Extension Roof Types Explained

Almost every extension roof is decided by a number rather than a preference, and the number is the height available between the paving outside and the underside of the first floor windows above. Work that out at the start and the list of roofs you can actually build is usually two long rather than six. This guide covers the forms, the coverings that suit each of them, what the choice does to the room underneath, and why the flat roof people assume is the budget answer is very often not.

It deals with the roof over a new extension. Altering the roof of the existing house, which is a different job under different rules, sits with the loft conversion pages.

The constraint stack, in the order it bites

Four dimensions settle the roof before anybody has said a word about how it should look.

The first is the sill of the first floor windows. A new roof has to finish below them, with enough room for the abutment detail and a gutter, or you lose a bedroom window and gain an argument with Building Control about ventilation and light.

The second is the eaves height near a boundary. Where any part of a single storey extension comes within two metres of a boundary, permitted development caps the eaves at three metres, and on a narrow Colchester plot that is the rule doing all the work. The height limits and where each is measured from are worth reading before the design is fixed rather than after.

The third is the overall cap, four metres for a single storey extension under permitted development, measured from the ground outside rather than from your new floor.

The fourth is arithmetic, and it is the one that surprises people. A pitched roof rises by its depth multiplied by the tangent of its pitch. A lean-to over a four metre deep extension at 30 degrees, which is roughly what a plain tile wants, rises 2.3 metres above its eaves. Set the eaves at 2.4 metres so the room is usable and the top of that roof is 4.7 metres above the paving, straight through the first floor windows and well over the cap. Drop it to 15 degrees and the rise is barely a metre, which fits, but 15 degrees rules out most tiles. That sum is why so many rear extensions here have flat roofs, and it has nothing to do with budget.

The forms, and what each one is for

Six shapes cover almost everything built on a house in this part of Essex.

A lean-to, or mono-pitch, runs one slope from an abutment against the house down to a gutter at the garden end. It is the simplest roof there is and the cheapest to frame, and its weakness is the one in the sum above: past about three metres of depth the pitch has to flatten until the covering options run out.

A duo-pitch with a gable puts a ridge down the middle and lets both slopes stay steep enough for tile, which is why it appears on wide, shallow extensions and on anything meant to read as part of the original house. It buys pitch at the cost of a gable wall, and that gable is usually the elevation a neighbour sees.

A hipped roof slopes on three sides instead of one. It sheds height at the boundary, which is exactly what the eaves rule wants, and it reads sympathetically on interwar and post-war housing where the main roof is hipped already. It costs more in cutting and in valley or hip detailing than a gable does.

A crown roof is a hipped perimeter with a small flat area in the middle. It is a compromise that solves a real problem: you get a pitched roof from the ground, tiles that match the house, and a top that stops well below the height limit. It has a flat roof hidden in it, so it wants the same care and the same access for maintenance as an honest flat one.

A flat roof is a deck laid to a fall with a jointless covering over it. It is the only form that gives a full height ceiling right up to the rear wall, it takes rooflights easily, and it keeps the whole extension below the sills. It is the default for deep rear extensions and for anything with glass in the roof.

Hide that same roof behind a parapet and you get the crisp square top of a contemporary addition, with no visible eaves and no gutter on the elevation. The price is that the rainwater now has nowhere to go except through the parapet or through the building, so the outlets, the overflow and the coping become the most important details on the job.

Pitch and covering are one decision

A covering has a pitch below which it stops being watertight, and that number chooses the roof as much as the roof chooses it.

Those minimums move with the product, the lap and how exposed the site is, so the specification confirms them rather than a table on a website. What the table does show is the gap between about 10 and 17 degrees, where nothing tiles and nothing quite reads as a flat roof either. Designs land in it more often than you would think, and the honest fix is to go one way or the other rather than argue a tile down below what it is rated for.

Why a flat roof is not the cheap option

It is chosen for height, and the saving people expect mostly is not there.

The intuition is reasonable. A flat roof has no rafters to cut, no ridge, no hips, and its area is the footprint rather than the larger sloping surface a pitch produces. All true, and it is where the saving stops.

The deck has to be firm enough not to deflect between joists, which means a decent board. The fall has to be built in, with firrings cut to a taper or with tapered insulation, and both are labour a pitched roof gets for nothing from its geometry. The covering is a specialist product laid by somebody who does only that, and the perimeter is where the cost concentrates: upstands at every abutment, a drip trim on the open edges, a proper outlet, and a kerb around anything that penetrates it.

Set against that, a tiled pitched roof buys a ventilated void you can run services through, cheap insulation at joist level, and a covering that outlasts any membrane. A flat roof usually needs recovering at least once in the lifetime of a tiled roof laid the same year.

None of which makes a flat roof wrong. It is the right answer on most deep rear extensions here, for the reason at the top of this page. It is just not the right answer because it is cheap, and the quotation well under the others usually turns out to be a cold deck with a two layer felt on it.

The fall, and the number that never appears on a quotation

A flat roof that is actually flat will hold water, and standing water finds every weakness a covering has.

The flat roofing standard works to a finished fall of at least 1 in 80. Because timber deflects under load, firrings are cut to a tolerance and decks are laid by human beings, the design fall is normally set at 1 in 40 so that the finished one arrives. Over a four metre depth, 1 in 40 is a hundred millimetres of drop, and that hundred millimetres has to come from somewhere in the height you already worked out you did not have.

Ponding is the symptom. A puddle still there two days after the rain holds dirt, freezes in January and sits on a lap or a seam for years, and it is the commonest thing we are asked to look at on a roof not yet ten years old. Which way the fall runs matters too. Back towards the house puts the water at the abutment, which is the hardest detail up there. Out to the garden edge puts it in a gutter you can see and clear.

Warm deck, cold deck, and the height it costs

The insulation goes above the deck or between the joists, and the two behave completely differently.

In a warm deck the insulation sits on top of the structure with a vapour control layer under it and the covering above. The timber stays at room temperature, there is nowhere inside the build-up for moisture to condense, and there is nothing to ventilate. In a cold deck it goes between the joists, which leaves the deck itself cold, and the void above the insulation has to be cross ventilated for the life of the building. That ventilation gets blocked by insulation pushed too high, by a later alteration, or by never having been adequate. The failure is rot in a structure nobody can see.

Warm deck is the standard answer and it is what raises the roof. Reaching the flat roof target under the current Part L edition takes real thickness above the deck, and that thickness pushes the finished level up towards the sills and the height cap. A thermal decision quietly becomes a planning one, which is why the U-value targets and what they mean in millimetres belong in the conversation at drawing stage rather than at ordering stage.

Vaulting the ceiling of a pitched extension raises the same question upside down. Insulation at rafter level needs either a ventilated void above it or a build-up designed to work without one, and either way it takes the ceiling down with it.

Where the new roof meets the old wall

The abutment is a hundred millimetres of detail that decides whether the room is dry.

Water running down the existing wall arrives at the new roof. Something has to catch it inside the wall and put it back out above the covering, and that something is a cavity tray, stepped up the wall where the roof is pitched, with stop ends at each finish and weep holes in the joint below it so what it collects can escape. Above the tray sits the flashing, dressed over the upstand or into the tiles.

Where the existing wall is solid rather than cavity, which covers most Victorian and a good deal of Edwardian Colchester, there is nothing to tray. The detail becomes a chased and wedged flashing with the covering turned up behind it, and the height of that upstand is what matters.

Damp appearing in a first floor room a year after handover is nearly always this junction, and nearly always because the tray was left out, stopped short, or had no way of discharging. It is invisible once the scaffold is down and not repairable without taking the roof edge apart.

Rainwater, and where it is allowed to go

The roof collects the water. Something downstream has to accept it.

An extension adds roof area, and that water needs a legitimate destination: a soakaway sized for the ground it sits in, a surface water drain, or the combined system with the water company’s agreement. Essex clay is poor at soakaways, and the trial pit tells you before the concrete does. Gutter and outlet sizing gets skipped more than any other calculation on a small job, and a parapet roof with one outlet and no overflow is a bath waiting for a blocked leaf guard. Where the new roof drains near an existing run, whose drain it is and what consent that needs is worth settling early.

What the roof does to your planning position

Two of the permitted development conditions are about the roof specifically.

Class A caps the height and, separately, the eaves height near a boundary, which is what pushes a deep extension flat. Class A also carries a condition that where the enlarged part has more than a single storey, the roof pitch must so far as practicable be the same as the roof pitch of the original house. Note the word: the pitch is measured against the original, while the materials condition beside it is judged against the house as it stands today. On a two storey extension that pitch condition frequently decides the whole form.

In a conservation area the officer will have a view on covering and form as well as on size, and a flat roof on a visible elevation is a harder conversation than a pitched one. Which conditions apply to which class is worth checking against your own drawing rather than against a summary.

What to ask before you sign

Ask what the finished height is at the abutment and at the outer edge, and what that leaves under the first floor sills. Ask whether a flat roof is warm deck or cold, what the design fall is and which way it runs, what the covering is and who lays it, and how the abutment is being trayed. A roof priced properly has answers to those before the scaffold goes up. We set the roof height first on every drawing, because it is the constraint everything else lives inside, and it is far cheaper to find the problem on paper.

Permitted development limits and the energy standards referred to here are revised from time to time, and how they apply depends on your property, its planning history and the edition in force when your work is notified. Treat this as an explanation of how the choices interact rather than as a specification for your job.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.