Do flat rooflights need to be sloped?

Two slopes, doing two different jobs

The first is the fall of the roof itself, designed at around 1:40 so that after joist deflection and timber movement the finished fall is still no shallower than 1:80. That gets water off the field of the roof to an outlet. Its minimum figures are worked through on the minimum fall page.

The second is the pitch of the glass, which is a much steeper number, roughly five degrees against the 1.4 degrees that 1:40 represents. It gets water off the glass. A roof can have perfect falls and a rooflight laid dead level on top of it, and that unit will still stain. The two are set out independently, and on a raking kerb the glass pitch is deliberately steeper than the roof under it.

Five degrees, and why the datasheets all land near it

Five degrees is roughly 1:11, or about 90mm of drop across a metre. At that angle a film of rainwater has enough component of gravity along the glass to move rather than sit, and surface tension no longer holds a sheet in place once it thins.

Some manufacturers quote three degrees as an absolute minimum and five as recommended, and a few quote seven for units over a certain size, because a large pane deflects slightly in the middle under its own weight and a shallow rake can end up nearly level at the centre of the span. Where a unit is over about two metres in its shorter direction it is worth asking the manufacturer for their figure in writing rather than assuming the general one.

What happens on glass laid nearly level

Three things, in order. Water sits in a shallow sheet and evaporates in place, leaving behind everything that was dissolved or suspended in it: dust off the road, pollen, and the calcium in Gloucestershire’s hard water. That leaves drying rings, which are visible from inside as pale outlines against the sky.

Then organic growth follows the standing water, because algae need it. Then, in the corners where the frame meets the glass, silt collects into a permanent dark line. None of that harms the sealed unit, but a homeowner who chose a rooflight for the view of the sky is not going to be pleased with it, and it is entirely avoidable at the setting out stage.

How the rake is built into the top edge of the kerb

The kerb is set out from a datum rather than measured up off the deck, because the deck is already falling. Two opposite sides of the timber box are cut to a taper, so the top edge runs from its high point at one side down to its low point at the other, and the two remaining sides are cut on the rake to match. The frame then seats flat on that plane.

Getting this right is a straightedge job, not an eye job. A twist across the top edge means the frame’s gasket compresses unevenly, and on a bonded glass unit that shows up as a whistle in wind long before it shows up as water.

Which way to fall the glass, and where the water is meant to go

The glass is normally fallen in the same direction as the roof, so water leaving the unit joins the flow already heading for the outlet rather than being sent back upslope into the dead area behind the kerb. On a roof with a cross-fall the two directions are worked out together with the tapered insulation set-out.

Where the roof falls towards the house wall, which happens on plenty of older rear additions in Gloucester, falling the glass the same way puts a concentrated run of water into the worst place on the roof. That is a case for correcting the deck first rather than for falling the glass against the roof.

Units sold as flat, and what they actually depend on

There are frameless and low profile products marketed as flat or zero pitch, generally with a bonded outer pane, a drained perimeter and sometimes a hydrophobic coating. They exist and they work in the right context, usually on a walk-on or terrace installation where a raked glass surface would be a trip hazard. That case is set out under walk-on rooflight installation.

What they depend on is a drainage detail around the perimeter that positively removes water rather than relying on it running off, and on the roof around them being genuinely well drained. On an existing roof with tired falls they are the wrong specification, and a raked unit is the honest answer.

Pitch, rain noise, and how fast the glass clears

Horizontal glass takes the full force of a raindrop, and drumming during heavy rain is one of the more common complaints about a first flat rooflight. Pitch helps a little, because the water starts moving instead of striking and sitting. It helps far less than the glass build-up does: a laminated inner pane with an acoustic interlayer makes a much bigger difference than another two degrees of rake, and that is worth specifying over a bedroom or a home office.

Coatings that only earn their keep if the water runs off

Self-cleaning outer coatings work in two stages: a photocatalytic layer breaks down organic dirt in daylight, and the surface is hydrophilic so rain sheets rather than beads and carries the loosened dirt away. The second stage needs the water to move. On a unit at two degrees the coating is doing half its job at best, which is one more reason the rake is not optional. Where a rooflight is high above a stairwell and cleaning is unrealistic, the coating plus a proper pitch is what keeps it clear. Glass options generally are covered under energy efficient rooflights.

Shallow roofs in the vale, and the rake designed into the unit

Severn Vale rear additions and interwar garage roofs across Longlevens and Hucclecote were built shallow to begin with, and a tall raking kerb on a low deck eats headroom in the room below. Where that is tight, the alternative is a unit manufactured with the pitch built into the frame, so the kerb stays level and the glass sits on the rake. It costs a little more at the order stage and it saves the reveal. That decision is taken at survey, before the kerb size is confirmed in writing, because the unit is made to that size and it cannot be trimmed on site.