Skylights, Decks and Add-Ons on a Pitched Roof

Adding a skylight, lantern, or deck to a pitched roof depends partly on the pitch, so knowing the pitch as a number helps you plan. The Roof Pitch Calculator turns a rise and run, or an x/12 ratio, into the pitch and angle, so you can check whether your roof suits a skylight, a lantern, or a deck structure.

What you can add to a pitched roof

You can add several things to a pitched roof: a skylight (rooflight) fits into the slope readily and is the easiest and commonest; a roof lantern, which is a raised glazed feature, needs a flat or flattened base rather than a slope, so it suits a flat roof or a flattened section; and a deck can be built on a pitched roof but needs a level structure over the slope, so it is more involved and more naturally suits a flat roof. So skylights are easy, lanterns need a flat base, and decks need a level frame over the pitch.

Each is possible with the right approach, but they differ in how well they suit a pitched roof. A skylight is designed to sit in a sloping roof and is the go-to for light. A roof lantern is really a flat-roof feature but can be added over a flattened part of a pitched roof. A deck over a pitch needs a supporting structure to create a level surface, which is why decks are more common on flat roofs. This guide covers each, starting with skylights, and what the pitch means for them.

Can you put a skylight on a pitched roof

Yes, you can readily put a skylight, or rooflight, on a pitched roof, and it is the most common way to bring light into a room under a pitched roof or a loft conversion. Skylights are designed to fit into a sloping roof, set between the rafters and flashed into the covering, so they suit a pitched roof naturally. A skylight in a pitched roof also self-cleans and sheds rain well, since it sits on the slope, which is one reason it works so well there.

So a skylight is the easiest and best add-on for a pitched roof, letting light into the space below while shedding rain on the slope. It is far simpler than a roof lantern or a deck on a pitched roof, since it is made for exactly this. Skylights come in sizes to fit between rafters, and larger ones may need a rafter trimmed and supported. So if you want to add light to a pitched roof, a skylight is the natural choice, fitting into the slope and flashed to stay watertight, which the next sections cover.

Installing a skylight

Installing a skylight in a pitched roof means cutting an opening in the roof between (or trimming) the rafters, fitting the skylight into the opening, and flashing it into the covering so it is watertight. The skylight sits on the slope, and a flashing kit dresses the covering up to and over its edges, so rain sheds around and off it. Inside, the opening is lined to form the light well down to the ceiling, splayed if wanted to spread the light.

The key to a leak-free skylight is the flashing, which must be correct for the covering and the pitch, since this junction sheds the rain around the skylight. Larger skylights that interrupt a rafter need the rafter trimmed and the load carried around the opening, a structural detail. So installing a skylight is cutting and framing the opening, fitting the unit, flashing it into the covering, and lining the well, with the flashing the critical watertight detail. Done right, a skylight in a pitched roof is a reliable, light-giving feature.

Skylights and pitch

Skylights suit a range of pitches, but most have a minimum pitch they are rated for, below which they may not shed rain reliably, so check the maker’s minimum for your roof. Most pitched roofs are well within the range, but a very low-pitch roof may need a skylight designed for low pitches or a kerb to raise it. On a steep roof, skylights work well and shed rain readily. So the pitch rarely rules out a skylight, but a very low pitch calls for a low-pitch-rated unit.

The pitch also affects how the skylight admits light and sheds rain: on a steeper roof the skylight faces more outward, on a shallow roof more upward. For a flat or very low roof, a flat-roof rooflight or a kerb-mounted unit is used instead of a sloping skylight. So match the skylight to your pitch, checking the minimum, and use a low-pitch or flat-roof rooflight if the roof is very shallow. The minimum roof pitch guide covers low-pitch roofs, where a standard skylight may need a low-pitch alternative.

Can you put a roof lantern on a pitched roof

A roof lantern is a raised, glazed feature that sits up above the roof plane, usually on a flat roof, so putting one on a pitched roof needs a flat or flattened base to mount it on. On a pitched roof, you cannot sit a lantern directly on the slope, so you either fit it to a flat-roofed section (such as a flat-roofed extension with a lantern) or create a flat base within the pitched roof, which is more involved. So a roof lantern suits a flat roof, and on a pitched roof it needs a flat mounting.

This is why roof lanterns are usually seen on flat-roofed extensions rather than pitched roofs: they are designed to sit on a flat roof and rise above it as a glazed pyramid. If you want a lantern with a pitched roof, the common approach is a flat-roofed section (perhaps a flat-roofed extension) carrying the lantern, alongside the pitched roof. So can you put a roof lantern on a pitched roof? Only on a flat or flattened part of it, since the lantern needs a flat base, which is why it is really a flat-roof feature. A skylight suits a pitched roof far better.

Fitting a roof lantern

Fitting a roof lantern means building a flat, level kerb (an upstand) around an opening in a flat roof, and sitting the lantern on the kerb, flashed and sealed so it is watertight. The kerb raises the lantern above the roof so water sheds around it, and the glazed lantern sits on top, letting light down through the opening into the room below. This is a flat-roof detail, which is why a lantern needs a flat base, whether that is a flat roof or a flat section built into a pitched one.

On a pitched roof, creating that flat base means either using an adjacent flat-roofed area or building a flat section, which complicates the roof. So in practice, a roof lantern goes on a flat roof or flat-roofed extension, not directly on a pitched slope. If you have a pitched roof and want overhead glazing, a skylight set into the slope is the natural choice, while a lantern is for a flat roof. So fitting a lantern is a flat-roof job, needing a level kerb, which is why it does not sit directly on a pitched roof.

Can you build a deck on a pitched roof

You can build a deck on a pitched roof, but it needs a supporting structure to create a level deck surface over the sloping roof, so it is more involved than a deck on a flat roof. The deck is built on a frame of adjustable pedestals or bearers that sit on the roof and rise to a level top, over which the decking boards are laid, so the finished deck is level even though the roof beneath slopes. So a deck on a pitched roof is possible, using a level structure over the slope.

This is more complex than decking a flat roof, where the deck can sit closer to the roof, because the pitched roof’s slope has to be taken up by the supporting structure. The roof beneath must also be watertight and strong enough to carry the deck and its users. So can you build a deck on a pitched roof? Yes, with a level supporting structure over the slope and a sound, strong roof beneath, though it is more involved than on a flat roof, which suits decks better. The next sections cover how it works.

How a deck on a pitched roof works

A deck on a pitched roof works by building a level platform above the sloping roof, carried on a frame that takes up the slope. Adjustable pedestals or a framed structure sit on the roof (usually on a low-pitch or flattened area) and rise to a level top, and the decking is laid on that, so the deck surface is flat and usable while the roof beneath drains under it. The roof stays watertight underneath, and the deck sits above it, with water draining off the roof below the deck.

So the deck and the roof are two layers: the watertight roof below, draining as normal, and the level deck above, on its supporting frame. This works best on a low-pitch or nearly flat roof, where the slope to take up is small; on a steeply pitched roof, the structure to create a level deck would be tall and impractical. So a deck on a pitched roof is really suited to a low-pitch roof, with the deck built level over the modest slope, above a roof that still drains beneath. A steep pitch makes a deck impractical.

Framing a level deck over a slope

To frame a level deck over a sloping roof, you use adjustable supports, pedestals or a graded frame, that sit on the roof and are set to different heights so their tops are all level, taking up the slope. The decking joists or boards sit on these level tops, giving a flat deck. Adjustable deck pedestals are made for exactly this, screwing up or down to level the deck over an uneven or sloping surface, and they spread the load without penetrating the roof covering.

The supports must sit on the roof without damaging the covering, spreading the load, and the deck must not trap water or block the roof’s drainage beneath. So framing a deck over a slope is about creating a level top on adjustable supports over the pitch, with the roof draining underneath. This suits a low pitch, where the supports vary only a little; a steep pitch needs tall supports and is impractical. So the level deck sits on graded or adjustable supports over the slope, a neat solution on a low-pitch roof.

Decks suit flat roofs better

Decks are far more common and practical on flat roofs than pitched ones, because a flat roof needs little or no structure to create a level deck. On a flat roof, the deck sits just above the roof on low pedestals, level and usable, with the roof draining its slight fall beneath. This is why roof terraces and roof decks are a flat-roof feature: the flat roof gives a ready base for a level deck. A pitched roof, by contrast, needs a taller structure to take up the slope.

So if you want a roof deck or terrace, a flat roof is the natural place for it, and a flat-roofed extension or a flat section is often built specifically to carry a deck. On a pitched roof, a deck is possible on a low pitch but more involved, and impractical on a steep one. The flat versus pitched comparison notes that a usable roof surface is a flat-roof advantage, and a deck is exactly that. So for a deck, a flat roof suits best, while a pitched roof can carry one only on a low pitch with a level structure.

Weight and structure

Any deck on a roof, pitched or flat, adds weight, both the deck itself and the people and furniture on it, so the roof structure must be strong enough to carry it. A roof built only to carry its covering may not be strong enough for a deck and its users, so the structure often needs strengthening or designing for the extra load. This is a structural matter that should be checked, since overloading a roof is dangerous. So a deck on a roof needs the roof designed or strengthened for the load.

On a pitched roof, the deck structure also adds its own weight, so the roof below carries both. The load of a deck full of people is significant, so the roof and its supports must be sized for it, ideally designed by a structural engineer for a deck. So before building a deck on any roof, ensure the structure can carry the load, strengthening it if needed. The weight is a key reason a roof deck is a designed feature, not a casual addition, whether on a flat or a pitched roof. Safety depends on the structure being adequate.

Keeping it watertight

Whatever you add to a pitched roof, keeping it watertight is essential, and the junction where the addition meets the roof is the critical detail. A skylight is flashed into the covering; a lantern sits on a sealed kerb; a deck sits above a roof that must stay watertight beneath it. In each case the roof covering must be sound and the junction with the addition sealed, so water sheds around and off, not in. A leak at one of these features is a common failure if the detail is poor.

So the waterproofing is as important as the feature itself: a skylight’s flashing, a lantern’s kerb seal, or the intact roof under a deck. For a deck, the deck supports must not penetrate or damage the covering, and the roof must drain freely beneath. So add these features with the waterproofing detail done properly, keeping the covering sound and the junctions sealed. The low-pitch roofing guide covers keeping low roofs watertight, relevant where a deck sits on a low pitch. Watertightness is what makes an add-on last.

Access and safety

A roof deck needs safe access and edge protection, since it puts people on the roof. Access is usually from inside the building, through a door or a large rooflight onto the deck, and the deck needs a railing or balustrade around it to stop falls, meeting the safety rules for a raised platform. So a roof deck is not just a structure but a safe, guarded, accessible space, which adds to its complexity and cost, on a pitched or flat roof alike.

Skylights and lanterns, by contrast, are not walked on (except walk-on rooflights made for it), so their safety concern is mainly the installation work at height. A deck, though, is used by people, so its balustrade, access, and structure must all be safe. So building a roof deck means providing safe access and a guarded edge, as well as the structure and waterproofing, which is another reason a deck is a designed project. The safety of people on the deck is paramount, requiring proper edge protection and sound access.

Skylight, roof lantern or rooflight

The terms skylight, rooflight, and roof lantern overlap but differ. A skylight or rooflight is a glazed unit set into the roof plane, flush or slightly raised, suiting a pitched roof (sloping) or a flat roof (flat or kerb-mounted). A roof lantern is a raised, multi-faceted glazed structure that rises above a flat roof like a small glass pyramid, a flat-roof feature. So for a pitched roof, a skylight or rooflight set into the slope is the fit; for a flat roof, either a flat rooflight or a raised lantern.

So choose by roof type and effect: a skylight or rooflight for a pitched roof, letting light in flush with the slope; a flat rooflight for a flat roof, flush glazing; a roof lantern for a flat roof where you want a raised, decorative glazed feature. The what is a pitched roof guide covers roof types. So for a pitched roof, the skylight or rooflight is the natural overhead glazing, while the lantern belongs on a flat roof, which clarifies which feature suits which roof.

Dormers as an alternative

If you want more than light, such as headroom and a vertical window, a dormer is an alternative to a skylight on a pitched roof. A dormer projects from the roof slope with its own small roof and a vertical window, adding usable headroom and floor space in the roof as well as light, which a flush skylight does not. So where a loft conversion needs standing room and a proper window, a dormer does more than a skylight, at more cost and work.

The dormer has its own small roof at its own pitch, as covered in the roof types guide, and where it meets the main slope it is flashed like any junction. So a dormer is a bigger addition than a skylight, adding space and a vertical window, suited to a loft conversion needing headroom. A skylight is simpler and cheaper for light alone; a dormer adds room. So consider a dormer where you need headroom and floor space, and a skylight where you only need light, on a pitched roof.

Solar panels as an add-on

Solar panels are another common add-on to a pitched roof, mounted on the slope facing the sun, and the pitched roof suits them well since it presents the panels at an angle to the sun. The panels are fixed on rails above the covering, and the pitch and orientation affect how much energy they capture, as the best roof pitch for solar panels guide covers. So a pitched roof is a natural mount for solar panels, holding them at a good angle without extra structure.

Unlike a deck, solar panels add little weight and need no level structure, just fixing rails on the slope, so they are a simple pitched-roof add-on. The pitch of the roof sets the panel angle, which affects the yield, so the roof pitch matters for solar, as the solar guide explains. So solar panels are a simple, common addition to a pitched roof, mounted on the slope at the roof’s pitch, which the solar guide covers in detail including the best pitch for capturing energy. The pitched roof’s slope is an advantage for solar.

Light wells and spreading the light

A skylight in a pitched roof lets light down through a light well, the lined opening between the roof and the ceiling, and how that well is formed affects how much light reaches the room. A well cut straight down gives a narrow beam, while a well splayed wider at the ceiling than at the roof spreads the light across the room, brightening it far more. So on a thick roof or a room needing lots of light, splaying the light well is a simple way to get more from the same skylight.

The direction the skylight faces also matters: a skylight on a south-facing slope gets strong direct sun, which is bright but can overheat a room, while a north-facing one gives softer, even light. So consider both the splay of the well and the orientation of the slope when placing a skylight, choosing the position and well shape for the light you want. A well-placed, well-splayed skylight transforms a dark loft room, which is why they are so central to loft conversions, doing far more than a small opening might suggest.

Ventilation and opening rooflights

Many skylights open, which adds ventilation as well as light, and this is worth planning for in a room under a pitched roof. An opening rooflight lets warm air escape at the high point of a sloping ceiling, drawing fresh air in from lower windows, which cools and ventilates a loft room naturally. In a bathroom or kitchen under a pitched roof, an opening rooflight clears steam and moisture, helping prevent condensation in the roof space.

So when choosing a skylight, consider whether you want it to open, and where the ventilation would help most, such as at the top of a room or over a wet area. Opening rooflights come with manual, pole, or electric operation, and some close automatically in rain. The ventilation a skylight gives is a real bonus on top of the light, particularly in a loft where opening windows may be limited. So an opening rooflight is often worth the small extra over a fixed one, adding natural ventilation to the light it brings into a pitched-roof room.

Green roofs and other additions

Beyond skylights, lanterns, and decks, a pitched roof can carry other additions, though the slope shapes what works. A green roof, planted with sedum or wildflowers, is more naturally a flat or low-pitch feature, since a steep slope makes the growing medium hard to retain, but low-pitch roofs can be planted with the right retention. Antennas, satellite dishes, and vents are common minor additions, fixed to the slope or through the covering with proper flashing.

Each addition shares the same two concerns as the bigger ones: the load it adds and the watertightness where it meets the roof. A green roof adds significant weight when saturated, so the structure must carry it, while a vent or dish adds little but must be flashed to keep water out. So the principles are consistent across everything you add to a pitched roof: check the structure can carry the load, and seal the junction so it stays watertight. Whether it is a skylight, a deck, a green roof, or a vent, those two things decide whether the addition works and lasts.

Mistakes to avoid

The common mistakes when adding to a pitched roof are these.

Avoiding these means flashing skylights properly, using a skylight (not a lantern) on a slope, decking only low-pitch roofs with a level structure, designing for the load, and guarding a deck. Do that and the add-on works and lasts.

Working out the pitch

Whether an add-on suits your roof depends partly on the pitch, and the roof pitch calculator gives it from a rise and run, or an x/12 ratio, as a ratio and an angle. A moderate to steep pitch suits skylights and solar panels well; a low pitch suits a deck (with a level structure) or needs a low-pitch skylight; a roof lantern needs a flat base rather than a slope. So knowing the pitch tells you which add-ons fit your roof and how to detail them.

The connected guides cover the roof and its add-ons: best roof pitch for solar panels for solar, roof types for dormers, flat versus pitched for where decks suit, and minimum roof pitch for low-pitch roofs. The roof pitch tool sits with the other estimators in the construction calculators category, near site tools like the dirt calculator, and the full roofing library is in the roof pitch blog off the homepage. Work out the pitch, then choose the right add-on.

Can you build a deck on a pitched roof: frequently asked questions