outer shell is composed of hard paper mache covered with a paintable waterproof membrane such as roof mastic. In between the two layers would be a drainage mat -perhaps the Stuc-o-Flex WaterWay Rainscreen 9120 - which would allow moist air to flow up to the dome peaks to vent via small cupola like structures. The rainscreen would also allow the mache to "float" free of the plywood, helping to prevent cracks. That's the theory anyway. If you see problems with this approach please share your concerns!

Here are a couple of sketches that may help you visualize the double-shell approach.

There are two components for which I could use some help. These are the waterproof roof membrane and something that could be added to the mache to help it dry faster (the mix I'm using now takes about a week to dry). Bill Hulstrunk suggested I try duct mastic for the mache mix and I'll give that whirl in the next few weeks. Any other suggestions are much appreciated!