Do flat rooflights need to be sloped?
Yes, in almost every case, and the usual number is five degrees. A flat rooflight that sits genuinely level holds water, holds the dirt that water carries, and dries in streaks that are visible from the room below within a season. Five degrees is enough for rain to run off under its own weight and take most of the surface deposit with it.
A handful of units are engineered to sit level and drain across a carefully designed frame, and they are a real option on the right roof. Everything else wants a fall. The wider installation is covered on flat roof rooflight installation.
Five degrees, and where the figure comes from
Five degrees is roughly a fall of one in eleven, which over a metre and a half of glass is about a hundred and thirty millimetres of drop from one side to the other. It is shallow enough that nobody standing in the garden reads the unit as pitched, and steep enough that a raindrop landing in the middle of the pane keeps moving.
Manufacturers set it as a minimum rather than a recommendation, and several make their weathering warranty conditional on it. Below that angle, surface tension starts winning: water beads, stalls, and evaporates where it stopped.
What actually happens on level glass
Three things, in order, and they compound.
Water sits in a shallow film across the pane instead of leaving. Airborne dust, pollen and the fine grit that comes off a tiled roof settle into that film. Then the water evaporates and leaves the solids behind as a tidemark. Repeat that through a spring and the glass carries a permanent bloom that washing removes and the next fortnight replaces.
There is a second consequence at the edges. Standing water at the frame perimeter keeps the gasket and the sealant line permanently wet, and permanently wet materials age faster than materials that get a chance to dry.
Self-cleaning coatings need water to move
Self-cleaning glass works in two stages. A titanium dioxide coating on the outer surface breaks down organic deposit under daylight, and the same coating makes the surface hydrophilic, so rain sheets across the pane rather than beading. That sheet then carries the loosened deposit off the edge.
The second stage is the one that needs a fall. On level glass the sheet forms and then stays where it is, so the coating breaks the dirt down and nothing takes it away. Specifying self-cleaning glass on a level unit buys half a product.
How the slope is built: sloped kerb or sloped unit
Two ways to get there, and the choice affects the ceiling as much as the roof.
The common method is to build the fall into the upstand: the kerb is set higher on one side, or tapered along its length, so the unit lands on an inclined plane while the roof deck stays as it is. The frame is then a standard rectangular unit sitting at an angle.
The alternative is a unit built with the fall inside its own frame, so the outer profile sits level on a level kerb and only the glass is inclined. It looks tidier from below and costs more, and it is the usual answer where the internal ceiling line has to stay square.
Which way to point the fall
Not arbitrary, and worth settling at survey rather than on the day.
The fall should run with the fall of the roof, discharging towards where the roof itself drains, so water leaving the glass joins the route it was always going to take. Pointing it against the roof fall pushes run-off into the high side of the kerb, which is precisely the condition that finds a weak flashing lap.
Where a rooflight sits near a parapet, the fall points away from it. Where two units sit side by side, both fall the same way rather than into the gap between them, since that gap is where debris collects and where the covering has least room to work.
Units designed to sit level
Flush level units do exist, and on a contemporary extension where the roof is meant to disappear entirely they are the only way to get the look. They rely on a frame that drains outward at every edge and a glass surface specified in the knowledge that it will not self-clean.
They ask more of the roof around them. The falls on the deck have to be right, the outlets have to be clear, and the maintenance expectation has to be realistic: the glass wants washing more often, and somebody has to be able to reach it safely to do that. The question of standing water on the roof around a unit is dealt with on whether flat rooflights hold water after rain.
Slope and the internal reveal
A sloped unit on a flat ceiling produces an internal opening where one side of the reveal is deeper than the other. Done carelessly this reads as a mistake. Done deliberately it is one of the better effects available on a flat roof, because the deeper side can be splayed harder and throws light further into the room.
The decision belongs at design, not at plastering. On a deep warm deck build-up the reveal can be three hundred millimetres or more, and the difference between a square reveal and a splayed one is roughly a doubling of the apparent size of the opening.
Snow, debris and the slope in winter
Essex does not get much lying snow, but it gets enough that the point is worth making. Snow slides off a five degree pane very slowly and mostly melts in place, so the fall does little for snow load. What it does do is clear the meltwater as it forms rather than letting it refreeze at the low edge.
Leaves are the more constant issue, particularly on flat roofs overlooked by mature trees, which describes a great many of the older streets in Chelmsford, Maldon and Saffron Walden. A fall moves leaf litter towards one edge, where it collects against the frame, so autumn clearing is the small habit that keeps the drainage path open. Five minutes in October avoids a longer conversation in February.
Getting the angle agreed before anything is ordered
The angle interacts with the kerb height, the glass specification and the reveal, so it is settled at survey along with them. Sunspire builds the upstand and sets the fall as part of the installation, and the workmanship carries a ten-year guarantee.
Where a roof has no usable fall at all, the sensible order of work is to establish the falls first and set glass into a roof that drains. That is a statement about the building rather than about the glass, and it is nearly always cheaper to hear it before the unit is ordered.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements