A garage roof is the job most owners meet first, because it was built cheaply, it is easy to reach, and it fails visibly. What makes the price is almost never the covering. It is whether the boarding underneath has gone soft, whether the old sheet is asbestos cement, and whether the edges and the upstand against the house are being reformed or simply covered over again.
The deck decides the price
Felt garage roofs nearly always fail from below. Water gets past a split or a lifted edge, sits in the chipboard or the softwood boarding, and the deck loses its stiffness long before the surface looks finished. By the time the ceiling inside is stained the boards are usually spongy underfoot, and a new covering laid straight over them will follow the same dips and split in the same places. A quote that does not say whether the deck is being renewed is a quote for half the job.
Check for asbestos cement before anyone climbs up
Corrugated sheeting on a garage built before the year 2000 is very often asbestos cement. Left sound and undisturbed it is not a hazard, and the guidance is explicit that it is the breaking, cutting and drilling of it that releases fibres. That makes it a question to answer before the work is priced rather than during it, because removal is regulated waste with its own handling and disposal cost and no legitimate contractor will quote as though it were ordinary rubbish.
What the covering choices actually buy you
A single-piece rubber membrane suits a small rectangular garage because it has almost no joints to fail. A reinforced bitumen system suits an awkward roof with several upstands, because it can be dressed into detail that a sheet cannot. A profiled metal sheet suits a long open span where appearance matters less than shedding water quickly. On any of them the trims at the perimeter and the flashing into the house wall are where the money should go, because that is where outbuilding roofs leak.
Questions people ask about garage roof replacement
Does a garage roof replacement need building regulations approval?
Replacing the covering of a detached garage like for like normally does not. Replacing more than a quarter of the covering of an attached garage, or altering the structure, is controlled work and needs either building control or a contractor registered on a competent person scheme.
Can a garage roof be replaced in a day?
A small single garage with a sound deck often can. Once the boarding is being renewed, or the old sheet is asbestos cement, it becomes a two-day job with a separate disposal arrangement.
Is it worth converting a flat garage roof to a pitched one?
On a garage that has leaked repeatedly it often is, because a pitch removes the standing water that caused the failures. It is a structural alteration, so it needs approval and sometimes planning permission depending on the height.