I'll be returning to London tomorrow to start working on the next phase of the roof construction, which is the insulation & waterproofing. I thought it might be helpful to briefly describe what kind of roof we're going to do and what it's made of:
The type of roof we're decided to do is known as a "hot deck", which means that the insulation is located on top of the "deck", which in our case is the wood planks. Normally, the insulation is located underneath the deck, in between the rafters - here that's called a "cold deck". The disadvantage of the cold deck is that one can get condensation on the interior, and there may also be issues of pipes freezing. With a hot deck, all the insulation is on the exterior, so it minimizes any water or condensation inside. A "hot deck" also allows the underside of the wood planks (our deck) to be visible--which is a key aspect of the design.
So here's the composition of our "hot deck" roof:
- Directly on top of the wood planks is a heavy black plastic sheet known here as a "damp-proof course". While actually watertight, it is intended to prevent the accumulation and seepage of any condensation as a result of the insulation.
- Above the black plastic is our rigid insulation which is required by the new building regulation to reduce the amount of heat and energy needed in homes. We're required to achieve a very high level of insulation, so it works out to be 120mm (4 inches). These sheets are large (4ft x 8ft) but fortunately don't weigh much and can be easily cut.
- Above the insulation we have our primary waterproof membrane known as the "breathable membrane" As it's name suggests, it's breathable, which helps to prevent mold within the roof cavity. As far as I know this is not really used in the US, so I'm interested to see how it works.
- The breathable membrane is held down by wooden battens which are narrow strips of treated wood. The battens are fastened to the rafters using a special helical nail that's 200mm long (8 inches) and designed specifically for hot roofs. These batten are installed vertically to allow any water to flow down the slope towards the gutter - with a vertical orientation, they become known as "counter-battens".
- Above the vertical "counter-battens" are the horizontal battens which the slates are nailed to. These battens must be perfectly straight and precisely installed otherwise the slates will go awry.
- Finally, we have the actual slates which are the traditional method of roofing here. The slates themselves are totally watertight and supposedly have a 200-300 year life expectancy (the modern membranes typically last about 20-30 years).
