Rooflights on a shallow pitch below 15 degrees

What happens to water on a ten degree slope

Water on a 45 degree roof leaves fast and leaves in a straight line. Water on a ten degree roof does neither.

Gravity’s component along the slope drops with the sine of the pitch, so at ten degrees the force pulling water down the roof is about a quarter of what it is at 45. The film of water on the surface is thicker and slower. Wind blowing across the roof can push that film sideways or briefly uphill. Surface tension holds it against edges instead of letting it drip. Every one of those effects works against a joint that assumes water travels downhill and only downhill, which is precisely what a standard flashing set assumes.

Capillary action, and the reason laps grow as pitch falls

Two surfaces close together with water between them draw the water into the gap regardless of gravity. That is capillary action, and it is the mechanism that puts water under a tile lap on a shallow roof.

The published headlap for a covering rises as pitch falls for exactly this reason. A concrete interlocking tile might want 75mm at 30 degrees and 100mm at 22.5, and drop below its stated minimum and the manufacturer simply stops warranting it. The same principle governs the metalwork around an opening: a head flashing tucked 150mm under the course above at 35 degrees needs 200mm or more at 15, and any hand-formed piece gets a welt or an anti-capillary bend to break the path rather than relying on length alone. The individual components and their laps are set out on soakers, aprons and head flashings explained.

Wind uplift and driving rain arriving nearly flat

On a shallow slope, rain driven by a south-westerly arrives at a low angle relative to the roof surface, which means it hits laps and upstands close to head on rather than glancing off them.

Wind also generates more suction over a shallow roof than a steep one. Air passing over the ridge separates and creates negative pressure across the upper part of the slope, which lifts the tail of every tile it passes and lets air, and the water in it, get under the covering. That is why mechanical fixing requirements increase on shallow roofs, and why an opening formed in one has to have every cut tile at its sides clipped or nailed rather than left to sit on its own weight.

Low pitch kits, and the two things they change

Some manufacturers publish a low pitch flashing kit that takes a standard roof window down to around ten degrees on a suitable covering. It is a real product and it is worth knowing what it does.

It changes two things. The apron is deeper, so water leaving the bottom of the frame lands further down the slope on covering that is itself properly lapped. And the head detail is extended, so the tuck under the course above is longer and better welted. What it does not change is the sash drainage or the covering’s own rating, so a low pitch kit on a covering already below its minimum solves nothing. It also cannot be improvised: it is a different part number, ordered at the outset, and a standard kit cannot be stretched into it.

Kerb-mounted units, and the upstand measured on the vertical

Below the low pitch kit’s range, the answer is a kerb-mounted rooflight: a made-to-size unit set on a timber upstand built off the deck, with the covering dressed up and over the kerb.

The upstand is 150mm measured vertically from the finished surface of the covering at the highest point, which on a sloping roof means the kerb is not the same height all the way round. The downslope face has to be taller than the upslope face for the top of the kerb to sit level, or the kerb is built level in itself and packed at the low side. Either way, the 150mm is checked at the point where the covering surface is highest against it, because that is where water will be deepest in a storm.

The glass on that unit is then set at a minimum of about five degrees of its own, independent of the roof pitch, so that rain sheds and dirt washes off. On a ten degree roof, laying the unit in the plane of the roof already achieves that. On a three degree roof it does not, and the rake gets built into the top of the kerb. The kerb detail generally is on the flat roof rooflight installation page.

The high side of a kerb on a sloping roof

Any kerb standing above a slope creates an obstruction, and water arriving from above has to divide and pass around it.

Directly upslope of the kerb the flow slows and deposits whatever it is carrying: moss off the ridge, grit from the covering, leaf litter. Over a few winters that builds into a dam sitting against the one face of the upstand that is already the lowest relative to the water. The detail that answers it is a formed diverter or a small back gutter upslope of the kerb, throwing flow to both sides before it reaches the upstand, plus enough clear covering above the unit that the diverter has somewhere to discharge. On a wide unit that back gutter needs a fall of its own and cannot simply be a flat tray.

Underlay and the drainage layer below the covering

On a shallow pitch the layer beneath the covering stops being a safety net and becomes a working drainage plane, because a meaningful quantity of water gets past the tiles in any strong wind.

That means the underlay laps get increased in line with the pitch, typically to 150mm or more horizontally, and it means the membrane must be properly supported so it forms a drape between rafters that carries water down rather than pooling in a slack bay. Around an opening the membrane is dressed in a shaped tongue over the head of the kerb or frame and taken into the drainage route at the sides, never slit. Where the roof has a fully boarded deck the water has nowhere to drape into, so the detail changes again. That layer is covered in full on the underlay and sarking around a pitched rooflight page.

Snow, silt and moss at the head of the unit

Snow sits on a shallow roof rather than sliding off it, and it sits longest against anything that stands up out of the surface.

A drift banked against the upslope face of a kerb melts from the bottom, and the meltwater has a head of standing water above it that a 150mm upstand exists to resist. That is the load case the figure was chosen for. Silt and moss do something similar over a longer timescale, and on a north-facing shallow slope in the Stroud valleys moss growth is heavy enough to change the effective pitch of the covering within a decade. Clearing the strip above a rooflight when the gutters are done is a small habit with a real effect.

Sheet coverings, and the units they suit

Lead, zinc, single ply and built-up felt are all designed to work at pitches a tile cannot approach, and a good number of shallow Gloucestershire roofs are covered in one of them.

On lead or zinc the unit sits on a kerb and the sheet is dressed up over it, with a welted or clipped detail at the top rather than a lapped one, and the standing seams and rolls have to be set out so none of them runs into the kerb at an awkward angle. On single ply the membrane is welded up the kerb and over, and the corners are prefabricated. On built-up felt the layers are dressed up the kerb in sequence with the cap sheet last. In every case the covering becomes continuous with the upstand rather than butting against it, which is the whole reason a kerb works where a flashing does not.

The 150mm projection limit on a roof that barely slopes

Permitted development for a roof alteration to a house sets a limit: the alteration must not project more than 150mm beyond the plane of the existing roof slope.

On a steep roof an in-plane window sits comfortably inside that. On a shallow roof, where the correct product is a kerb-mounted unit standing 150mm up plus the depth of the frame and glass, the total projection can exceed the limit, and then a planning application is needed rather than assumed. This catches people out because the shallow roof is often the least visible one on the house. Flats and maisonettes have no permitted development rights for roof alterations at all, and Article 4 directions remove rights street by street in parts of Gloucester and Cheltenham. The local planning authority is the only body that can confirm the position for an address, and a written answer before ordering is cheap.

Cheltenham mansard tops, hidden behind a parapet

Central Cheltenham is full of mansards: a steep lower slope visible from the pavement in Welsh slate, and a shallow upper slope at five to fifteen degrees behind a parapet, usually in lead, that nobody on the street can see.

That upper slope is where the rooflight goes, and it is almost always below the roof window threshold, so it is a kerb-mounted unit on a lead flat rather than a window in the slate. The real constraints there are the parapet gutters and their outlets: the position has to leave the falls alone, and on a terrace the outlets are frequently shared with next door. Much of this stock is listed, and consent is required for an alteration whether or not it can be seen, though visibility strongly affects what an officer will accept.

Vale lean-tos and the low pitch estate roofs

Two other shallow situations turn up constantly around Gloucester.

The first is the rear addition or lean-to on a Severn Vale terrace, often at 15 to 22 degrees in clay tile and sometimes lower where a later extension was built with a low pitch concrete tile. The run from gutter to abutment is short, commonly 1.8 to 2.4 metres, so the flashing laps a shallow pitch demands eat a large share of the available slope and the unit gets smaller than people hope. The second is the 1960s and 1970s estate roof at Quedgeley, Abbeydale or Bishops Cleeve, built at 12.5 to 17.5 degrees in a low pitch interlocking tile over trussed rafters. Those roofs are within their rating, they will take a low pitch kit on a suitable profile, and the trusses underneath decide the width, which is set out on the rooflights in a trussed roof page.