The short answer on cost (and why quotes vary so much)

When customers ask me on a driveway in Brighton, Hove, or Worthing, “What will my flat garage roof cost?”, I give a straight, realistic range and then explain what moves the number up or down.

  • Single garage (around 12–18 m²): basic overlay on a sound deck typically £1,100–£2,000; full strip with new deck and a robust two-layer felt system typically £1,600–£3,200.
  • Double garage (around 20–36 m²): basic overlay typically £1,900–£3,600; full strip with new deck and two-layer felt typically £2,800–£5,600.
  • Warm roof insulation upgrade: usually adds £600–£2,000 depending on size and details.

If you want a tailored figure right now, use my garage roof cost calculator. It’s instant and commitment‑free.

The 9 price triggers I check on site before I confirm a fixed quote

1) Size and shape of the roof

Bigger areas usually bring the cost per square metre down a touch, but awkward shapes, lots of steps, or multiple joins can add labour time that eats the savings. I measure properly so you’re not paying for “guesstimates”.

2) Access and working height

Can I get materials to the roof safely? Is there clear side access, or am I hand-balling everything through a narrow alley? Simple tower access is one price, full scaffold around a double garage is another. If you want to run the numbers, my scaffolding price calculator gives a realistic ballpark.

3) Condition of the existing deck

In my 18 years on the roof, I’ve noticed this is the biggest swing. If the OSB/ply is solid and dry, an overlay may be an option. If it’s spongy, swollen, or blackened, I’ll strip and re-deck. New boards, fixings, and disposal add cost but prevent a failure in year two.

4) Insulation (warm roof vs. none)

Garages that are attached, used as workshops, or under a room benefit from warm-roof insulation. It raises the roof build-up, which means extra trims and detailing, but it stops condensation and heat loss. If your garage is detached and unheated, I’ll be honest if insulation won’t pay you back.

5) Edges, upstands, and flashings

Good detailing is where leaks are won or lost. Drip trims, verges, and any upstands around walls or doors take time. If your roof meets brickwork, I often need to chase in and renew the lead flashing. It’s not the cheapest line item, but it’s the one that stops water tracking down your walls.

6) Skylights, hatches, and pipe penetrations

Each opening needs careful weathering—proper collars, preformed corners, and correct laps. Replacing an old brittle skylight while I’m there is usually cheaper than doing it as a separate job later.

7) Drainage, gutters, and outlets

If water ponds, I may need to add a tapered layer or reshape the falls. Tired gutters or tiny outlets keep roofs wet and shorten lifespan. If yours are past it, I’ll price in guttering and downpipe replacement so the system works as one.

8) Tear-off, waste, and any hazardous materials

Old felt, rotten boards, nails, trims—disposal is part of the job. If I suspect asbestos (more common on old corrugated roofs than flat felt, but it does crop up), I’ll flag it straight away and explain safe, compliant options before any costs mount up. No surprises.

9) Timing and weather window

A calm 48-hour window lets me phase strips and make every layer count. Rushed “patch-and-hope” work in bad weather is false economy. If you ask me, I’ll tell you straight when to book to avoid weather premiums.

Three real‑world scenarios with honest price ranges

Scenario A: Single garage overlay on a sound deck

Area: ~14 m². Deck solid; no insulation; simple edges; tower access. Two-layer torch-on felt overlay with new drip trims and one outlet. Typical total: £1,300–£1,900. Time on site: 1–2 days.

Scenario B: Double garage full strip and warm roof

Area: ~28 m². Old felt and rotten boards stripped; new OSB; warm-roof insulation; new trims and one new outlet; easy driveway access. Typical total: £3,800–£5,200. Time on site: 2–3 days.

Scenario C: Tight-access single garage with new leadwork

Area: ~16 m². Full strip, new deck; no insulation; chased into brickwork with new lead flashing; hand-carry access. Typical total: £2,000–£2,900. Time on site: 2 days.

How to keep the budget sensible without cutting corners

  • Clear access before I arrive: bins, bikes, stored timber—every minute saved is money saved.
  • Decide early on insulation: it affects trims, falls, and leadwork. Planning it in is cheaper than adding it late.
  • Fix small extras while I’m there: a new outlet or short gutter run is cheaper during a re-roof than months later.
  • Pick a realistic weather window: it protects your deck and lets me deliver a proper, dry installation.

My process: from instant estimate to fixed price with no surprises

I do things simply and transparently. First, get a no‑pressure figure with my garage roof cost calculator. If that looks sensible, book a free roof inspection and I’ll come out, check the deck, access, and details, then issue a fixed‑price quote. You’ll know exactly what’s included—materials, waste, trims, leadwork, and any scaffolding if needed.

Once we start, you get full visibility through my client portal: schedule, daily photo logs, and all your paperwork in one place. I’m on the job from first estimate to final handover—no middlemen.

Want a firm number for your garage?

Tell me your size, access, and whether you want insulation, and I’ll pin the price down. Start with the instant calculator for a quick guide, then I’ll confirm everything on a free site visit and lock it in with a fixed quote for your peace of mind.