The short answer
Not always. The deck is the layer of boards under the covering, and it only needs replacing where it has gone soft, rotten or sagged. A sound deck can stay and take a new membrane. We check it from above and below before any price is given, because new covering laid on a bad deck fails early, and no one can see the problem until the old roof is off.
The covering gets the attention, but the deck and joists under it carry the whole roof. Checking them is part of any honest garage re-roof.
What the deck is and what it does
A flat garage roof is built in layers. Timber joists, the beams that span the garage from wall to wall, carry a deck of boards or sheets on top. The waterproof covering, whether felt, rubber or GRP, lies on that deck. The deck gives the covering a flat, firm surface to be stuck or laid on, and it spreads the weight of anyone standing on it.
Why does a failing deck matter?
A membrane follows whatever is under it. If the boards have dipped, water collects in the dip and sits there. If they have rotted, the covering loses its support and punctures or splits when you step on it. Fixings lose their grip in soft timber, so edges lift in wind. In every case the new covering inherits the fault.
That is why a low quoted figure from someone who has never lifted a corner of the old roof is a warning. A proper price is written after the survey, once we have looked at what is underneath. The garage roof cost page, garage roof cost, sets out what moves the figure.
How we check the boards and joists
The survey starts indoors. Standing on the garage floor, we study the ceiling line and the joists for dark staining, dripping, white fungal growth and a sagging line between the walls. Daylight showing through a joint is plain evidence of a gap above.
Can you tell if boards are rotten from above?
Partly. On the roof we feel for spongy patches underfoot, look for ridges where the covering has lifted and press on suspect areas with a screwdriver. A sound board resists. A rotten one lets the tip sink in. Where the covering is thin or already torn, we open a small section to see the timber directly, which is the surest test.
Moisture meters help too. Timber that reads high in several places has been wet for some time, and the cause has to be found as well as the damage, otherwise a new deck rots just as the old one did. A blocked gutter, a poor drip edge or a ponding area are the usual causes, and each is dealt with as part of the job.
You can see how a ponding roof is handled on our leaking garage roof page.
Repair a section or replace the lot
Where rot is local, such as one corner under a failed gutter, a section of deck can be cut out and renewed with new boards, and the joists beneath are checked and treated or doubled up where needed. The new boards are nailed so that their ends meet over a joist, and the joint is supported, not floating.
When does the whole deck come off?
When damage is widespread, when the boards are thin and flexing, or when the deck is a type that does not suit the new covering. Many flat roofs need a firm board with joints supported to take a bonded membrane. Sagging joists get a closer look still, since replacing a deck on joists that are too weak simply covers up the real problem.
The size of the saving from keeping a sound deck is real, and the cost of ignoring a bad one is bigger. In MyJobQuote's guide (Oct 2026) a garage roof averages £1,500, with £750 to £3,500 as the typical UK range, and a deck replacement pushes a job toward the upper part of it. Falls matter here: BS 6229:2025 keeps a minimum finished fall of 1:80, and new timber firrings, the tapered strips fixed on top of joists, can correct a flat deck.
Ventilation, insulation and the next covering
A new deck is the moment to deal with the air space and insulation beneath it. A cold roof keeps an air gap above the ceiling that outside air can pass through. A warm roof puts insulation above the deck instead, so the timbers stay warm and dry. BS 5250:2021 is the British Standard we use for the moisture side of that choice.
Which covering goes on a renewed deck?
That is your choice, guided by the garage. Typical lives, as published, are 10 to 20 years for torch-on felt, 25 to 40 for GRP and 30 to 50 for EPDM (Less, Oct 2026), and GreenSpec gives EPDM 20 to 30 years. None of those is a promise, and a firm deck is the first condition for reaching them.
Edges and details finish the job. A neat drip edge, the metal strip that carries water into the gutter, and a sealed upstand at any wall keep the new covering working. Our garage roof drip edge page shows the point where many failures begin, and the garage and asbestos roofs page covers the rest of the service. Wherever we open the roof, the exposed timber is photographed so you can see its state before anything is decided.
More on timber under the roof
Can new boards be laid over the old deck?
Sometimes, where the old boards are dry, flat and firmly fixed. Laying over a soft or uneven deck traps the fault underneath. We test the old boards first, and if they pass, a new layer can improve the surface and sometimes the strength.
Will a new deck change the height of my garage roof?
Only slightly. A deck board adds a few millimetres, and added insulation or firrings add more. At doors, walls and gutters that can matter, so we check the heights on site and adjust the details before the work starts, not halfway through.
The full picture: Garage and asbestos roof replacement in Taunton · More short answers