Guide · Roof Window Installation
Rooflights on a shallow pitch below 15 degrees
Fifteen degrees is the number that decides which set of rules applies. Above it, a roof window in a tiled or slated slope is standard work with a standard flashing kit. Below it, water leaves the covering slowly enough that wind can push it back uphill, and every detail around the opening has to be reconsidered: the unit itself, the kit, the underlay laps, and sometimes the covering.
Shallow slopes are common in Essex. The new town housing across Harlow and Basildon, the flat roofed and low pitched bungalows on Canvey and the reclaimed land, mono-pitch rear extensions everywhere, and garage and outrigger roofs on almost every street all sit in the range this page is about. A roof window on one of them is achievable. It is simply a different job, and pretending otherwise is how a slow leak arrives in year three.
Measuring the pitch before anything else
Guessing the pitch from the ground is unreliable, and the errors run in the direction that causes problems: a slope people call twenty degrees is very often fourteen.
The measurement is taken off the rafter line inside the loft with a digital angle finder, or calculated from the rise and the run. Rise over run gives the tangent: a slope rising 1m over a 4m run is 14 degrees, one rising 1m over 3m is 18. On a mono-pitch extension the check is quick and it changes the specification, so it is done before a unit is discussed rather than after one has been ordered.
| Rise over run | Pitch | What applies |
|---|---|---|
| 1 in 2 | Around 27 degrees | Standard roof window work |
| 1 in 3 | Around 18 degrees | Standard, with attention to laps |
| 1 in 4 | Around 14 degrees | Low pitch rules, low pitch kit |
| 1 in 6 | Around 9.5 degrees | Below most roof window minimums |
| 1 in 12 | Around 5 degrees | Flat roof detailing, kerb mounted unit |
What changes for water below 15 degrees
Water on a roof is driven by gravity down the slope and pushed by wind in whatever direction the wind is going. The steeper the slope, the more gravity dominates. Reduce the pitch and the two forces come closer to parity.
Three specific consequences follow. Water travels more slowly, so a given volume of rain sits on the surface longer. It spreads laterally rather than running in defined paths, so it finds joints that a fast running sheet would pass over. And the horizontal component of wind driven rain becomes large relative to the slope, so water can genuinely travel uphill under a tile for a considerable distance. That third effect is the one that catches out a flashing detail designed for a steeper roof.
Capillary action, and joints that fill up
The other mechanism is capillary. Two surfaces close together with water between them will draw that water along the gap regardless of which way the gap slopes, and the narrower the gap the stronger the effect.
On a steep roof capillary movement is overwhelmed by gravity and the water drains before it can travel far. On a shallow one, gravity is barely winning, and a tight joint between a flashing and a tile becomes a wick. This is why low pitch details use anti-capillary features: a turned edge, a welt, or a deliberate air gap that breaks the continuous film of water. It is also why simply sealing a joint tighter makes the problem worse rather than better.
At low pitch, a tighter joint draws water in; a broken joint lets it fall out.
Manufacturer minimums, and what they actually say
Most roof windows are rated for installation from 15 degrees as standard. A number of models are rated from 10 degrees provided a specific low pitch flashing kit is used, and some are rated to 5 degrees with a proprietary kerb or upstand kit beneath them.
Those figures are conditions of the warranty, not suggestions. A unit installed at 12 degrees with a standard kit is outside what the manufacturer will stand behind, and it is outside it whether or not water ever comes in. Since the workmanship carries a ten year guarantee, the specification has to sit inside the manufacturer’s stated range for the whole thing to mean anything. Checking the model’s minimum pitch is the first thing done once the pitch is measured.
The low pitch kit, and what is different about it
A low pitch flashing kit is not the standard kit with a different label. Three things change.
The apron below the unit is longer, so water leaving the frame is released further down the slope, past the zone where it might be driven back. The upstand behind the head flashing is taller, giving more height for wind driven water to be pushed up before it can get over. And the side soakers have deeper turn-ups against the frame. Some kits also include an underfelt collar with extended skirts. The components themselves are described on soakers, aprons and head flashings explained; what a low pitch version does is give every one of them a larger margin.
Laps on the layer beneath
The underlay under the covering catches whatever gets past, and its laps are set by pitch for exactly the same reasons the flashing laps are. On a 40 degree slope a 100mm horizontal lap is conventional. At 15 degrees that increases substantially, and at 12.5 degrees and below most underlay manufacturers require a taped or sealed lap rather than a loose one.
Around a rooflight this matters twice over, because the underlay is being cut and re-formed at the very point where the roof is most vulnerable. The sequence and the dressing are covered on underlay and sarking around a pitched rooflight. The short version is that on a shallow slope the secondary layer is doing considerably more work than it does higher up, and it is detailed accordingly.
Whether the covering itself is rated that low
A frequent discovery on a low pitch survey is that the rooflight is not the only element sitting outside its range. Coverings have minimum pitches too, and they vary widely.
- Plain clay tiles want a steep roof, conventionally 35 degrees and above. They are rarely found on a shallow slope, and where they are, something has been done that was not intended.
- Natural slate works down to around 20 degrees at standard headlap, and lower with an increased headlap and larger slates.
- Concrete interlocking tiles vary by profile. Many are rated to 17.5 degrees, several low pitch profiles go to 12.5, and a few are rated lower still with a specific underlay.
- Single ply, felt and GRP membranes have no lower limit in this sense and are the usual covering below 10 degrees.
Where the covering is already below its own minimum, adding a rooflight into it is building on a detail that was marginal before anyone arrived. The honest sequence is to sort the covering first.
Below 10 degrees, where it becomes a flat roof problem
Under about 10 degrees the whole approach changes. The correct answer is usually not a roof window in a slope at all but a kerb mounted unit detailed as a flat roof installation: an upstand built off the structure, the covering dressed up the sides of it, and the unit landing on top.
That is a well understood detail with its own rules, covered on flat roof rooflight installation. It brings the 150mm upstand convention with it, along with the requirement that the roof around the kerb actually drains. Approaching a 7 degree roof as a shallow pitched roof rather than as a flat one is the single most common specification error in this range.
Building a fall where there is not enough
Where the pitch is genuinely too low for the unit that is wanted, the fall can sometimes be built rather than accepted. A tapered insulation scheme, a firring layer on the deck, or a purpose built kerb with a slope formed on top of it all raise the effective pitch of the glass without touching the roof structure.
This is standard practice for flush units on flat roofs and it works on a shallow slope too. What it costs is height, so the interaction is with the ceiling below and with any parapet or abutment above. Where a rear extension roof runs into a first floor wall, the height available at the abutment is the constraint that decides how much fall can be built.
Debris, moss and what a low slope collects
Shallow roofs collect. Leaves that would blow off a 40 degree slope sit on a 12 degree one, moss establishes where water lingers, and silt washes down and banks up against anything standing proud of the surface. A rooflight frame is exactly that.
The consequence is a dam forming along the head of the unit, holding water against the flashing upstand for hours rather than seconds. Over a few autumns that is enough to find any shortfall in the detail. Ten minutes clearing the perimeter each autumn is the whole maintenance requirement, and on a shallow slope it is worth actually doing rather than intending to do.
Ponding on a slope that has settled
Low pitch roofs deflect, and an old one has already done most of its deflecting. A roof designed at 10 degrees that has sagged 30mm over a 4m span in the middle now has a section that is close to level, and water stands there.
Standing water more than a couple of days after rain is worth taking seriously before a rooflight goes anywhere near it. It indicates the fall is not delivering, and a unit placed in or immediately below a ponding area is being asked to sit in water rather than shed it. The survey looks for the tide marks that show where water has stood, since they are visible long after the water has gone.
Snow, and the load a shallow slope holds
Snow slides off a steep roof and sits on a shallow one. That difference shows up in the structural load, since the design snow load on a roof is reduced as pitch increases and is at its full value below 30 degrees.
Around a rooflight the practical concern is drifting rather than uniform load. Snow accumulates against anything projecting from the surface, so it banks up along the head of a unit and can sit several times deeper there than on the open roof. That load is carried by the trimming around the opening, and it is one of the reasons the trimming on a low pitch roof is not skimped. The carpentry itself is covered on trimming rafters for a pitched roof rooflight.
Wind uplift on a low slope
Shallow roofs experience suction rather than pressure over most of their area. Wind flowing over a low slope behaves like air over a wing, and the resulting negative pressure tries to lift the covering off rather than press it down. The effect is strongest at the edges and corners of the roof, which in Essex terms means the exposed estuary side of a bungalow on Canvey or a coastal plot near Maldon.
For a rooflight this means the fixings and clips holding the flashing are working harder than they look, and mechanical fixing of the covering around the opening is not something to leave to the weight of the tiles. It also means a unit placed near the edge of a low pitch roof is in the worst zone available, and moving it a metre towards the middle materially reduces what it has to resist.
How light behaves through a shallow unit
A low pitch rooflight behaves optically much more like a flat one than like a steep roof window. It sees most of the sky dome rather than a slice of it, which means more light in general and considerably more summer solar gain when the sun is high.
The practical effect is that a shallow unit needs its glazing considered more carefully than a steep one of the same size. On a south facing 12 degree slope the July sun is close to normal to the glass in the middle of the day, and solar control glass moves from being optional to being the default. Specification is covered on energy efficient rooflights.
Rain noise, and the angle of the drop
Rain hitting glass at close to a right angle transfers more energy than rain glancing off a steep pane. A shallow rooflight is therefore noticeably louder in heavy rain than a steep one of the same specification, and directly above a bedroom that is a real consideration.
The two effective answers are both glass choices made at order. A laminated outer pane damps the drumming because the interlayer absorbs vibration a single sheet transmits, and an asymmetric unit with two different pane thicknesses breaks up the resonance instead of reinforcing it. Neither can be added afterwards.
Where these roofs are across Essex
The shallow pitch stock in the county falls into a few clear groups. The new town housing in Harlow and Basildon used low pitch and flat roof forms extensively, often with a concrete interlocking tile on a mono-pitch. Bungalows on Canvey and the low lying land around the estuary carry shallow hipped roofs, frequently exposed to wind off the water. Garage roofs, outriggers and rear additions across the county sit at whatever pitch the original builder could achieve under a first floor window. And nearly every contemporary rear extension built in the last fifteen years is a shallow mono-pitch or a flat roof by design.
The weatherboard villages along the Blackwater and around Stock add another case, where a shallow lean-to roof runs off a two storey elevation and the abutment height is the governing dimension. Each of these has a workable answer. None of them has the same answer.
What the survey measures on a shallow roof
Six figures settle the specification. The measured pitch, taken off the rafter line rather than estimated. The covering and its own minimum pitch. Whether the underlay laps are sealed or loose. Whether there is evidence of standing water. The height available at any abutment above, which caps how much fall can be built. And the exposure of the position, since edge and corner zones on a low roof are the worst place to put an opening.
Those six determine whether the job is a low pitch roof window with the correct kit, a kerb mounted unit detailed as flat roof work, or a case for building the fall up first. All three are ordinary work. Choosing between them from a photograph is not, which is why the pitch measurement comes before the product conversation rather than after it.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Two rooflights side by side on one slope
Read the guide →Underlay and sarking around a pitched rooflight
Read the guide →
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