Skylights for hillside terraces in Dewsbury, and rooflights for the old mill buildings

Read the lease first

The lease, or the managing agent, tells you who has to agree to work on the roof. It helps to arrive at that conversation knowing which room the rooflight lights, which slope it goes in and how big the unit is, because those are the three things anyone approving it will ask.

Which unit suits which flat

• A sun tunnel for a hallway or bathroom with the flat's own roof straight above it.
• A roof window in the pitch above the bed of an attic room, or above the living space under the roof.
• A conservation rooflight where the building sits in a conservation area and the slope can be seen from the street.

A flat that is not under the roof

A flat on a lower floor has other homes between it and the slates, and a sun tunnel cannot pass through someone else's rooms. We look at what sits above the room before suggesting a unit, and a landing under an open stairwell is a different matter, because the stairwell gives the light a clear drop from the roof.

Chapel conversions bring their own roof shapes, with tall pitched slopes and sometimes a gallery level inside. A roof window high in a slope like that is reached from the inside by pole or by an electric motor, so it is worth saying at the start if the ceiling is out of arm's reach.

Between the rafters

Making the opening changes how the roof is carried. Where a rafter stops at the gap, short timbers called trimmers take its load round the frame, and the opening is sized to suit how far apart your rafters sit. The spacing can differ from one terrace house to the next, so the size of unit is settled on site and written into the quote.

Keeping the weather out

Slate lies flat and thin, so the frame takes a flashing kit made for slate. Slates above the frame are trimmed and laid so they overlap the top edge of the flashing, so rain is guided around the frame and returned to the slope below. Flashing sets made for deep profiled tiles leave the wrong gaps on slate and are not used.

Between two neighbours

In a terrace the chimney breasts and party walls rise through the roof at regular spacing, and each one limits where a frame can sit. Clean slate and a clear run of rafters are needed on all four sides of the opening, so the unit is placed in the bay between stacks. Where a bay is narrow, a narrow roof window, or a sun tunnel, takes less room than a wide frame.

Where the slope faces

On a hillside terrace the back slope may face the valley or the hill behind, and that changes how much direct sun a room gets through the glass. A roof window on a slope that faces the afternoon sun lights an attic generously and warms it, so a blind or solar-control glazing is worth thinking about at the outset. A slope facing the other way gives softer, steadier light all day.

Finishing indoors

Under the roof, the lining is insulated and plaster finishes the reveal. Old ceilings give off a fine dust when they are cut, so the floor and the stair are sheeted before the saw comes out.

Rooflight or lantern

A flat roof rooflight sits low on a kerb and gives an even spread of light. A roof lantern rises in a pitched glass shape and suits a wider kitchen where there is height for it. A lantern normally stands well proud of the roof, past the 150mm projection limit for permitted development, so put the question to Kirklees Council, or see if the permission for the extension already takes it in.

What the roof is covered with

Rear wings tend to carry felt, rubber sheet (EPDM) or fibreglass (GRP). Each calls for a different upstand and edge detail, and the covering is taken up the side of the kerb as a single piece. If the covering is old and near the end of its life, the best moment to fit a unit is when the roof is being re-covered, so the new kerb and the new covering are built together.

Where in the roof it goes

The unit usually sits over the working end of the room, between the cooker and the sink, or over a table at the far end if the family eats there. Rafters or joists run one way across the wing, so the opening is built to match them, and the size and the position are fixed together on site. A long narrow kitchen often does best with one unit placed centrally rather than two small ones at either end.

Keeping the kitchen comfortable

A kitchen under glass can warm up on a sunny afternoon, so the glazing is worth choosing with that in mind. Solar-control coatings cut the heat that passes through, while two or three panes with a warm-edge spacer help the room hold its heat through winter. Rooflights, lanterns included, are held to a U-value of 2.2 W/m²K for new and replacement units in existing homes; the smaller the figure, the less warmth escapes through the glass.

An opening rooflight lets steam out of a kitchen as well as light in. A switch or remote moves the glass, and many models include a rain sensor which shuts it at the first drops of rain.