How Do You Fit a Flat Rooflight Without It Leaking?
Fit a flat rooflight so it cannot leak by working in order: set the roof falls so water runs away from all four faces, build the upstand at least 150mm above the finished roof at its lowest point, dress the covering up the face and over the top edge, then mechanically fix the unit onto that dressed covering. Sealant seals nothing structural. It is the fifth line, never the first.
That ordering is the whole discipline. Every stage above the one below it can fail without water getting in, because the stage beneath still holds. A leak needs the whole stack to fail at once, and on a correctly built installation it never does. Where we are called to look at somebody else’s work, the stack has usually been collapsed down to a single layer: a unit dropped on a timber box and a bead of mastic run round it.
Layer one: the fall, before anything else exists
Water that moves does not leak. Water that stands finds a way in eventually, because standing water is a continuous hydrostatic load on a bond line, and it is hot in July and frozen in January.
So the first thing that makes a rooflight watertight happens before the opening is cut. The roof around the intended position has to fall away from where the upstand will sit, on all four sides, to an outlet. Not fall correctly on average across the roof. On all four faces individually. A roof can drain perfectly overall and still have a dead flat pocket in the exact spot the architect drew the rooflight, and firrings cut to a single direction will happily create one on the upslope side of a new upstand.
Where we survey a retrofit, we laser the fall on each face before quoting. If the existing roof already ponds, that gets corrected with firrings or tapered insulation as part of the job, or we say plainly that it will not be corrected and what the consequence is. The numbers themselves live on our minimum fall for a flat rooflight page, and the reasons roofs end up flat in the first place are covered under why flat rooflights pond water.
Layer two: upstand height, measured at the right end
The upstand lifts the glass out of the water plane and gives the covering something vertical to climb. We build to a minimum of 150mm above the finished roof surface, and the measurement is taken at the lowest point of the fall, not the highest and not the average.
This is where a lot of otherwise decent work quietly goes wrong. Say the roof falls 45mm across the width of a 1.5m opening. Someone measures 150mm at the high side, builds a level box, and the low side is 105mm. The low corner is precisely where water arrives, where it lingers longest, and where wind pushes it up the face. That corner is the one that stains a ceiling in year three.
Manufacturers will often state 100mm as an acceptable minimum, and inland on a well-drained roof at Stowmarket or Needham Market it genuinely is. On the open-sea belt at Felixstowe, Harwich, Frinton or Southwold we build to 200mm as standard and do not offer the alternative. Driven rain on that coast does not fall, it travels, and it climbs a vertical face further than most people believe.
A quick check on an existing installation. From a window or a ladder at the eaves, sight along the roof surface to the base of the rooflight frame. If the visible timber or aluminium at the low corner looks noticeably shorter than at the high corner, the upstand was built level onto a falling roof. Do not go onto a wet or frosty flat roof to check this. Photograph it from where you are safe.
Layer three: how the covering is dressed
The covering has to come up the full height of the upstand and turn over the top edge, so the rooflight frame sits on top of the membrane rather than beside it. Get that and water has to travel uphill twice and past a mechanical fixing line to reach the inside of the building.
The detail differs by material and the differences matter. EPDM gets a wall-grade adhesive on the vertical face, not the deck adhesive, with preformed moulded corners bonded and rolled in. GRP is laminated continuously over an angle fillet at the internal corner, never into a sharp 90 degree junction, with extra mat at the corners rather than less. Single-ply is hot-air welded with proprietary corners and terminated under a manufacturer’s bar, and PVC and TPO are not interchangeable. Felt is dressed in stepped layers so the laps do not stack, over a reinforcing fillet, with the cap sheet turned over the top. Our rooflight flashing and membrane page carries the full detail for each system.
The single most common defect we find on other people’s flat rooflights is a covering stopped 20mm short of the top edge with the gap filled by mastic. It looks finished. It is watertight on handover day. It has about five years in it.
Layer four: mechanical fixing
The unit is bedded onto the dressed covering and then held down mechanically. Bedding compound is a bedding compound. It fills the tolerance between two manufactured surfaces and it compresses evenly under fixing load. It is not what stops the unit lifting in a gale on the Orwell estuary at Nacton or Pin Mill.
Fixing grade is a specification decision, not a van-stock decision. A2 grade 304 stainless is fine across inland Suffolk. Within a mile of open sea it is not, because chloride concentrates at fixing heads through repeated wetting and drying and pits the surface. There we use 316L throughout, where the molybdenum content is the thing doing the work, along with isolation washers between aluminium and steel so the assembly does not set up a galvanic cell. A rust stain running down a two-year-old frame at Felixstowe is a specification error, not weather.
Layer five, and only then: the gasket and the sealant
Modern flush glass units are bonded, with the outer pane oversailing the frame, so the water path is designed rather than sealed. Gaskets compress against the frame to exclude wind-driven rain that gets past the geometry. Sealant fills the last small gaps at the perimeter.
Both are consumables. Gaskets harden and shrink, most noticeably at corners. Sealant loses elasticity, typically at ten to fifteen years on a good product and much sooner on a cheap one facing straight up at the sky. That is fine and expected, because on a correctly built installation neither is holding water out on its own. When either fails on a badly built one, water arrives inside the same winter.
What this means on a Suffolk or north Essex roof
Two local patterns show up repeatedly. The flat-roofed rear additions behind the Victorian terraces in central Ipswich often have joists that are old, irregular and slightly sagged, so the finished roof plane is not what the drawing says. Falls have to be set from a laser reading, not from the deck. And the 1960s to 1980s garage and utility extensions at Kesgrave, Rushmere St Andrew, Chantry and Whitton generally carry a dome on a bare timber box with no insulation and no dressed top edge, built to a standard that was normal then. Replacing the unit alone on one of those solves nothing.
Note that this page is about preventing a leak at installation. If you already have water coming in, the diagnosis is a different job and it is covered on flat rooflight kerb leak, and storm-specific symptoms on rooflight leaking after a storm. If you are checking whether an existing kerb was built correctly, start with how to build a rooflight timber kerb. The wider installation picture sits on our flat roof skylight installation page.
Every installation we carry out is hose tested before handover and carries a 10-year workmanship guarantee. Request a fixed quote and we will laser the falls, check the upstand height at the low corner and tell you what your roof actually needs before anyone quotes a price.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.