Reliability answer
Do skylights leak in the end?
The fear is that every skylight is a hole in the roof waiting to drip. Where water actually gets in, what is often mistaken for a leak, and what a correct fitting does to keep it out.
The answer
No. A skylight fitted correctly for its roof has no reason to leak, and age alone does not make one let water in. Where water does show up, the weathering is the usual culprit: the wrong flashing for the covering, or a low upstand on a flat roof with too little fall. Condensation is the usual false alarm.
Where does water actually get in?
At the joint between the frame and the roof, not through the glass. The flashing on a pitched roof and the upstand and membrane on a flat roof are the places that decide whether a rooflight stays dry.
Glass is not what lets rain in. The glass is rarely the problem. What counts is the junction at the frame, and the fitter builds that junction on site. A good frame set badly will let in water; a plain frame weathered correctly will not.
Pitched roofs
Rain running down a pitched roof arrives at the frame of a roof window. A matched flashing kit steers it round the frame and back on to the slates or tiles. Slate and plain tile need a different kit from concrete tile, and fitting the wrong one is the classic way for a window to be wet within a season.
Flat roofs
Over a flat roof, the unit rests on a timber upstand wrapped in the roof membrane, which finishes beneath the frame. If the upstand is too low, or the membrane stops short, water that lies on the roof can find the gap.
Fixings and seals
Screws and sealants are the last line, never the first. A sealant bead on its own cannot do the job a flashing does, which is why the order of work matters more than the tube of mastic.
Is it a leak, or is it condensation?
Droplets on the room side of the glass on a cold morning are condensation. A damp mark around the frame after rain is water getting past the weathering. Misting between two panes is a failed sealed unit.
Homeowners often report a leak where the water in question has come from the room. Telling the three apart is simple once you know what each looks like.
Condensation
Warm, damp air from cooking, washing and breathing meets a cold surface and turns to water. A rooflight is a cool surface in a warm room, so it shows this first. The drops form on the inside of the glass, clear in daytime and worst on cold mornings. It points to ventilation and glazing quality, not to a hole.
A sealed unit that has failed
Double glazing is two panes sealed together. Once that seal lets go, damp air enters the gap and leaves a cloudy or streaky film that no amount of wiping will shift. The answer is a replacement unit, not a flashing job. Our misted rooflights page covers swapping the glass or the whole unit.
Water past the weathering
A stain on the ceiling or reveal that appears after rain, and gets worse with wind, points at the joint with the roof. That is the only one of the three that needs the roof opening up.
| What you see | Usually means |
|---|---|
| Drops on the inside of the glass, cold mornings | Condensation from room air |
| Cloud or streaks between the two panes | A failed sealed unit |
| A stain around the frame after heavy rain | Water getting past the weathering |
| Damp at the lowest edge only | Water collecting at the foot of the frame |
| Damp on the ceiling away from the frame | Something other than the rooflight |
Why is a flat roof the harder case?
Water on a flat roof only leaves if the roof has enough fall. A rooflight on a low or badly set upstand sits where that water gathers, so height and fall have to be right.
On a slope, gravity does the work. On a flat roof, the roof has to be built so that it does.
The fall
A flat roof is never completely flat. It is laid with a gentle fall towards the gutter, so rain runs off. If the fall is weak, a puddle can sit beside the rooflight, and a puddle is water with time to find any gap. Our survey looks at the existing fall before a rooflight is specified.
The upstand
Manufacturers set minimum upstand heights, commonly around 150mm above the finished roof. An upstand set higher than a puddle can reach keeps the junction clear of standing water, and the membrane dressed up it forms a continuous skin. Where a house is covered by Class C, the projection of the upstand counts towards the 150mm limit, so height and permission have to be planned together.
The joint at the edge
The membrane has to run under the edge of the unit's frame, with the frame lapping over it, so that water runs from one to the other in the direction it falls. The build-up is shown on the upstands and kerbs page.
What does a dry installation involve?
Weathering chosen for the roof covering, laid in the right order over a properly trimmed structure. The workmanship then sits behind a ten-year written guarantee.
A dry rooflight is the result of a sequence, and the sequence is the same whatever the product.
Match the weathering to the roof
The first decision is the covering. Slate, plain tile, concrete tile and single-ply or felt flat roofs all take different details, so the survey records what is on the roof before a kit is chosen.
Build from the bottom up
Water-shedding layers overlap so that water always runs over the top of a lower layer. The lowest flashing goes in first, and the upper ones lap over it. Done in the opposite order, a flashing sheds water into the roof instead of off it.
Back it with a guarantee
A written guarantee of ten years covers the workmanship, and the guarantee page gives the terms. For an older rooflight that has failed, we replace it with a new unit and fresh weathering.
The homepage shows every product we install.
Our view: a skylight stays dry because of how its edges are built, not because of the glass.
Leaks and damp: other worries
Will a rooflight leak when the wind drives rain sideways?
A correctly lapped flashing handles wind-driven rain as it handles ordinary rain, because the water is shed over overlapping layers. Poorly set flashings are the ones that show up in a storm.
Do plastic domes leak more than glass units?
The material of the glazing does not decide it. The junction with the roof does. An old dome can fail through ageing of the plastic or its seal, and our page on changing a plastic dome to glass covers that swap.
Does a skylight make a loft damp?
A dry, well-weathered rooflight does not. Damp in a loft room is more often a ventilation issue than water entering, which is why opening units and good airflow matter in bedrooms and bathrooms.