Roof answer
What fall does a flat rooflight need?
Fall means the slope that lets water run off. With a rooflight there are two to think about: the fall of the roof around it, and any slope the glass needs for itself. Each has a different source.
The answer
The roof deck around a flat rooflight is laid to a fall so water runs to an outlet, and the figure comes from the roof covering maker. The upstand itself is built level so the glass sits true. Some glass units also have a minimum slope of their own, printed in the rooflight maker's instructions, which we check at the survey.
Where does the main fall come from?
From the roof structure and covering. The deck is set to slope toward a gutter or outlet, and the covering system sets the lowest fall it will accept. The rooflight does not change that.
A flat roof is never truly flat. Its joists or deck are set at a slight angle so that rain runs off to a gutter, a downpipe or an outlet instead of sitting. The size of that angle is set by the roof covering, because each system has its own lowest fall under which water may be left standing, and by how the extension was built.
The rooflight is a passenger
Cutting an opening for a rooflight must not undo the fall. The joists round the hole are trimmed and doubled but kept on the same line as the rest of the deck. If one side of the opening ends up lower than its neighbour, the water that was meant to pass it will pool at its edge. On site we trace where the water leaves the roof and which way the deck runs, so the opening and upstand can go in without changing that route.
How can the upstand be level on a sloping roof?
The deck falls but the top of the upstand is held level. The rim therefore stands taller on the low side and shorter on the high side, and the covering is dressed up whichever height results.
Glass should sit true. A frame set on a rim whose top edge slopes will twist and pull its seals unevenly. So the rim is built to a level top, and the difference in the deck is taken up in its height.
Taller on the low side
On a deck that falls one way, the side lower down the slope needs more rim to reach the same level as the high side. Every side must still meet the maker's minimum above the finished roof, which means the shortest side is the one to measure first. Heights and builds are covered in the guide on upstands and kerbs for flat rooflights.
Water behind the high side
An upstand sits across the path of water running down the roof. On the high side, the rim can dam that water, which then needs to be steered round the opening. A small sloped section behind the rim, often called a cricket, sends it to either side. Without one, a leaf or a puddle can collect against the rim. Where the roof is wide enough for this to matter, it is part of the build that gets planned before the covering is laid.
Does the glass need a slope of its own?
Some do. Glass units that are close to flat can hold water on the surface, so makers specify a minimum pitch for the pane. The figure is on the product data sheet and decides which models suit a given roof.
A pane lying exactly level collects rainwater, dirt and then algae. Makers deal with this in different ways. Some build a slight slope into the sash or glass. Others set a minimum on the roof fall the rooflight can be fitted into. Others use a curved or domed top that sheds water by shape.
Reading the data sheet
The sheet for the rooflight gives the lowest slope for which it is designed. If the roof is at or below that, it needs a different model or a built-up slope under the rim. On a deck with plenty of fall, the question rarely comes up. On a very shallow roof, it decides the choice of unit.
Cleaning and staining
Even with a correct slope, glass on a flat roof collects dirt. A little more pitch helps rain wash it off. If the water leaves a tide mark at the lower edge, that is a sign the pane is holding water at the bottom. Finding the cause is worth the effort, because water that sits also feeds the damp described in our answer on wet glass on a flat rooflight.
How is the fall checked before we fit?
At the free survey we read the deck line, find where the water leaves, note the covering and compare it with the rooflight maker's minimum. That tells us whether the opening can go in as planned.
A free survey comes ahead of any price. On a flat roof, the fall is one of the first things we look at, because it affects the model of rooflight, the height of the upstand and whether any build-up is needed behind it.
What the roof tells us
We look at which way the water runs, how many outlets there are and whether any area ponds after rain. We note the type of covering and its age. We then compare the results with the lowest fall the chosen rooflight allows.
What goes in the quote
A small sloped section, or a taller rim on one side, goes into the written quote as a fixed item. The quote then stands unless the job itself alters. Cutting a new opening is notified to Building Control, and we deal with that where the work is notifiable. For the full range of options, see our flat roof rooflights page.
Slope and drainage, briefly
Is a roof with no fall at all suitable for a flat rooflight?
It depends on the covering and the unit. A roof that holds water needs its fall sorted for the covering to last, and many glass units need a minimum slope. The survey shows whether the roof can take the unit as it stands.
Is a ponding patch next to a rooflight a problem?
Water that stays beside the rim is a warning that the fall or the outlet is not doing its job. The rim should stand clear of the pool, but the cause is better dealt with in the build than left to the upstand.
Does a bigger rooflight need more fall?
Not in itself. Bigger glass spans more of the roof, which can interrupt water paths and call for a cricket behind the rim. The slope the glass needs comes from the maker, and the roof fall comes from the covering.