A flat rooflight is never level. The roof around it is laid to a slight slope, and the glass either follows that slope or sits on a upstand that does its best to ignore it. This guide explains the flat rooflight pitch that a roof needs, how a flat skylight fall for drainage is built up and measured, and what it tells you when water lingers on the glass after the rain has stopped. It also covers the part people rarely ask about: what the same water does to the sound of the room underneath.
A roof that looks flat and is not
Every flat roof carries a built-in slope so that water has somewhere to go. The slope is shallow enough that your eye reads it as level, and steep enough that a bucket of water tipped onto the deck runs to the outlet.
That slope is the reason a flat rooflight is never perfectly level. Whatever sits on the roof, whether a dome, a fixed pane of flat glass or a long lantern, is sitting on a surface that falls away from one edge towards another. The glazing has to work with that fall. If it fights the fall, the water stops at the upstand, settles, and leaves a tide line of dirt. If it works with the fall, the rain crosses the roof, passes the glass and carries on to the gutter.
In a typical Crawley extension, the kind built onto a New Town semi from the 1950s to 1970s, the roof is a single plane drained to one side. The rooflight goes into the middle of that plane. So the question is never whether the roof falls. It is how much, in which direction, and what the rooflight does to the water on its way.
Reading a fall as a ratio
Builders and roofers write a fall as a ratio, such as 1:80, meaning one unit of drop for every eighty units of run. It is a dull piece of arithmetic that gives you a number you can check with a level and a tape.
A fall of 1:80 over a span of four metres drops the surface by 50 millimetres. A fall of 1:40 over the same span drops it by 100 millimetres. As angles, 1:80 is under three quarters of a degree and 1:40 is under one and a half. Neither looks like a slope. Both move water.
The figures that matter are these:
| Fall | Drop over 3 m | What it means |
|---|---|---|
| 1:40 | 75 mm | A common design fall, with room to spare |
| 1:60 | 50 mm | A comfortable working fall |
| 1:80 | about 38 mm | The finished fall to treat as the floor |
| 1:120 | 25 mm | Too flat: expect standing water |
The reasoning behind designing steeper than you need is simple. Timber moves, decking deflects a little under load, and a membrane is laid by hand. A roof drawn at 1:40 can end up at 1:60 or 1:80 once it is built, and still drain. A roof drawn at 1:80 can end up flatter than that in places, and hold water in a shallow dish. Design generously, then measure what was actually built.
Where the slope is built in
A fall is made in one of three ways, and which one you have decides what a rooflight can do with it. You can usually tell from a photograph of the roof edge and a look in the loft.
The first is a sloping structure. The joists themselves are laid to a fall, either by setting one wall plate lower than the other or by notching the joists to a slope. The deck on top is then flat to the joists and the whole roof falls together. This is the cleanest arrangement, because the structure under the rooflight opening already slopes in the right direction.
The second is firring. The joists are level, and tapered timber strips are fixed on top to give the deck its fall. This is common on older extensions and on roofs altered over the years. The fall then sits in a thin layer, and it is easy for that layer to be uneven.
The third is tapered insulation, used on warm roofs. The boards are cut to a slope, so the insulation does the job that firring would do, and the waterproof layer follows it. This suits a flat roof that is being upgraded at the same time as the rooflight goes in, because the fall is designed once, in one layer.
Before we quote, we work out which of these the roof uses. The answer changes how the opening is framed, and it changes how the upstand is shaped. A level kerb on a sloping roof has to be cut taller on one side, and that extra height has to be planned, not improvised on the day. The guide to upstand height, insulation and finish sets out what that height is made of.
What a rooflight does to the water on the deck
A kerb is a small wall across a sloping roof, and water running downhill meets it head on. It divides around the sides, and on the uphill face it has to go somewhere.
On the high side of the rooflight, the membrane turns up the face of the upstand. Water arriving there slows, and has to travel sideways before it can continue downhill. If the roof has a good fall, that sideways path is short and quick. If the fall is marginal, the water takes its time, and the strip of deck at the top of the kerb can hold a thin layer of rain long after the rest of the roof has dried.
The fix is geometry, not product. A small sloping feature built behind the high side of the upstand, often called a cricket or a saddle, splits the flow and pushes it round the corners. It is a wedge of insulation or timber under the membrane, and it turns a stalled strip into two small channels. On wider rooflights it is often the difference between a dry high side and a permanently damp one.
The same applies on the low side. The membrane leaves the downhill upstand and the water has to keep going. If an outlet sits close to the low face, the water gathers quickly and leaves. If the outlet is far away, or sits in a dip, the low side collects the flow from the entire roof plane above it. That is the moment when the glass starts to look dirty at one end.
Outlets, gutters and the route out
A fall is only half a drainage system. The water has to leave the roof, and the rooflight should never sit in the way of that route.
On a roof drained to one side, the water ends in a gutter or a hopper, often behind a fascia. The best position for a rooflight is upstream of a clear stretch of deck, so the water passes the glass and then reaches the gutter without turning a corner. On a roof with an internal outlet, the water travels to a single point, and the rooflight needs to stay well clear of it. A kerb close to an outlet makes the water funnel round the glass in a tight arc, and any debris gets pushed straight into the grating.
We check three things at the survey:
- Which edge the roof falls to, and whether it falls to one edge or two.
- Where the outlet, gutter or downpipe is, and whether it is big enough for the added flow round the kerb.
- Whether the route between the rooflight and the outlet is clear, straight and free of obstacles such as a flue, a vent pipe or an aerial bracket.
A gutter that was sized for a small extension can be slow to clear a storm, and a rooflight changes the picture a little. The glass sheds water faster than a rough membrane does, because it is smooth, and that flow arrives at the gutter in a short burst. Looking at the gutter before the work begins is part of the job. So is leaving the route for leaves and moss clear, which the guide on snow and falling leaves on flat rooflights covers in more detail.
Flat glass, and the pitch of the glass itself
The fall of the roof is one thing. The pitch of the glass is another, and they should not be confused.
A dome or a flat-glass unit sits on its upstand with the glass close to horizontal. Many flat glass rooflights are made with a slight built-in slope on the glass, so that rain runs to one edge instead of resting in the middle. Others sit dead level on the kerb and rely on the roof fall and the upstand shape to do the work. Manufacturers state the minimum pitch they expect for their glass, and a flat skylight must be installed within that figure, because the warranty and the performance of the seals both depend on it.
If the roof is steeper than the glass needs, the kerb can be shaped to carry the pane level, which means the upstand is taller on the downhill side. If the roof is flatter than the data sheet wants, the upstand is shaped to tip the glass, and the kerb is taller on the uphill side. In both cases the kerb is where the difference is made up. This is why the upstand is cut to a particular shape and not simply ordered to a standard height.
A very shallow roof is the hard case. Below a certain point, the roof cannot be made to drain well by the rooflight detail alone, and the answer is to adjust the roof. For the boundary between a flat roof and a shallow pitch, read when a flat rooflight can go on a shallow pitch.
Water sitting on the glass: what it tells you
A little water on a rooflight after rain is normal. A film that clears within hours is surface tension, nothing more. Standing water that stays for a day or more is information.
Start by noticing where it sits. Water in the middle of the pane points to glass that has sagged a little or a pane sitting dead level on a poorly shaped seal. Water along one edge points to the low side of the kerb, where the roof fall is weak. Water round the whole perimeter of the upstand points to the deck itself, and a shallow dish in the roof around the rooflight.
| What you see | Likely cause | What it points to |
|---|---|---|
| Film that clears in hours | Normal surface tension | Nothing to do |
| Puddle on one edge of the glass | Low side of the kerb is holding water | A weak fall at the downhill face |
| Water on the deck at the high side | Flow stalled against the upstand | A saddle or cricket behind the kerb |
| Puddles across the wider roof | Deck has dished or fall is too shallow | A roof question, not a rooflight one |
Standing water is not an instant failure. Glass and a sound upstand will sit in water for a good while. It does, however, age the seals, leave a scum line on the glass and invite moss, and it is a sign that the roof was built flatter than it should have been. We would rather you understood that early. The page on why water sits on a flat rooflight goes through the same signs in a question-and-answer form.
If a unit has already failed, with misting between the panes or water marks inside the frame, the conversation changes from drainage to replacement. That is covered on the misted and failed rooflight replacement page. When we replace a unit, we look at the fall again and correct it where the roof allows, because fitting new glass into an old puddle only repeats the problem.
Dirt, rain lines and keeping the glass clear
A good fall is what keeps glass clean. Rain crossing a pane carries dust and pollen with it, and on a well-drained rooflight the water leaves before the dirt settles. On a poorly drained one, the water dries in place and leaves a ring.
That ring is the drainage map. Look at a rooflight a few days after a dry spell following rain, and the pattern of dried marks shows you exactly where the water sat. A tidy, even film of dust is healthy. A crisp band along one edge shows the low side. A grey patch at each corner shows where the flow gathered against the upstand.
Two habits help. First, clear leaves and debris from the route between the rooflight and the outlet before autumn, because a blocked gutter turns a good fall into a pond. Second, wash the glass with plain water and a soft brush from a safe position. The guide to cleaning a flat rooflight explains the method and what to avoid.
Rain on flat glass: the drainage you can hear
Water that drains well also drums less, and this is where the fall meets the quiet spec. A sheet of rain landing on a large pane produces a steady tapping. A thin film of standing water changes the sound, and a run of water around a hard frame can trickle and tick in a way you notice in a quiet room.
The glazing build matters most. A laminated acoustic inner pane damps the drumming, and the thickness and cavity of the unit set how much of it reaches the room. The upstand and seals matter too, because a hard, empty kerb can ring. We write the choice into the quiet spec that comes with every quote, along with the noise we heard at the survey. You can read how that works in the guide to quieter glazing for a flat roof rooflight over a bedroom, and on the quiet spec page.
The fall plays a part in this, though not a dramatic one. A roof that sheds water promptly gives the glass fewer minutes of heavy running water to transmit, and a clean, well-seated kerb gives the seals less to work against. It is one more reason to settle the drainage before the glass is chosen, not after.
What we measure before we quote
We do not guess the fall. At the survey we lay a long straight edge and a level on the roof, or, where the deck is safely reachable, read the fall with a string line between known points.
The notes we take are simple and specific:
- The direction of the fall and its ratio, taken across the span where the rooflight will sit.
- Any dips, ridges or soft areas within a metre of the opening.
- The position of the outlet, gutter or hopper, and the distance from the rooflight to it.
- The build-up of the roof, so we know whether the fall sits in the structure, the firring or the insulation.
Those readings decide the shape of the kerb, whether a saddle is needed behind it, and whether the rooflight should move a little along the roof. If the fall is too shallow for the detail to work, we say so in writing, before any work starts, and explain what would need to change. The steps on the day are laid out in how a flat roof rooflight is installed.
The work is installed to current Building Regulations, and where it is notifiable we handle the Building Control notification. Drainage and weathering are part of what that covers, together with the insulation and structure around the opening. Everything comes with our 10-year workmanship guarantee. When you are ready, get a quote and we will book the survey.
Questions about falls and standing water
These are the points that come up most when a homeowner looks at the roof from an upstairs window.
Can a flat roof be too flat for a rooflight?
Yes. If the finished fall is flatter than 1:80 in places, water will sit, and a rooflight will make that more obvious. The answer is to correct the roof fall, by tapered insulation or new firring, before or while the rooflight goes in.
Does the upstand height change with the fall?
It can. On a sloping roof the upstand is usually cut taller on the downhill face so the glass sits at the right pitch. Height is also set by insulation and the weathering needed above the membrane, which is why the figure is written on the quote and not assumed.
Is a puddle on the glass a sign of a leak?
Not by itself. Standing water shows a drainage problem. Water inside the room is a different matter, and points to the seals, the membrane dressing or a failed unit. The two are worth telling apart, and the survey does that.
Can I add a rooflight to a roof that already holds water?
You can, if the fall is dealt with as part of the job. Start with the guide to putting a rooflight in an existing flat roof, then return to the parent page on flat roof rooflight installation in Crawley for the full picture.