Roof · Thermal Integrity

Table of Contents

Cottage Deep Retrofit - This article is part of a series.

Part 1: Intro

Part 2: Structure

Part 3: Electrics

Part 4: Insulation

Part 5: Flooring & ceiling

Part 6: This Article

Part 7: Bathroom

Part 8: Windows

Part 9: Details

Having a flat roof is asking for trouble. So: sheathing for a clear drainage plane; battens for a ventilation path; ribbed metal roofing.

Sheathing
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Sheathing creates a rigid plane that will drain water reliably. I used the same boards as on the ceiling: 6mm OSB (Egger OS’Brace OSB3). It is hard to find clear information about the vapour permeability of board materials. But according to LifePanels this one is usefully vapour permeable.

Better options have appeared on the market recently. Specialist boards that function as both rigid sheathing and vapour permeable membrane: Hardie RAB Board and maybe Gyprock Glasroc X.

Battens
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Battens along the slope of the roof (sitting on each rafter) allow plenty of ventilation for the roof to dry out, as well as drainage of any liquid water.

Roofing battens run perpendicular to the slope, and support the metal sheet roofing.

Roofing
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With a roof this flat, you can’t mess around. You need a continuous run of something rigid and waterproof. I went for steel in a TrimDek profile, which allows for slopes down to 2º. My slope is about 2.5º. It is easy to fasten with screws through the ribs.

To finish the roof around the edges I ordered custom folded flashing.

At the front, the flashing is cut short and also slightly overhanging the edge of the roofing. This is to maintain a ventilation pathway, while also acting as a drip edge.

At the sides, the flashing folds back in under the roofing overhang.

Meanwhile I had installed a fascia board of Cypress Pine (visible above).

Soffit
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For lining the underside of the eaves, I used plywood. Fun fact: this plywood was repurposed after being used as formwork on the walls of my hemp-lime hut. I gave it a light sanding and a coat of decking oil.