Side by side or apart: two ways to add the second window
A second roof window either sits tight against the first as a matched pair, or goes in further along the slope as a separate window with its own flashing.
A close pair is the neater result from inside and out. The two frames sit at the same height with a narrow strip of roof between them, usually a single rafter, and read as one wide band of glass. The outside is dressed with a combination flashing, a kit made for two or more windows that runs one continuous apron along the bottom and one gutter along the top, with a narrow connecting piece where the frames meet. VELUX sets the gap between frames for these kits, and the standard spacing is around 100mm, so the new frame has to land at a precise distance from the old one.
Spacing the second window further away is simpler on the roof. Each window gets its own single flashing and its own trimmed opening, and nothing about the existing window has to change. The trade off is inside the room: two separate pools of light rather than one broad one, and two separate lining boxes to plaster. On a long loft room in a 1930s semi that can be exactly right, one window over the bed and one over a desk. On a narrow terrace loft the gap between them can look like an afterthought.
What the rafters allow
The easiest second window is one that fits between the next pair of rafters without cutting anything, and that comes down to width and rafter spacing.
VELUX widths are coded by letter: C is 550mm, F is 660mm, M is 780mm, P is 940mm and S is 1140mm. Our guide to VELUX window sizes and codes shows how to read the code on the data plate of the window you already have. Rafters in older Cambridge houses are commonly at around 400mm centres, sometimes wider in Victorian roofs, and newer homes built with trussed rafters are usually at 600mm centres. A window wider than the gap between rafters means cutting one, doubling the rafters either side and fitting trimmers top and bottom.
A combination pair is set out so the shared rafter runs through the strip between the frames, which keeps structural work down. Where the first window was originally set off centre in its bay, the second one may not line up without trimming, and that is something we check in the loft before any size is agreed. Trussed rafter roofs on estates such as Great Kneighton or Cambourne need more care: a truss should not be cut without the designer's or an engineer's input, so the second window may have to be a width that fits the existing bay.
| Layout | Flashing | Existing window |
|---|---|---|
| Close pair, same height | Combination kit | Usually re-flashed |
| Separate, same slope | Single kit each | Left alone |
| One above the other | Combination kit | Usually re-flashed |
Matching the existing window
Two windows side by side only look right if they match in size, finish and generation, so the age of the first window matters.
If the existing window is a current model, the new one can be ordered to match it exactly: same size code, same pine or white polyurethane finish, same opening type. If it is older, the frames will differ in depth, profile and glazing, and a pre-2013 window beside a new one is easy to spot from the room. Identifying your VELUX window generation takes a few minutes with the data plate. Where the old window is near the end of its life anyway, replacing it at the same visit gives a true pair, one combination flashing and matching blinds, and saves opening the same area of roof twice. Replacing an old VELUX window covers what changes when a unit is renewed.
Flashing, insulation and the inside lining
The work on the roof is about keeping one weather line across both windows; the work inside is about making the pair read as one opening.
The combination flashing has to suit the roof covering, just as a single flashing does: profiled concrete tiles, flat plain tiles and natural slate each take a different kit. VELUX flashing types explained sets out which is which. Each frame gets its own insulation collar and vapour barrier so the strip between the windows does not become a cold bridge; our page on insulation collars and installation kits explains why the gap between two frames is a common place for condensation if this is skipped.
Inside, we line the two openings with a shared head and sill where the rafter between them allows, so the plastered reveal runs across both. The head is kept level and the sill plumb, which spreads the light down into the room instead of onto the ceiling.
Building Control, planning and the light
A new roof window is normally notifiable under the Building Regulations, and the planning position depends on the house and where it is.
Cutting or trimming rafters brings in Part A (structure), and the new window has to meet Part L for thermal performance. On terraced houses, fire guidance under Part B generally keeps rooflights out of a strip roughly 1500mm either side of the party wall, which can limit where a second window goes on a narrow Romsey or Mill Road roof. Where the work is notifiable, we handle the Building Control notification. For planning, a rooflight on a house usually falls under permitted development if it projects no more than 150mm and stays below the ridge, but conservation areas, listed buildings and Article 4 directions change that. Read do you need planning permission for VELUX windows before you commit to a front slope.
Doubling the glass on one slope also doubles what that slope brings into the room. On a north slope a second skylight is almost pure gain: more of the same steady light. On a south slope it doubles the summer heat and can put a patch of sunlight across a bed or desk for several hours, so solar control glass or external blinds from the VELUX blinds guide earn their place. Our page on how we plan the light shows how orientation shapes this, and every written quote carries a light plan with the position, size and glass of the new window and the reason for each.
Questions people also ask
Can a second VELUX be fitted without scaffolding?
A single new window in a straightforward roof can sometimes be worked from inside, but a combination flashing means stripping and relaying tiles across both windows, so safe roof access by tower or scaffold is usually needed.
Does the new window have to be the same size as the old one?
For a close pair on one combination flashing, the heights should match so the top and bottom lines run through. Widths can differ, although matching sizes look calmer from inside the room.
Is it better to fit one bigger window instead of two?
Not always. A wide single window often means cutting several rafters, while two windows in neighbouring bays keep more of the roof intact. Can you fit a bigger VELUX window compares the structural side.
How do I find out what my roof will take?
A survey checks the rafters, the covering, the existing window and the direction of the slope. See VELUX window installation in Cambridge, or book a light survey.
