When insulating a flat roof wood structure, 475’s goal, as always, is to do it in the most healthy, structurally sound, durable, and ecologically friendly way — achieving truly high performance. This means we try to generally avoid using foam above or below the roof deck (for more on that, see our Foam Fails series).
With foam out of the equation, we examine how to avoid condensation, rot and mold in our structures. Based on the research noted in our 475 blog post Unvented Flat Roofs: A Technical Discussion, we have established The Ten Golden Rules. By following the Ten Golden Rules you are ensured a robust roof assembly.
The Ten Golden Rules are:
1. Minimum flat roof pitch 2% (1/4″:12) and 3% (3/8″:12) when considering potential deflection
Shedding water is always the essential first step in preventing leaks and keeping your roof safe. The New York architect Chris Benedict leads in the right direction by going further, using 1/2″:12 flat roofs.
Ponding water from inadequate pitch.
2. Roof Membrane Should be Dark (in Climate Zone 5 and Higher)
The colder the climate and the higher the percentage of cloud cover, the higher the absorption value of the roof should be. This sounds counter intuitive, as we’ve all been directed towards white roofs. However, the additional heat provided by the sun will allow any humidity within the wood structure/insulation to be driven inwards (the only way the assembly can dry). In Germany >80% absorption is recommended. (In the US heating climates (>5), or as you head to southern climates (zone 4,3,2 — east of the Rockies), you could have a lighter roof, because of higher solar radiation levels, but we do recommend a WUFI calculation to determine how light.)
3. No shading of roof membrane
This means no pavers, no terraces, no gravel, no PV, no green roofs and no surrounding structures that might shade the roof. Any shading will prevent the sun from driving the humidity out of the assembly.