Do rooflights leak?
A properly detailed rooflight stays dry for the whole service life of the covering around it, which on a tiled or slated roof is decades. The unit itself is a manufactured, tested, factory-glazed object, and it is very rarely the thing that lets water in. What decides whether an installation is dry in twenty years is the perimeter: the height of the upstand, the direction of the laps, how the underlay is dressed around the frame, and where the covering terminates. Four details, all of them buried by the time anyone sees the finished job. Get those right and water has nowhere to go except off the roof. The full installation sequence is on the roof window installation page, and this is the part of it that keeps the ceiling clean.
The unit is the small half, and the perimeter is the large one
A modern roof window arrives as a sealed frame with a factory-glazed sash, gaskets on every closing face, and drainage channels moulded into the outer frame that collect the small amount of water that always gets past a gasket and carry it back out onto the covering. It is tested to weathertightness standards in a rig before it is ever sold.
Everything that goes wrong on a roof happens in the 300mm around it. That is where the covering has been disturbed, where two systems meet, where the underlay has been cut, and where somebody was working at height in the afternoon. Which is why the useful question is never whether rooflights are reliable. It is whether the perimeter was formed the way the drawings show.
The underlay is the second line, and it is the one that saves the ceiling
Every pitched roof works on the assumption that some water crosses the covering. Wind drives it under a lap, snow gets blown up a slate, a tile lifts in a gale. The underlay beneath is there to catch that water and run it down to the eaves.
At a rooflight, the underlay is cut, and how it is cut is the single most important detail on the job. Done properly, a tongue of membrane is taken up over the head of the frame and stapled to the batten above, the side pieces are dressed down into the frame’s own drainage channel, and the piece below the cill runs out over the covering. Water crossing the tiles then lands on the membrane and is carried around the outside of the frame. Done as a straight slit with the frame pushed through it, that same water arrives at the head trimmer. This detail is invisible for the entire life of the roof and it is the one that decides whether a mark ever appears on a ceiling.
Apron, soakers and head flashing: three parts, one direction
Above the underlay, the flashing kit does the visible work, and it is not one component but three. The apron sits at the cill, over the covering below it, so water running off the glass lands on top of tiles. The side pieces, either individual soakers interleaved course by course or a continuous side flashing, take water arriving from either side of the frame and carry it out onto the covering. The head flashing goes underneath the course above, so water coming down the slope crosses onto metal and is thrown sideways past the frame.
Kits are matched to the covering profile, and the kit for a plain tile is not the kit for a pantile, which is not the kit for a slate. Head lap is a minimum of 150mm and more on a shallow pitch. Nothing relies on adhesion, nothing relies on a bead of sealant, and every joint is an overlap in the direction of flow. That is the whole principle, and it is the same principle the roof was built on.
Lead fixed at the head only, so it can move
Lead is code 4 for most flashings around a rooflight and code 5 where it is exposed or forming a gutter. It expands and contracts a surprising amount across a summer day, several millimetres in a metre run between a frosty dawn and a July afternoon on a south-facing slope.
So every piece is fixed at the head and left free along its length. Lead nailed down both sides has nowhere to go, and it tears itself along the fixing line within a few winters. Pieces are kept short, typically no more than 1.5 metres, and lapped 150mm. Cracked lead along a row of nail holes is one of the most common findings when a thirty-year-old unit comes off a Gloucestershire roof, and it is entirely a fixing decision rather than a material failure.
Cut tiles at the sides, and what holds them down
Forming an opening means cutting tiles or slates down both sides of it. A cut tile has lost its nibs, its nail holes or both, so it no longer hangs on the batten the way a whole one does.
Those cut pieces get mechanically fixed: clipped, nailed through a new hole, or on a slate roof held with a copper tingle. A cut tile bedded in mortar and nothing else will loosen, and when it lifts in a gale the water goes straight onto the underlay in the one place where the underlay has already been cut. On the concrete interlocking tile roofs across Longlevens, Hucclecote and Warden Hill this is the detail worth looking at closely, because interlocking tiles are long and a cut one can be carrying a lot of leverage in a wind.
Flat roofs, where the height of the upstand does the work
On a flat roof the arithmetic is different because water can sit rather than run. The standard upstand is 150mm measured from the finished surface of the waterproofing at the highest point of the fall, not from the deck and not from the lowest corner. That figure exists because standing water, snow and wind-driven rain all climb, and 150mm is where they stop.
Falls matter as much as height. A flat roof is designed to fall at around 1:40 so that after deflection the finished fall is still no shallower than 1:80. Anything flatter ponds, and standing water against a kerb for six months of the year will find any weakness in it. Where an existing deck already ponds against the position somebody wants, the honest answer is that the falls have to be sorted before a unit goes on, and that gets said at survey rather than afterwards. Kerb heights are covered on the flat roof rooflight installation page.
Rain off the south west, and the laps that get increased for it
Gloucestershire’s weather arrives from the south west, and on the exposed sides of the county it arrives with force. The Forest of Dean around Coleford and Cinderford is wetter and windier than the vale, the tops of the wolds catch everything, and a Stroud valley funnels it.
Driving rain against a slope travels sideways across the covering rather than straight down it, and it climbs laps that would be perfectly adequate in still conditions. On an exposed elevation, head laps go beyond the manufacturer’s minimum and side flashings are taken wider than the standard kit dimension. It costs very little at the time and it is the difference between a detail that works in a normal week and one that works in a February gale.
Water on the inside of the glass that never came through the roof
A large share of the water people find under a rooflight arrived from inside the house. Warm air holds moisture, a house generates plenty of it from cooking, showers and drying clothes indoors, and that air rises. The coldest surface in a loft room is the glass overhead, so that is where the moisture lands first.
The two are easy to tell apart once you know what to look at. Condensation appears on cold clear mornings whether or not it has rained, forms as fine droplets across the whole pane or as a band around the edges, and is gone by midday. Water from outside appears during and after rain, often only when the wind is from one direction, and runs in a line rather than sitting as droplets. A damp mark in the plaster at the top corners of the reveal, worst in winter, is a third thing again: a cold bridge where insulation stops short of the frame. Ventilation and reveal geometry answer the first two, and continuous insulation taped to the frame answers the third.
The four details the ten year guarantee is written around
The workmanship guarantee runs for ten years, and it is written against the things workmanship controls: the upstand height measured from the right place, the falls, the flashing laps and the fixing pattern, and where the covering terminates with the underlay dressed properly around the frame. The glass and the frame carry the manufacturer’s own warranty, which is a separate document with its own terms, and the survey notes set out which is which.
None of those four can be inspected once the job is finished, which is exactly why they get skipped by anyone in a hurry. Writing the guarantee against them is a way of saying that they were done, and photographs of the opening at each stage go to the homeowner with the completion paperwork so there is a record of what is under the tiles. On the Severn Vale, where people have watched water come into their houses and are rightly sceptical of anyone cheerful about cutting a hole in a roof, that record is worth having.
Next on this subject.
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- Question Can you fit a rooflight to a tiled roof? Yes. A tiled roof is the most straightforward substrate a roof window goes into, and it is what the manufacturers…
- Question Does fitting a rooflight weaken the roof? No, not when the opening is trimmed properly. Cutting a rafter does remove structure, but the load it was carrying…
- Question How much does a pitched roof rooflight cost? The price of a rooflight in a pitched roof is decided by the roof far more than by the window.…