Guide · Skylight & Rooflight Installation & Replacement Chelmsford

Flat roof against pitched roof rooflights

16 sections 10 minute read

Before any conversation about products, one question settles most of the job: is the glass going into a flat roof or a pitched one. The two are not variations on a theme. They weather differently, they are built differently underneath, they behave differently in sun, rain and snow, and they age differently. Anyone who has both on the same house will tell you the rooms feel nothing alike.

This page compares them properly, on the things that actually change day to day and year to year rather than on appearance. It assumes you may have a genuine choice, which in practice happens on a new extension or on a house with more than one roof type. Where the choice has already been made for you, the relevant detail sits on the rooflight and skylight installation hub.

The fork, and where the line sits in degrees

Flat means anything below about ten degrees, which includes almost every rear extension, side return, garage and dormer top in the county. Pitched means fifteen degrees and above, which covers the main roof of nearly every house.

The band between ten and fifteen is genuinely awkward and it is where most arguments happen. Some roof windows are rated down to ten degrees with a specific underlay detail beneath them. Below that, the water no longer runs off reliably and manufacturers stop supporting the weathering. If your roof sits in that band, the pitch needs measuring rather than estimating, because the difference between eleven and nine degrees changes which family of product is available.

The upstand in section: 150mm from the finished surface to the top of the kerb, with the covering dressed up the face and terminated under the frame.

What each roof does with water

A pitched roof sheds. A flat roof drains, and the distinction is not pedantic.

On a pitch, gravity moves water down the slope continuously and the covering is a series of overlapping pieces arranged so that each one sheds onto the one below. Water is never asked to stay still. On a flat roof, the covering is a continuous membrane and the water moves because the deck has been laid to a fall, conventionally a minimum of 1:80 as designed to allow for deflection and construction tolerance. Where that fall is absent or has been lost to sagging joists, water stands, and standing water around a glazed opening is the condition that finds any weakness in the perimeter detail.

The build-up under a flat roof rooflight

A flat rooflight sits on a kerb, an upstand built off the deck, and the covering is dressed up the face of that kerb before the unit lands on top.

In a warm roof, the insulation sits above the deck and the kerb has to be built up through it, so the total height from ceiling to glass can be three hundred and fifty millimetres or more. In a cold roof, the insulation sits between the joists with a ventilated void above, and the kerb is shorter but the ventilation path around the opening has to be maintained. Either way the governing number is the height from the finished roof surface to the top of the kerb, and one hundred and fifty millimetres is the figure quoted across the industry. On an exposed or genuinely level roof that is the floor rather than the target.

The build-up under a pitched roof rooflight

A roof window is trimmed into the rafters, so timber is cut and load is redistributed. In a traditional cut roof with rafters at four hundred or six hundred centres that is routine: the rafters either side are doubled, trimmers go in above and below, and the frame is fixed to the resulting box.

Above the frame, the underlay is cut and dressed to shed onto the flashing rather than behind it, which is a detail that goes wrong more often than the flashing itself. Then the kit weathers the outside. Below, the insulation returns around the frame and the reveal is formed. There is no kerb, no membrane termination and no upstand height to argue about. There is a flashing kit that must match the covering exactly.

Daylight compared, hour by hour

A flat rooflight faces the whole dome of sky. A pitched one faces a single compass direction and roughly half of it.

In practice that means a flat unit delivers a steadier level through the day, high at midday and falling off gently at either end, with no strong directionality. A roof window in a south facing slope produces a moving band of light that tracks across the room and is dramatic rather than even. On a north slope it produces something very close to studio light: cool, shadowless and constant. The working comparison used in daylight design is that roof glazing of any kind delivers in the order of two to three times the illuminance of a vertical window of the same area, and the flat orientation sits at the upper end of that.

Solar gain compared across a summer day

This is where the two diverge most sharply and where regret usually originates.

In midsummer the sun reaches around sixty degrees above the horizon at noon in Essex, which means it is falling almost square onto a flat rooflight and glancing across a shallow pitched one. A flat unit therefore takes the peak overhead gain at exactly the hottest part of the day, which is why solar control glass is close to standard on flat installations. A pitched unit takes its worst gain in the afternoon on a south west plane, when the sun is lower and the angle of incidence is more direct on a vertical-ish surface. The mitigations differ too: external shading is straightforward above a pitched window and awkward above a flat one.

Condition	Flat rooflight	Pitched roof window
Midsummer noon	Peak gain, sun nearly square on	Glancing, less intense
Winter midday	Modest gain, low sun angle	Good gain on a south slope
Overcast day	Best available, full sky view	Around half the sky view
Overheating control	Solar control glass, near standard	Glass plus external awning if needed

Ventilation and air movement compared

Warm air collects at the highest point in a room, so an opening at roof level lets it leave while cooler air is drawn in through windows lower down. That stack effect works with both, but not equally.

A roof window can be swung wide, and a top hung sash at a good angle presents a large free area to the outside. A flat rooflight generally lifts a modest amount all round, so the free area is smaller for the same unit size. Against that, the flat unit sits at the true top of the room, which is where the warmest air actually is, whereas a roof window in a slope sits partway up. In a tall extension with a flat roof, a single opening unit moves a surprising volume of air with no fan and no power.

Rain noise on each of them

Rain on glass is louder than rain on tile, and the angle decides how much louder.

A flat rooflight takes rain square on, so the drumming is at its most direct and it is noticeable in a quiet room below. A pitched unit takes it at an angle and the drops glance rather than strike, which is measurably quieter. Two specification choices reduce it meaningfully on either. Laminated glass in the outer pane damps the drumming because the interlayer absorbs vibration a monolithic pane transmits, and an asymmetric unit with panes of differing thickness breaks up the resonance rather than reinforcing it. Both are ordered with the unit and neither can be added afterwards.

Snow, leaves and what collects

Anything that lands on a pitched roof eventually leaves. Anything that lands on a flat roof stays until something moves it.

Leaf fall from the mature trees along the older Chelmsford streets, moss shed off a tiled main roof onto a lower flat extension, blown grit and the general debris of a year all accumulate against a kerb, and the kerb is the thing they collect against because it is the only obstruction on the roof. That build-up is what closes the drainage path and turns a fall into a puddle. Snow behaves the same way: it slides off a pitch and sits on the flat, and a settled load of snow around a kerb is a temporary rise in the effective water level against the upstand.

Cleaning and reach compared

Roof windows were designed to be cleaned by the person who owns them. Flat rooflights largely were not.

A centre pivot unit rotates about its middle so the outer face can be turned into the room and washed from a standing position indoors, with the sash locked in the cleaning position. That is a genuine practical advantage and it is why centre pivot units remain popular on upper storeys. A flat rooflight has to be reached from the roof itself, which on a single storey extension is a step ladder and a careful decision about whether the deck will take a person, and on anything higher is not a homeowner job. Self cleaning coatings reduce the frequency on both without removing the need.

Daylight from a single large rooflight over a kitchen extension

Structural work compared

The structural question is different in kind, not just in degree.

On a pitch, the constraint is what the rafters are doing and whether they are cut rafters or trusses. In a trussed roof, common across the estate housing at Great Notley, South Woodham Ferrers and Beaulieu Park, the webs and bottom chord carry load and cannot simply be cut, which caps the practical width at around five hundred and fifty millimetres between trusses. On a flat roof, the constraint is the joist span and where the opening sits relative to the bearing. Below about a metre and a half, timber trimming is usually enough. Beyond that, and particularly across the span, a steel is the usual destination and it brings a calculation and a longer programme with it.

Time on site compared

For a straightforward roof window in a sound pitched slope, one day is normal. For a flat rooflight into an existing flat roof, one to two. For a lantern with a structural opening, two to four depending on the steel.

The reason the flat job runs longer is the kerb. It is joinery built on a roof, it has to be square and to dimension with the diagonals checked, and the covering then has to be dressed up it and terminated correctly. A roof window has no equivalent stage: the opening is framed, the unit goes in, the kit weathers it. On either, the roof is closed and watertight at the end of every working day whatever the forecast is doing.

How cost behaves on each

Neither is dependably cheaper, but they have different shapes of risk.

A pitched job is predictable. The main variables are access and the covering, since re-laying hand-made clay peg tiles around an opening is slower and more skilled than dealing with concrete interlocking tiles. A flat job carries the condition of the existing deck as an additional unknown, and that unknown can be significant: a deck holding water wants the fall sorted before glass goes into it. This is precisely why a range comes first and a fixed figure comes after a survey.

A house with both: extension and loft

A great many Chelmsford houses end up with both, and the order in which they happen is worth thinking about.

The common pattern is a rear extension with a flat roof over the kitchen and a pitched main roof over a loft room. Where both are planned, the flat roof glazing is generally the bigger transformation, because it lights the room the household actually lives in, while the loft window makes an unused space usable. Where budget forces a sequence, doing the flat roof first while the extension is being built avoids opening a finished roof later, and running a cable at first fix for a future electric opener costs almost nothing at that stage.

Which roof suits which room

Some rooms have a clear preference regardless of what the house offers.

Kitchens and dining spaces suit flat roof glazing, because the even overhead light falls on worktops and tables rather than tracking across a wall.
Bedrooms suit pitched roof windows, because a view out and a fitted blackout blind both matter more than raw illuminance.
Bathrooms and utilities take either, with the ventilation path mattering more than the daylight.
Studios and offices suit a north facing pitch above all else, for light that does not move or glare.
Hallways and landings often have no roofline directly above at all, which is a different problem entirely.

Choosing when both are available

On a new extension the roof itself is still a decision, and the glazing should be part of making it rather than a consequence of it.

Ask three questions in order. Which part of the room is dark, since that decides position before it decides type. Which way does the roof face, since that decides how hard the glass has to work. And what is the ceiling doing, since a vaulted ceiling under a pitch and a flat ceiling under a flat roof produce very different reveals and very different spreads of light. Answer those and the fork usually resolves itself, at which point the conversation moves on to sizes, specification and a survey.

Keep reading

More on skylight & rooflight installation & replacement chelmsford

Get a quote

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