What pitch does a roof lantern need?

The pitch a lantern needs, in two figures

Fifteen degrees is the shallowest angle at which a lantern behaves like a lantern. Twenty-five is the angle most manufactured frames are set to unless something is specified, and it is the one that reads correctly from the garden: high enough to be a shape rather than a lid, low enough to stay in proportion over a normal rear extension.

Above twenty-five the lantern gains volume in the room and presence on the roof. Thirty to thirty-five degrees suits a large span or a room with a high ceiling where a shallow lantern would look squashed. Beyond about forty the frame becomes an object visible from a long way off, and the panes get long enough that weight and handling start to drive the price.

What the fifteen degree floor is protecting

The figure is not arbitrary. Three things stop working underneath it, and they stop working in a way that shows up in the second or third winter rather than on the day.

Rainwater stops carrying dirt off the glass and starts pooling in the lower corner of each pane against the glazing bar, where it dries and leaves a mineral ring. Self cleaning coatings, which rely on a sheeting flow of water rather than droplets, generally want fifteen degrees before the manufacturer will stand behind the performance. And snow that would slide off a twenty-five degree plane sits on a fifteen degree one for the length of a cold spell, which is a load case rather than an inconvenience.

Condensation on the inner face, and the angle it needs to drain

A lantern frame is drained rather than sealed. There are channels along the inside of each rafter and the eaves beam, and any moisture that forms on the room face of the glass is meant to run down the pane, reach that channel, and leave through weep holes at the bottom.

That only happens if the glass is steep enough for the water to run rather than bead. On a shallow pane the droplets grow until they are heavy enough to fall, and they fall onto whatever is beneath them. Fifteen degrees is roughly where a wetted pane starts to sheet reliably, which is the same threshold as the self cleaning coating and for the same reason. Kitchens make this more noticeable than any other room, because they put more water vapour into the air than anywhere else in the house, and the glass is the coldest surface above it. Better glazing narrows the gap without removing it, which is covered on the do roof lanterns lose a lot of heat page.

A long narrow room against a square one

Pitch is measured across the short dimension, so a lantern over a long narrow room behaves very differently from a square one of the same area. A hipped lantern at 3.0m by 1.4m has a short span, which means twenty-five degrees produces a rise of only 326mm and the unit reads as a low ridge running the length of the room. The same twenty-five degrees on a 2.4m square lantern gives 560mm and a proper pyramid.

On a narrow rear addition, going steeper is usually the right call. Thirty-five degrees over a 1.4m span still only rises 490mm, and it gives the long ridge enough height to be read as a deliberate shape rather than a slight bump in the roof.

How steep a lantern reads from a Cheltenham garden

Conservation officers in Cheltenham and across the Cotswold District look at bulk and visibility more than they look at angles, but pitch is how you control both. A rear extension in Lansdown, Pittville or Tivoli sits in a long garden overlooked by the upper windows of its own house and its neighbours, so the lantern is seen from above and at a distance rather than from the pavement.

A shallower lantern at fifteen to twenty degrees keeps the ridge below the sightline from those windows and reads as a glazed slot in the flat roof. A thirty-five degree pyramid on the same extension becomes a structure in its own right. Where the extension sits behind a stone parapet, which is common on Cotswold garden rooms through Painswick and the wolds villages, the parapet sets a hard ceiling: measure from the finished roof surface to the top of the coping, subtract the kerb, and the remainder is all the rise the pitch is allowed to produce.

Three Gloucestershire situations where the roof decides the angle

On a Gloucester or Tewkesbury Victorian rear addition, the lantern usually goes on a small flat roof where the addition meets the back wall of the house. The abutment is a two storey brick wall with a first floor window in it, and the ridge has to stay below the cill of that window with room for a flashing above. That measurement frequently caps the pitch at twenty degrees regardless of what anybody wanted.

On the trussed estates at Quedgeley, Abbeydale and Bishops Cleeve the flat rear extension roofs are shallow and the fascia line is low, so a steep lantern overhangs its own eaves visually. Twenty to twenty-five degrees suits them.

In the Stroud valleys the constraint is weight rather than height. A steeper pitch means longer panes, longer panes mean heavier glass, and heavier glass has to be carried up a flight of steps from a shared lane in Chalford or Rodborough. Splitting the same lantern into more panes at a shallower angle sometimes turns a mechanical lift into a two person carry.