Flat to pitched
Can a flat garage roof be converted to a pitched roof?
A flat garage roof can be rebuilt as a pitched one, but the new roof raises height, weight and planning questions. Here is what each one turns on.
The short answer
Yes, a flat garage roof can be rebuilt as a pitched one, subject to building control and, often, planning. A new roof structure is checked for strength, and where the weight changes an engineer or surveyor checks the structure before work, as the Planning Portal advises. A pitched roof also stands well proud of the old flat surface, and permitted development for a house roof does not allow an alteration that protrudes more than 0.15 metres beyond the plane of the original roof.
5 min read · Part of Garage roofs
Planning and the height limit
Class C of the permitted development rules covers "any other alteration to the roof of a dwellinghouse". It is not allowed where the alteration would protrude more than 0.15 metres beyond the plane of the original roof slope, or rise above the highest part of the original roof. A pitched roof built up from a flat one stands far more than 0.15 metres proud of the old surface, so a conversion on a garage that counts as part of the house is likely to need a planning application.
Whether your garage counts as part of the dwellinghouse or as a separate outbuilding is a question for the council that covers your address, because outbuildings have their own rules that this site does not set out. Flats have no permitted development at all, and an Article 4 direction may take other rights away.
Listed homes keep Class C, but they need listed building consent for anything that would change that special character, and that consent is a separate application from planning. Ask the council early, in writing, and keep its reply with your papers. Our planning permission guide covers the conditions in one place.
Building control and the new load
A new pitched roof structure is a different job from changing the covering, and building control looks at it. The Planning Portal lists approval triggers for re-roofing, among them a covering 15 per cent or more heavier than the existing one. Approved Document A treats a roof loading raised by more than 15% as a significant change in loading. The walls that will carry a pitched roof, and any opening beneath it, have to be checked for the new load.
Part L of the Building Regulations deals with the conservation of fuel and power, and it is the next question. Where more than 50% of a roof surface is renovated, AD L 2021 expects the whole element to reach Table 4.3's improved 0.16 W/m²K (against a 0.35 threshold) where that is feasible. Whether a garage that stays a garage falls within it is a point for building control to confirm for your address, so ask before the design is fixed.
Building control can be done by the council or by a registered approver, and either can tell you which drawings they want to see before work starts.
How steep the new roof must be
The covering sets the pitch. Published minimum pitches range widely, so the roof is designed around the chosen covering, never the other way round.
| Covering | Minimum pitch |
|---|---|
| Marley Mendip 12.5 concrete tile | 12.5° |
| Marley Plain concrete tile | 35° |
| Marley Ashdowne clay tile | 35° |
| Natural slate, moderate exposure, 500mm slate, 115mm overlap | 20° |
Always check the product's own data sheet. A low garage may not have the height for a steep pitch against the house wall or a neighbour's boundary, which feeds straight back into the planning question above. A shallow-pitch tile gives the lowest ridge, and the survey shows whether the structure can carry it.
Below 25°, Cupa advises against hook fixings on slate. Fixings, like pitch, are chosen to suit the covering, so the covering has to be settled before the carpentry is drawn. At the junction with the house wall the roof needs a flashing too, and the survey and the data sheet settle that detail together. A garage that has to keep its doors clear of the eaves may also limit how far the roof can overhang, which is one more reason to measure before you choose.
Neighbours, order of work and the quote
If the new roof meets a wall shared with a neighbour, the Party Wall etc. Act 1996 may apply. The party wall notice comes from the building owner, with at least two months to run before work to a party wall begins. Only if the neighbour dissents, or has not replied within 14 days, is a surveyor needed, and the neighbour can agree in writing to an earlier start.
Take the steps in this order:
- Ask the council whether the garage counts as part of the house, and whether planning is needed.
- Ask building control about the structure and the energy rules.
- Settle any party wall question before work starts.
- Choose the covering and set the pitch from its data sheet.
A survey comes first, and you get one total price in writing once it is done. If the existing flat roof is sound, replacing the covering may be the better answer, because it avoids a new structure, a height change and an application. See our garage roof planning page, or the garage roofs hub.
More questions
Will a pitched garage roof always need planning permission?
Not in every case, but where the garage forms part of the house, a pitched roof standing more than 0.15 metres proud of the original flat roof is not covered by Class C. Outbuildings and listed or Article 4 homes follow other rules. Ask your council before you design anything, and we will check the route again at survey.
Is a flat roof with a new covering the simpler option?
For many garages, yes. A new membrane on sound boards avoids a new roof structure, a height change and a planning application. Cost and lifespan differ by covering, and our guide to the best material for a garage roof sets the figures out one by one.
Tell us what's wrong with the roof.
You'll get one written total price.