Costs and practicalities for roof windows on a pitched slope and rooflights or lanterns on a flat roof, including the parts of the job that are not the window.
Fitting a standard roof window into an existing pitched slope typically costs £1,100 to £2,200 supplied and fitted, before internal finishing. That covers forming the opening, trimming the rafters, the window, the correct flashing kit for your covering and reinstating the tiles or slates.
The flashing kit is specific to the roof covering, with different profiles for slate, plain tile and deep profile interlocking tile. Using the wrong kit, or worse, mortar in place of a kit, is the single most common cause of a leaking roof window.
Flat roof rooflights and lanterns
A flat glass rooflight on a new or existing flat roof generally runs £1,600 to £3,500 fitted, depending on size and glazing specification. A multi pane lantern on a larger extension is often £2,500 to £5,000 or more.
Almost all of them need an insulated timber upstand kerb, typically 150mm high, built and covered before the unit goes on. That kerb is where flat roof rooflights leak when it is skimped, so it is worth as much attention as the glass.
Plasterboarding and plastering a reveal internally usually adds £250 to £600 per opening. Structural work where a rafter has to be cut and trimmed, or where a large opening needs a beam, is additional and may need a structural engineer's input.
Electric opening, blinds, solar controls and specialist glazing all add to the unit price. It is worth deciding on these before ordering, because retro fitting a motor later is rarely straightforward.
Regulations and neighbours
Rooflights on an existing roof are usually permitted development if they do not project more than 150mm beyond the roof plane and are not above the highest part of the roof, though listed buildings and conservation areas are treated differently.
Building regulations apply to the structural opening, thermal performance and, in a habitable loft, escape provision. A window on a boundary elevation may need obscured glazing. Our planning and building regulations guide covers this in more detail.
Getting it right first time
Position matters more than size. A rooflight placed between existing rafters avoids structural work entirely, and a well placed smaller unit often delivers more usable light than a larger one badly sited.
Where several openings are planned, doing them in one visit shares the access cost and means the roof is only opened up once.
How long does it take to fit a roof window?
A single window into an existing pitched roof is generally a day for the roofing work, with plastering following once the opening is weathertight.
Not if the correct flashing kit for your covering is used and the underlay is dressed in properly. Nearly every leaking rooflight we are called to was fitted without the right kit.
Can a rooflight go into any flat roof?
It needs an insulated kerb and the surrounding structure has to support it, so on an old or soft deck the roof usually needs re covering at the same time.
Do I need permission for a rooflight?
Often not on an ordinary house, but listed buildings, conservation areas and flats have different rules, and building regulations apply to the opening itself. Always check before ordering.
Do you supply the window or do I?
Either. We are happy to fit a unit you have bought, and we will tell you before ordering whether the flashing kit supplied matches your roof covering.
Thinking about more daylight?
Tell us the room and we will advise on position, size and the flashing detail, then price the roofing side in writing.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.