Start with the unit you have
Replacing a rooflight is one of the more satisfying jobs on a house, because the old unit has usually been letting the room down for years. It might be a roof lantern over a rear kitchen, a flat rooflight on a hallway extension, a roof window in a slate slope, or a plastic dome that has gone the colour of weak tea. Anyone searching for how to replace a skylight is really asking the same question: what does the whole job involve, from the first look at the roof to the last sweep of the floor?
Every job opens with a survey, and no price is given until it has been done. Someone comes to the house, looks at the unit from inside and out, and works out what it sits on. A roof window sits in a timber opening trimmed between rafters. Flat rooflights and lanterns rest on a kerb, meaning a small timber box or an insulated upstand that rises from the roof deck. A sun tunnel sits on a short flashed collar over a tube. Each of these comes out and goes back differently, so the survey starts with identifying which one you have.
It also looks for the reason the old unit is going. Some are replaced because the glass has misted between the panes. Some leak at the frame, some are single-glazed and cold, and some are simply the wrong size for a room that has since been redecorated or extended. Is it only the glass that has failed? Then choosing between new glass and a whole new unit is the page to read. If the unit is sound but you want something different, this page is the one to read.
Measuring happens on this visit too. The surveyor checks the opening, the pitch and covering of the roof, the state of the timber under the old frame, and what the ceiling looks like below. Access gets checked as well, because a tall terrace with a rear yard and a bungalow with a garage behind it are very different jobs to get tools onto.
- What the unit sits on: kerb, trimmed opening or collar
- Why it is being replaced
- The roof covering around it
- The state of the timber underneath
- How the roof can be reached
Like for like, or a different size
The simplest replacement puts a new unit into the same opening. It uses the same timber, the same roof cut-out and the same ceiling hole, so the work stays on the roof and the room underneath is barely touched. When the old unit sits in good timber, this is the quickest and tidiest route.
Going bigger changes the job. A wider roof window may mean cutting a rafter and trimming a new opening. A larger flat rooflight needs a bigger hole in the deck, new joists trimmed around it, and a new kerb. A longer lantern is the same story with more weight to carry. At that point the work is structural, so it needs an engineer's detail where the span or load calls for one, and it is dealt with under the Building Regulations. Our page on rooflights and Building Regulations sets out why.
The most common reason to change size is that the old one is no longer made. Roof windows come in a range of fixed sizes, and an old one may be from a range that has been dropped. The choice is then between a nearby size with the opening adjusted, or a different make that fits. A VELUX frame carries a label that VELUX size codes explained shows you how to read, and the guide to replacing an old VELUX window covers that make in more depth.
The opening decides the job. Keep it and the work stays on the roof; change it and the structure comes into play.
Glazing is the other choice at this stage. Old units were often single-glazed or had thin sealed units. A new one is normally double-glazed with a laminated inner pane. Whether a third pane earns its keep is weighed in double or triple glazed rooflights, and the decision stays yours.
Ordering and lead times
What happens next depends on the unit. Standard roof windows in common sizes are stock items, and they can often be ready quickly once the size is agreed. Flat rooflights and roof lanterns are different. Many are made to order, built to the exact dimensions of your kerb, in the colour and glazing you chose, so there is a wait while they are made. A lantern with bespoke bars and a special finish waits longest.
That wait is the reason the survey comes first. Ordering a made-to-measure unit from a sketch is how people end up with a lantern a few millimetres too big for the kerb. We measure the actual opening, agree the exact size with you in writing, and only then place the order. The fixed written quote covers what has been agreed. When the wait is long, we tell you the day the order goes in and when the unit is due.
While the new unit is being made, the old one stays where it is. Nothing is stripped out until the replacement has arrived. In a lived-in house that matters: you never end up with a hole in the roof and a unit still weeks away.
| Unit | Usually | Because |
|---|---|---|
| Roof window, common size | Stock item | Popular sizes are held |
| Flat rooflight | Made to order | Built to the kerb |
| Roof lantern | Made to order | Bars, finish and glazing chosen |
| Sun tunnel | Stock item | Kits come in set diameters |
Taking the old unit out
Removal day starts with covering the room. Dust sheets go down under the opening, and furniture is moved or wrapped. Lifting a rooflight out lets down plenty of dust and grit from the roof, and a covered floor makes the clean-up quick.
On a pitched roof, the work begins at the covering. The slates or tiles around the frame are lifted and set aside so they can be reused, along with the old flashing. The frame is unscrewed from the rafters and lifted out, and then the opening is looked at properly. This is when timber that has been wet for years shows itself, since a leaking frame can rot the trimmer beneath it. Any soft timber is cut out and replaced before anything new goes in.
On a flat roof, the felt, single-ply or rubber is cut back from the kerb and the old unit is lifted off. The kerb itself is inspected. A sound timber kerb in good condition can sometimes be kept, but a rotten or thin one is replaced. The deck underneath is checked for damp, because an old unit that leaked often marks the board below it.
An opening is never left uncovered overnight. If the job cannot be finished in one visit, the opening is boarded and weatherproofed so the house stays dry until the new unit goes in.
A new upstand and flashing
This chapter is where a replacement either lasts or does not. Water finds its way into a house at the joint between the unit and the roof, so the new kerb, upstand and flashing matter more than the glass.
On a flat roof, the new rooflight sits on a kerb that stands up from the deck. The kerb is insulated, and the waterproof membrane is dressed up its sides so water running across the roof meets an unbroken surface, never a seam. How high the kerb stands and how the membrane finishes at the top decide whether the roof drains clear. Anyone wanting the full build-up can find it in kerbs and upstands for flat roof rooflights.
A roof window on a pitched roof arrives with a flashing kit made to suit the covering around it. Slate, profiled tile and plain tile each need a different kit, because water has to be led off over the roof covering and never under it. The kit is fitted from the bottom of the frame upwards, so each piece laps over the one beneath. Our guide to roof windows in slate and tiled roofs explains the differences.
If the roof covering is tired, this is also the moment to say so. Setting a new unit among slates that are about to slip is false economy. The slates around the opening are checked, any that are cracked or held on with rusted nails are replaced, and the surveyor will tell you if the wider roof needs attention.
The new frame is fixed, checked for square and level, and the insulation collar or surround goes in before the roof covering is laid back. A new unit only counts as finished once the flashing is complete and the roof covering is back around it.
Making good inside
With the unit in, the work turns to the room. The opening needs lining, so the plasterboard or timber that forms the splay between the ceiling and the window is built, filled and sealed. A splayed lining, wider at the bottom than the top, throws light further into the room, and it is worth doing well.
If the opening has changed in size, the plaster has to meet the old ceiling cleanly. Small changes can be skimmed over. Larger ones may call for a section of new plasterboard and a wider skim to blend the join. Decorating is left to you, so the lining is handed over ready for paint. Dust sheets stay down until the last visit, and the room is cleared and vacuumed before we go.
Blinds and controls come last. A roof window blind runs in channels fixed to the frame. Electric opening gear needs a cable run to a switch, which is easiest while the lining is still open. If you are changing from a fixed unit to an opening one, plan the cable route at the survey rather than after the lining is up. Our answer on swapping a fixed rooflight for an opening one lists what changes.
Building Control and the paperwork
Controlled work under the Building Regulations 2010 includes any new or replacement rooflight in England. Private inspectors have been called registered building control approvers since April 2024, and the notification may go to either one of them or the local authority's building control. Chasing paperwork is not your job: where the work is notifiable, we do the notifying. In Wales the route is similar, through the local authority or a registered approver there. The council signs off with a completion certificate, and an approver with a final certificate.
What the inspector looks at depends on the job. A like-for-like swap in an existing opening mostly concerns the thermal performance of the new unit. A larger opening brings in the structure as well. Each part is explained on our page about Building Regulations for rooflights.
Planning is a separate question. Rooflights on a house are usually permitted development, but not always, and flats and listed buildings are treated differently. Before work begins, planning permission for a skylight lays the rules out so you know where you stand.
Two things stay with you afterwards: the certificate, where the work is notifiable, and a written guarantee on the workmanship that lasts 10 years. File both with the deeds or the house papers, because anyone buying the house later will ask for them.
Timing is dealt with in how long it takes to fit a skylight, and the cost to replace a skylight covers what moves the price. Our main page on skylight and rooflight installation in Liverpool is the place to begin, or ask for a free quote and we will come and look at your roof.
Before you book a replacement
Will the replacement be the same size as the old rooflight?
It can be. Fitting a new unit into the existing opening is the simplest route and keeps the work on the roof. A different size is possible too, but a larger opening means structural work and the survey will say what is involved.
Do I need to move out while it is done?
No. The work happens at the opening, and the floor below is covered. Rooms next to the opening stay usable, and the unit is made weathertight each day before we leave.
Can a different type of rooflight go in the old opening?
Sometimes. A fixed unit can often become an opening one, and a dome can become a flat glass rooflight on a new kerb. A roof window cannot become a roof lantern, because the frame, the structure and the roof around it are all different. The survey will tell you what the opening allows.
What if the old unit is rotten underneath?
Soft timber is cut out and replaced before the new unit goes in. We find out as soon as the old frame is lifted, tell you what we have found, and carry on once the opening is sound.