Garage Roofs: Repair, Re-cover or Replace

The short answer: The boarding decides a garage roof. Where it is dry and firm, a new covering goes straight over the old one; where it is soft, covering and boards come off together and the roof is rebuilt from the joists up.

The difference

Why a garage roof is not a small version of the house roof

Because almost nothing that governs a roof over a room applies to one over a car. There is no warm damp air rising into the build-up from a kitchen or a bathroom, so the condensation problem that drives so much modern specification barely arises. There is usually no insulation to get wet, which removes the finding that normally forces a renewal. And there is nobody standing underneath to notice a stain, which removes the alarm that gets a house roof looked at in year one rather than year fifteen. On plots with room for outbuildings, which is most of Dulwich, that can mean several coverings nobody has looked at in a decade. Out along the interwar streets towards Wimbledon Chase one identical garage follows another, each with a covering of the same age and the same history of being ignored.

Nothing up there is ever warm

That absence of heat cuts both ways, and the second half is the half that matters. A roof over a heated room gets a slow assist from the building itself: the structure runs above outdoor temperature for months of the year, so a small amount of moisture that reaches the boards has a chance of leaving again. A garage gives its roof no such help. Water that gets into the boarding in October is still in it in March, working, and that is why a garage roof can look merely tired for a decade and then fail structurally in a single season.

It is the roof people patch themselves

Low, reachable from a ladder, with no ceiling below to ruin: the garage is where a homeowner tries the repair first, and where the previous owner tried one before that. So we routinely lift a garage covering and find bitumen emulsion brushed out of a tin over the splits, flashing tape across a lap, a second sheet nailed over the first. None of it is stupid and some of it bought real time. What it does is hide the surface from anybody trying to read it later, and add weight to a structure that was never asked to carry three coverings.

Failure

What fails on a garage roof, and in what order

Remarkably consistently, and almost always from the outside in. Knowing the order is useful, because the stage a roof has reached decides which of the three options below is honest.

1. The edge, first and always. On most garages the covering is turned over a bare timber edge with no drip trim beneath it. Water reaching that lip runs back along the underside instead of falling clear, and does so on every wet day for thirty years. The fascia softens, then the outermost few inches of deck behind it.
2. The surface. The mineral finish weathers away, the bitumen hardens, and the sheet stops being able to move with the boards underneath it.
3. The field splits. Timber moves with the seasons and a hardened sheet cannot follow it, so cracks open along the board joints in straight lines. That straightness is how a survey tells them apart from impact damage, which is where somebody put a foot or a ladder.
4. The boarding. Once water is through, it stays, for the reason given above. This is the stage at which the cost of the job changes character.
5. The bearing. Last, and worst: the joist ends where they sit on the wall. Rot arrives here latest and matters most, because at that point it has stopped being a covering repair and become structural carpentry, which we carry out and price on its own terms.

The boards

Why the boarding decides more here than almost anywhere

On a house roof the deciding question is usually the insulation: whether water has spread through it and whether it can be saved. A garage roof has no insulation to answer for, so the entire decision drops onto the boards, and unusually for roofing you can establish their condition without instruments. Sound boarding takes weight without movement. Boarding that has been wet gives underfoot, particularly along the outer row and around whatever passes for an outlet.

That single test is worth more on a garage than any amount of surface inspection, and it is also why the same roof can be a modest job or a substantial one with nothing visible from the ground to tell you which. Renewing failed boarding, and the wall plate under it where the wet has travelled that far, is structural work in its own right, priced as such and not tucked into a covering rate.

The fall, or the absence of one

Very few garages were built with a designed slope. The roof fell whichever way the joists were laid, by however much they happened to be out, and forty years of gentle sag has since had its say. Water therefore sits somewhere on most garage roofs, and what standing water actually does over time is the reason it matters rather than a matter of tidiness. A re-board is the one moment in a garage roof's life when the slope can be corrected for the cost of the timber, which is why it is worth raising before the boards are ordered rather than after.

The choice

Repair, re-cover or replace on this particular roof

The general version of this decision, for any flat roof, is an options guide of its own. On a garage it narrows fast, because two of the usual unknowns are absent.

Which system goes on, if the answer is re-covering, is settled at the survey rather than here: the cold-applied liquid route is what we recommend on most of them, with felt reserved for the roofs that argue for it, and the report names yours and gives the reason.

Renewal

What replacing a single-skin garage roof involves

Most garages are one brick thick with no cavity, and that changes what the roof is for. It is not only keeping the space below dry; its perimeter is also the weather protection for the top of the wall, because there is rarely an overhang worth the name and often no gutter that still works. A garage re-roof is therefore a perimeter job with a field in the middle, and the sequence follows that fact.

• Everything comes off, including whatever has been added since, and what is underneath is finally visible.
• The boarding is judged row by row. The outer row usually loses.
• The timber where the roof bears on the brickwork is checked, not assumed, because that is the one piece whose failure turns the job from a covering into a piece of carpentry, quoted on its own line.
• The slope is set in the layer above the joists, which is the cheap moment to fix it.
• Softened perimeter timber and any gutter that has stopped working are renewed at the same time, because the edge and its rainwater goods are what killed the last covering and the access is already paid for.
• A proper trim goes on all four edges with a drip beneath it, so the next thirty years of water leaves the building instead of tracking back under.
• The covering goes down, and where it turns up against the house wall the junction is detailed rather than tucked.

Where there is no old roof at all, because the garage is new or is being rebuilt from the walls up, none of the above applies and the work is a roof designed from the drawings instead.

A boundary

Corrugated cement sheeting: the one we will not touch

Some older detached garages are not felted at all. They are roofed in corrugated cement sheeting, and on a garage of that age the sheeting may contain asbestos. We do not work on it. That is a boundary rather than a preference: cutting, drilling, breaking or lifting that material is precisely what releases fibres from it, and taking it off a building is work for an asbestos contractor operating under the Control of Asbestos Regulations, with the training, the controls and the disposal route those require.

What we do is recognise it and stop. If we arrive to price a job and find cement sheeting rather than a felted deck, you are told on the spot and again in writing, and no part of any quotation from us will involve touching it. Nor will we tell you whether it definitely contains asbestos, because identifying that takes a sample and a laboratory, and a roofer's opinion on it is worth nothing to you. Once the sheeting has gone and the structure has been made good by people qualified to do that, the roof that replaces it is ordinary work and we will quote for it gladly.