Can You Add a Rooflight to an Existing Flat Roof?

On a new extension, the kerb goes in while the roof is still open, so the joists, insulation and covering are all built around it from the start. On a roof that already has a covering, a ceiling below and years of weather behind it, the opening has to be formed without weakening a structure that is already doing its job.

That difference shows up first at survey. Before a size or position is agreed, we check what the existing roof is actually built from: joist direction and spacing, the depth of insulation, whether the build-up is a warm roof or a cold one, and the age and condition of the covering itself. A flat roof over a Fletton-brick kitchen extension and one of the flat-roofed additions common across Bretton, Orton, Werrington and Hampton can look similar from the garden, yet be built to quite different specifications underneath, so the drawing follows the survey rather than the other way round.

The width and position of the new opening are set by what the existing timbers can carry, not by the size of glass someone would like to have.

• Which way the joists run, and the span between the walls that support them
• Centres and depth, checked against the width the opening would need to be
• Any sag, softness or damp staining in the deck near the proposed opening
• What the roof already carries above the deck: insulation, ballast or a heavier covering

A rooflight sized to fit between two existing joists, without cutting either of them, is the simplest version of this job. Go wider than that spacing and at least one joist has to be cut, with trimmers doubled up on each side to carry the load around the new gap. Our answer on whether a flat rooflight can be fitted between joists sets out where that line usually falls and what changes once a joist has to come out.

Felt, EPDM and GRP all get cut back further than the kerb line itself, not stopped at it, so the new lap lands on decking that is dry and firm rather than on a covering edge that could already be working loose.

Older felt, in particular, often turns out softer near a proposed opening than it looks from the ground, especially where the roof has held water against a poor fall for years. Where that softness turns out to run across the whole roof rather than just at the new opening, a rooflight is still possible, but the wider covering needs attention alongside it rather than being ignored. Replacing domed plastic rooflights with flat glass covers a related case: an old, clouded rooflight opening is often exactly where a covering has been letting water track sideways for years without anyone noticing from inside.

The kerb the rooflight sits on is built onto the trimmed opening to the height the roof's fall and covering demand, commonly at least 150mm above the finished surface, and not simply screwed onto whatever level the existing deck happens to offer.

An older flat roof does not always have the fall it was built with. Movement, added layers of covering over the years and settling timbers can leave a roof close to flat in places, or falling at less than the 1:80 a new roof would be set to. Forming a new opening is a natural point to correct that locally with firring pieces or tapered insulation around the kerb, so water is guided away from every side of the new upstand rather than left to find its own way. Kerbs and upstands for flat rooflights goes through the site-built and factory-made options in more depth, and both fit onto an existing roof in the same way.

The point where a new kerb meets an old covering is where most flat-roof problems start, which is why it gets more attention than any other single joint on the job.

Mixing materials at this junction, torching felt onto rubber, for example, tends to fail faster than matching what is already there, which is why the survey identifies the existing covering before the kerb detail is even drawn. Flat rooflights on EPDM, felt and GRP roofs covers how each covering behaves at a kerb in more detail, and how a flat rooflight is weathered into the roof follows the whole junction from upstand to finished flashing.

Once the covering is cut, the room below has no roof over that section until the kerb is built and dressed, so the opening is protected the moment it is formed rather than left bare between visits.

On a straightforward opening this is a short exposure: cutting, trimming and building the kerb typically fill the first day, with the covering dressed up and over it before anyone leaves the roof unweathered overnight. A forecast that turns wet mid-job changes the plan rather than the outcome, since temporary sheeting over an open deck holds the weather off until conditions allow the covering to go back on properly. How many days does fitting a flat rooflight take? breaks a typical job down day by day, including where an existing roof adds time over a new one.

Cutting a new opening into an existing flat roof, and trimming the joists around it, falls under the structural rules in Part A, and the new rooflight has to meet the same thermal standard as any other replacement or new unit in an existing home. We install to current Building Regulations and handle the Building Control notification wherever the work is notifiable, so the job ends with a completion certificate rather than a gap in the paperwork. What Building Control asks of a flat rooflight goes through each regulation that applies.

Planning is usually a shorter conversation, since a flat rooflight sits low in its kerb rather than rising above the roofline. Some homes still need a check before anything is cut: flats and maisonettes, listed buildings, and properties in a conservation area or covered by an Article 4 direction, which is more likely to apply around Stamford and the stone villages to the west. Will my flat rooflight need a planning application? covers when that check turns something up.

Occasionally the survey finds a covering old enough that adding one rooflight is not really the question; the whole roof is close to the end of its life regardless of where the opening goes.

That does not rule out a rooflight. It changes the order of work, since stripping the roof back to the deck and re-covering it in one pass gives the new kerb a continuous covering to sit in rather than a join tied into an ageing surface, and it means paying for access once rather than twice if a rooflight was already on your mind. Should I add a rooflight when my flat roof is re-covered? sets out how that timing decision is usually made.

Everything above is about the roof; what actually changes the room is the reveal that appears once the ceiling is opened up and lined to meet the new kerb.

A kerb built to the minimum height the fall and covering allow keeps the reveal shallow, which matters most where the eye position that counts is off to one side, a bed against a wall or a sofa set along it rather than centred under the glass. The sightline sketch shows what that reveal will actually frame before the roof is cut, drawn from where you sit or lie in the finished room. The full process, from first visit to finished ceiling, is covered on our flat roof rooflight installation page.

Does the existing roof always need a structural engineer involved?

Not always. Many openings sit comfortably within joist sizes and spans that a standard trimmed opening can carry safely. An engineer becomes part of the job where the opening is wide, sits close to a supporting wall, or where the existing joists are already close to their limit before any opening is cut.

Can a rooflight be added without disturbing the rest of the roof?

Usually, yes. The work is generally confined to the area around the new opening, cut back only as far as needed to reach sound decking and a firm lap for the covering. The rest of the roof is left alone unless the survey finds the covering or the joists need attention beyond that immediate area.

Is adding a rooflight to an existing roof more disruptive than fitting one during a new extension?

It takes a little more care rather than a great deal more time. On a new extension the kerb is built in from the start; on an existing roof, cutting back the covering and matching the new kerb to what is already there adds steps, but the room below is still only exposed for the part of the job when the roof is actually open.

What happens if the survey finds the joists are not suitable for the opening wanted?

The opening is resized or repositioned to suit what the structure can safely carry, or the trimming is designed with an engineer's input where a wider opening is still wanted. Either way, the size agreed on the drawing is one the existing roof can actually take.