What flush means on a real roof
Flush means the top of the rooflight frame lines up with the face of the surrounding slates or tiles, so from the street the glass looks like a dark pane laid into the roof rather than a unit standing on it.
It rarely means perfectly level to the millimetre. A slate is only a few millimetres thick, a clay plain tile is thicker, and a pantile rises and falls across its width, so "flush" is always measured against something. In practice a well fitted conservation skylight has its frame sitting roughly in the plane of the finished roof, with the flashing tucked beneath the covering instead of wrapped over the top of it. Compare that with a standard roof window, where the frame and its cladding sit on the battens and the unit stands proud of the tiles by a visible margin, casting a small shadow line all the way round.
That shadow line is what a conservation officer notices on a Victorian terrace. It is also why flush fit is now treated as a minimum expectation for a conservation rooflight rather than an optional extra. The wider set of features that make a rooflight a conservation unit, such as the slim steel frame and the glazing bar, all build on this starting point.
How the flush fit is achieved
The frame is set lower into the roof structure than a standard unit, and the flashing is designed to run under the covering so nothing sits on top of the slates or tiles.
The sequence on a pitched roof goes broadly like this:
- The opening is marked out between rafters, and trimmers are fitted above and below so the frame has a solid, square seat.
- The frame is set down to the depth the maker specifies for that covering, which brings its upper face close to the face of the finished roof.
- The flashing, whether a purpose-made kit or dressed lead, is laid so it passes beneath the surrounding slates or tiles, with the covering then cut and relaid tight to the frame.
- The underlay is lapped into the flashing so any water that gets past the covering is carried out onto the roof below the unit.
The detail that matters most is the depth setting. Set the frame too high and it stands proud; set it too low and water can sit at the head. Each manufacturer publishes its own installation depth for slate and for tile, and those figures differ, so the right answer for your roof comes from the drawing for the unit you choose rather than a general rule.
- The roof plane: the line of the tiles or slates. How far a window sits above it is what the eye picks up from the street.
- Standard roof window: frame and sash sit proud of the roof, the usual choice where appearance is not controlled.
- Standard flashing kit: a wide, visible collar around the frame.
- Conservation rooflight: set down so the glass sits flush with the slates, in a slim dark frame.
- Slim central glazing bar, echoing the cast-iron rooflights of Victorian roofs.
- Low-profile flashing tucked under the slates, so almost nothing shows around the frame.
Slate, plain tile and pantile compared
The thinner and flatter the covering, the closer a conservation rooflight can sit to a true flush line.
| Covering | How close to flush | What shapes the fit |
|---|---|---|
| Natural slate | Closest | Thin, flat units; flashing hides easily |
| Clay plain tile | Very close | Thicker tile and double lap |
| Pantile | Close to the tile's low point | Wavy profile needs dressed flashing |
| Concrete interlocking | Close, varies by profile | Tile height and roll shape |
Slate roofs, common on the gault brick terraces of Romsey, Petersfield and around Mill Road, are where a flush conservation rooflight looks most convincing: the frame edge disappears into a flat, dark field. Plain tile roofs on older cottages give a slightly deeper step because the tiles are thicker and overlap twice, but the result still reads as flush from the ground. Pantiles need more care, because the flashing has to follow the curve of each tile beside the frame, and the "flush" line is taken from the tile's lower trough. There is more on those coverings in our guide to conservation rooflights on slate and tile.
Flush fit, pitch and weathering
A flush rooflight relies on the pitch of the roof to shed water, so it needs a slope within the range the maker has tested it for.
Conservation units are generally rated for pitched roofs, with published ranges from roughly 17.5 degrees up to around 65 degrees for some ranges, though each product has its own limits. On a shallow slope, water moves off more slowly and the head of the frame works harder, which is one reason makers set minimum pitches rather than leaving it to the installer. If your roof sits near the bottom of a range, check the figure before choosing the unit; our answer on what roof pitch a conservation rooflight needs goes through this. A flat roof is a different case altogether: there is no plane to sit flush with, and a conservation unit is not the right product there.
What the flush fit means for the light inside
Sitting the frame lower does not reduce the glass area, but it does change the depth of the reveal around it, and that affects how the light spreads into the room.
Because the frame is set into the rafters, the lining between the glass and the ceiling is a little deeper than with a standard unit. A square, deep lining acts like a short tunnel and narrows the beam of light. Splaying the lining, angling the head upwards and the sill downwards, opens the reveal and lets the sun patch spread wider across the floor and walls. On a north slope that gives a softer, more even light across a loft room; on a south slope it spreads the direct sun rather than concentrating it. We work this out as part of the light plan that comes with every quote, alongside position and glass. Frame width and the glazing bar have their own small effect, covered in our answer on whether a conservation rooflight lets in less light.
Questions people also ask
Does fitting a rooflight flush make it more likely to let water in?
Not when it is set to the maker's depth and flashed correctly. The flashing kits for conservation units are designed for a recessed frame, with the covering lapping over them, and the underlay carries any stray water out below the unit.
Can an existing roof window be converted to sit flush?
Not by altering the old unit. Getting a flush line means taking it out and fitting a conservation rooflight set to the correct depth, usually in the same opening or one adjusted slightly to suit the new frame. If the old unit is an original cast-iron rooflight, see our guide to replacing cast iron rooflights.
Is flush fit what conservation officers check first?
It is one of several points, alongside size, position, the glazing bar and how many rooflights are on the slope. Our answer on what conservation officers look for sets out the full list.
Who decides the fitting depth on my roof?
The manufacturer's installation drawing for the chosen unit and covering sets it, and we confirm it at the survey. See our conservation rooflight installation page, or book a light survey to have your slope measured.
