ᐅ Roof structure for garage: sheathing or breathable membrane?

Hello everyone,

the roof structure of our garage is due soon.

It will be a single-pitch roof with a 15-degree slope to the rear. The covering will be trapezoidal metal sheets to allow for the installation of photovoltaic panels later on.

The garage will be unheated, so we don’t need any insulation; with 24cm (9.5 inches) hollow concrete blocks as the exterior wall, it wouldn’t make much sense anyway.

Should the roof structure be constructed with a solid sheathing and waterproofing using a bituminous membrane? Alternatively, we were offered a breathable underlay membrane, with battens and counter battens above it, and then the metal sheets.

What do you think is the best option? What would you recommend regarding the photovoltaic system?

Many thanks for your help.

First a waterproof membrane and then metal sheets? A double layer of waterproof membrane alone should be sufficient! I usually see this combination the other way around on industrial roofs: metal sheets, vapor barrier including separation layer, insulation, and then a double waterproof membrane.

From a straightforward construction perspective, I would lean towards a waterproof membrane and metal sheets. If the metal sheet leaks, you can spot it quickly that way. However, metal sheets can be quite noisy and, depending on the indoor climate, are prone to condensate forming underneath very quickly.

Which looks better as it ages (metal or bitumen felt) is probably very subjective. Neither really looks good after some time...

From experience, I was thinking of the roof frame, roofing underlay, battens, and metal sheet.

Would that be okay?

Are there any issues with watertightness?