Three layers, not the one people picture.

What we lay is a built-up system: a first layer fixed down to the deck, an intermediate layer bonded over it, and a mineral-finished cap sheet bonded over that. All three are polyester-reinforced and modified with SBS, which is the additive that lets bitumen flex in cold weather instead of cracking. Three separate skins between the weather and your decking, each one lapped so the joints never line up.

The first layer is fixed rather than torched straight onto timber. That matters twice over — it keeps a naked flame off a dry deck, and it lets the covering move independently of the boards underneath, which is what stops the whole thing splitting along a joint in the first hard frost. Anyone torching layer one directly onto ply is saving twenty minutes at your expense.

It goes on extensions, garages, dormers, porches and outbuildings, and it is the sensible answer where the roof is out of sight and the budget matters more than the last ten years of service life. Where you will walk on it or look at it, fibreglass is usually the better spend.