What the job actually involves
Replacing a skylight is a roofing job and a glazing job at once: the old unit comes out, the opening is checked, the new rooflight goes in, and the roof covering is weathered back around it.
On a pitched roof with a roof window, the sequence is usually: remove the sash, unscrew the frame from the rafters or trimmers, lift away the old flashing, loosen the surrounding tiles or slates, fit the new frame at the right depth, then fit the flashing kit from the bottom piece upward so each part laps over the one below. Inside, the lining and plaster around the reveal are made good, and the gap between the frame and the roof structure is insulated so there is no cold ring for condensation to form on.
On a flat roof the unit sits on a kerb, or upstand, and the roof membrane has to be turned up and sealed to it. That is often the harder half: felt, single-ply and GRP each have their own detailing, and the upstand should finish at least 150mm above the roof surface. A lantern adds more weight and more joints again. The guide to types of rooflight sets out how each one is built, and the rooflight replacement process walks through the stages as a fitting team runs them.
Working at height and handling the weight
The two physical risks are the height you work at and the weight you are holding while you are up there.
A roof window can sometimes be swapped largely from inside, with the sash removed and the frame worked from the loft, but the flashing and tiles still have to be dealt with from outside. That means a roof ladder hooked over the ridge at the very least, and on a two-storey Cambridge terrace or a steep slate roof, a scaffold or tower with edge protection is the sensible way to do it. Flat roofs feel safer underfoot, yet a single-storey extension roof still has an unguarded edge and, often, the old rooflight opening itself to fall through.
Glazed units are heavier than they look from the ground. A double or triple glazed pane with a laminated inner layer, in an aluminium or timber frame, is a two-person lift on a flat roof and an awkward one on a slope. Check the weight on the manufacturer's data sheet before you plan the lift, and plan where the old unit will go, because it has to come down too.
Flashing, fit and the light you get back
A replacement is only as good as its weathering: the flashing kit must match the roof covering and the fitting depth, or the new unit will let water in where the old one did not.
Roof window makers sell flashings for different tile and slate profiles, and a profiled tile flashing will not sit properly on flat slate. With older VELUX windows, those made before about April 2001, the makers offer replacement flashings designed to keep the original fitting depth, so the inside lining can stay as it is. Get the make, model and size off the existing frame first; how do I know what skylight I have shows where to find the plate.
This is also the moment to ask what the new glass should do. A like-for-like swap copies whatever the last installer chose, including a clear pane on a south slope that overheats a loft bedroom in July. Solar control glass, a better blind or a slightly different size can change how the room feels far more than the new frame will. The guide to which way a skylight should face covers what each slope gives a room, and skylight replacement options sets out the choices beyond straight like for like.
Building Control, planning and paperwork
Replacing a whole skylight or rooflight is building work under the Building Regulations, so it is normally notifiable, and a homeowner doing it themselves must tell Building Control before the work starts.
Installers registered with a government-authorised competent person scheme can sign off their own replacement windows and rooflights. A homeowner cannot, so the route is a building notice or an application to the local authority Building Control service, with a fee, and an inspection of the finished work. Your district council's Building Control team can confirm which service covers your address. Changing only the glass in an existing frame is generally treated differently from replacing the unit, so ask Building Control which applies to your job.
The new rooflight has to meet current standards, not the ones the old unit was made to. For thermal performance, Part L sets a limiting U-value of 2.2 W/m²K for rooflights, and Part K expects safety glazing overhead, which in practice means a laminated inner pane. Building Regulations for rooflights covers all the parts that apply.
Planning permission is not usually needed for a like-for-like replacement, but it can be if you enlarge the opening, raise the unit above the roof plane, live in a flat, or live in a listed building or under an Article 4 direction. The planning permission guide has the detail. Skipping the notification tends to surface when you sell: a buyer's solicitor asks for the completion certificate, and the route then is regularisation after the event.
Weighing DIY against having it fitted
If you are confident on a roof, have a second pair of hands and are replacing a standard roof window in a matching opening, doing it yourself is a reasonable project.
The balance tips the other way when the job grows: an upsized opening that means cutting or trimming rafters, a flat rooflight where the membrane needs re-forming around a new kerb, a heavy lantern, an electric opener that needs wiring, or a roof you cannot safely reach without scaffold. It also shifts if you want someone else to carry the risk of the weathering, since a professional installation comes with a workmanship guarantee and a DIY one relies on your own work alone.
Vision Skylights replaces skylights and rooflights across Cambridge and about 30 miles around. We look at the room first, write down the position, size, glass and shading in a light plan with the quote, install to current Building Regulations, handle the Building Control notification where the work is notifiable, and back the installation with a 10-year workmanship guarantee. Choosing a rooflight installer lists what to ask any firm, and you can see how we work on the skylight and rooflight installation homepage. If you would rather hand it over, book a light survey or message us on WhatsApp.
Questions people also ask
Can I replace just the glass in a rooflight myself?
Some roof windows and rooflights take a replacement sealed unit or sash from the maker, which is a lighter job than changing the whole frame. Frameless flat rooflights bonded to their kerb usually need the whole unit renewed. Check with the maker and with Building Control before you order.
Do I need scaffolding to replace a skylight?
Not always. A roof window can sometimes be changed from inside with a roof ladder for the flashing. For higher or steeper roofs, heavy units or flat roofs with open edges, a scaffold or tower with edge protection is the safe choice.
Will a DIY replacement affect my home insurance?
It can matter if water gets in later and the insurer asks how the rooflight was fitted. Keeping the Building Control completion certificate helps. See do skylights affect home insurance for what insurers usually ask.
How long does a skylight replacement take?
A like-for-like roof window swap can be done in a day by an experienced team; flat rooflights, lanterns and upsized openings take longer. How long does skylight replacement take gives the timescales by type.
