Flat garage roofs leak for dull reasons. Water that cannot run off finds the weakest lap or the oldest sealant, and a roof that is meant to fall at 1:80 has often settled to nothing. A pitched roof removes the cause instead of managing it: the slope carries the water away, tiles or sheets shed it, and the problem of standing water goes.
That is why many owners ask about conversion when a third felt roof is due. The answer depends on far more than the covering. A new pitched roof is a structure, with its own timbers, its own loads and a different outline against the sky, so it raises questions about the walls beneath it, about what the council will make of the new shape, and about whether building control needs to see the work.
This guide takes those questions in order: why people convert and when it is not worth it, the planning side, the structure, the choice of pitch and covering, the detailing that keeps a low roof dry in a sea gale, and what it typically costs. It is written for a single or double garage in a house of ordinary age, and every garage is different, so the survey decides the final answer.
Why owners convert, and when it is not worth it
The common reasons come in three. The flat roof keeps leaking and the owner wants an end to it. The garage looks out of place against a pitched house, and a matching roof would improve the street. Or there is a wish for storage space in the roof void that a flat roof cannot give. All three are fair, but they do not carry equal weight.
A slope does solve the drainage fault. It also adds expense and, in most cases, new approval steps. If the flat roof has simply reached the end of its felt and the structure is sound, re-covering may be the smaller job, and re-felting a garage roof is worth pricing alongside a conversion. A flat roof leaking at one junction may only need a repair; our page on a leaking garage roof shows how to find the cause first.
Where conversion makes most sense
Conversion earns its cost when the flat roof has failed more than once, when the garage walls are in good order, and when the new pitch can be set so that it fits comfortably under the eaves of the house or the neighbouring boundary. It also suits garages that will be kept for many years, because a tiled or sheeted pitched roof can last for decades.
Where it does not
If the garage walls are thin, cracked or built from panels that were never meant to carry a roof, the cost of strengthening them can overwhelm any saving. If the plot is tight and a ridge would rise above a boundary, the planning discussion becomes harder. And if the garage is likely to be knocked down within a few years, a cheaper covering is the sensible choice. The survey will tell you that plainly instead of selling the bigger job.
Planning permission and how the new roof will look
Planning law excludes minor works with no effect on a building's outward appearance, which is why replacing a roof with the same again rarely calls for permission. Swapping flat for pitched is a different matter. The outline changes, the height changes, and the garage may now be seen above a wall or fence that used to hide it. That is the point at which to ask the council before a roofer is booked.
In Worthing and Adur you deal with the team based at Worthing Town Hall, which serves both councils, although each council stays its own planning authority. Inside the South Downs National Park, the park authority fills that role in the areas it has recovered. Tell them the garage's position, its height now, the height after the conversion and whether it is attached to the house.
Listed buildings, conservation areas and Article 4
If your house is listed, or the garage stands within its grounds, listed building consent may be needed before the roof shape changes. Conservation areas add their own considerations, and an Article 4 direction can remove permitted development rights that would otherwise apply. These caveats are the reason we do not state a general rule here: the answer depends on the address.
A roof window in the new slope has its own rules, which our guide to skylights and roof windows explains.
Neighbours matter too. A taller garage on a shared boundary can lead to a complaint even where the rules are met, so a word beforehand costs nothing. If the garage wall is shared, see our guide to shared garage roofs before you start.
Building Regulations and the structure underneath
On Building Regulations the Planning Portal is direct: you will need approval if you carry out structural alterations. Building a pitched roof over a flat one is a structural alteration. New rafters or trusses, new wall plates and new restraint to the walls all count, so a building control application is the normal route. Our guide to Building Regulations for garage roofs goes through the wider thresholds and how an attached garage differs from a detached one.
Approval comes through a registered building control approver or through the council's own team. Where an application is needed, we guide you through it and deal with the Building Regulations notification, and building control may want to inspect the roof structure before it is covered.
What the walls must carry
A flat roof sits on the walls and mostly pushes straight down. A pitched roof also pushes outwards if it is not tied properly, and it adds wind uplift on the exposed sides of a coastal garage. The walls therefore need to be sound, of enough thickness and tied to the roof at intervals with straps. The Planning Portal's re-roofing page sends owners to an engineer or a surveyor once the loading changes, and for a conversion that is the route we would take. Single-skin blockwork, old precast panels and walls that have already moved are all reasons to stop and look closely.
Timber, trusses and the old deck
The usual solution is a set of lightweight timber trusses made to measure and fixed to a new wall plate on each side wall. The flat roof deck comes out, because it has served its purpose and its weight is no longer wanted. Where the garage adjoins the house, the new roof becomes a lean-to against the wall, and lead flashing closes the joint at the abutment.
The survey photographs and a written quote follow once the walls have been looked at.
Choosing the pitch and the covering
The pitch, meaning how steeply the roof slopes, decides which coverings are possible. Each tile and slate type has a lowest workable pitch, under which water is not shed reliably, and the maker's data sheet settles the figure. Marley's published figures give a flavour: its interlocking concrete tiles range from 12.5° to 25° depending on the profile, its concrete plain tile needs 35°, and its clay plain tiles need 30° to 40°. For natural slate, BS 5534:2014+A2:2018 indicates a minimum of 20 degrees in moderate exposure with a 500 mm slate and 115 mm overlap, according to Cupa Pizarras.
Why pitch matters at the coast
Marley cautions that a windward slope under 30° can suffer strong upward suction. Most garage roofs sit well below 30°, and a roof on our coast faces gales off the Channel, so the fixing specification is not something to guess. The tile maker's specification, with mechanical fixings, is how a low pitched roof stays on in a gale. Near Shoreham airport, the tile maker's own fixing advice may need special consultation.
Sheets, tiles or something lighter
Metal sheets or composite tiles are light, suit a low pitch and leave the walls less to carry. Concrete tiles are heavier but familiar, and they match many houses; clay costs more again. Many conversions start with an old corrugated roof, which brings its own questions that our page on corrugated garage roofs answers. If those sheets are asbestos cement, read about overcladding asbestos cement sheets first, because the old roof must be dealt with before a new structure goes up.
Pitch also affects looks: a steep roof on a small garage looks top-heavy, while a shallow one can look flat from the road. We sketch the options against the house on the survey.
Eaves, verges, ridge and the wall junction
On a small roof the details carry the weight. The eaves are the bottom edge, next to the gutter, and the verges are the sloping edges at the gable ends. The ridge runs along the top, where the two slopes meet. These are the places where gales get under a roof, and BS 5534 sets the fixing rules for them.
Since the 2014 revision of BS 5534 a mortar bed alone is not trusted to hold the ridge, and each tile is fixed mechanically as well. BS 8612:2018 is the test specification for ridge, hip and verge parts that are dry-fixed, with no mortar at all. At the perimeter of a roof, Marley says tiles get two fixings each, and we work that way on every pitched roof, garage or house. Never accept a ridge held only by a bed of mortar.
Underlay, ventilation and condensation
A pitched garage roof needs an underlay beneath the covering and a way for the roof void to breathe. Marley's reading of BS 5250 gives 10 mm of continuous eaves ventilation where the pitch exceeds 15° and a high-resistance underlay is used, and 25 mm at 15° or less. Skip this and condensation forms under the covering, dripping on cars and stored items. Our page on garage roof condensation goes into the signs.
Joining the house or the neighbour
Where the new roof runs into the house wall, the junction is flashed in lead and tied in with soakers or stepped flashings so water cannot track behind. This is the detail most likely to leak if rushed. Gutters change too: a pitched garage drains to the eaves, so the old flat-roof outlet is replaced with a new gutter and downpipe, sized and fallen to carry the water to a drain.
Wildlife matters too. Bats and nesting birds have legal protection, and old eaves can suit either, so we look before stripping begins.
Costs, sequence and what to ask for
For a pitched garage roof, MyBuilder (updated 21/08/2026) gives a typical UK range of £2,500 to £4,500. The same guide gives £700 to £1,000 for a single flat roof on a garage, and £900 to £1,400 for a double. Those are for replacing roofs of the type named, not for turning a flat roof into a sloped structure, so the real price of a conversion depends on the survey. Treat the figures as a guide to the scale of the job.
What moves the price
Cost is shaped by garage size, the wall work needed, whether trusses or cut rafters are used, the covering, the junctions and access for materials. Expect scaffold on a pitched roof, and ask whether scaffold and VAT sit inside each quote's total. If the old roof is asbestos cement, removal is a separate matter to price, and the typical ranges are in our page on removing an asbestos cement garage roof.
The order of the work
The sequence is stripping the old roof, checking and repairing the wall tops, fixing the wall plates, lifting the trusses, bracing them, laying the underlay and battens, fixing the covering, then dressing the ridge, verges and junctions, fitting gutters and clearing the site. Weather delays are normal. We take the waste away.
The survey is followed by a written quote, one figure for the whole job, plus photographs of your garage, and the quote names the covering and the felt or membrane beneath it. Our garage roof replacement page covers the other routes. To begin, send us photographs of the garage from the street and from inside, with its approximate width and depth, and say whether it joins the house.
