Wigan's Welsh-slate mill terraces and the skylights or rooflights they take

The River Douglas runs through the valley Wigan sits in, which leaves the town centre better sheltered than the coast or the open plain to the west. The land rises towards Billinge and Orrell, and roofs on that higher ground catch more of the weather from the south-west than the terraces on the valley floor. A skylight on an Orrell slope is specified with that in mind, with a deeper flashing lap and fixings chosen for exposure. A rooflight in a closed-in terrace street is a calmer proposition, although water still has to be shed well from a shallow gutter run.

The slate roofs of the terraces bring their own detail, and the surrounding mills and warehouses add tall brick walls that throw shade and shelter in odd patterns across the rooftops. A roof window on a lower roof beside a mill wall may get less sun than the orientation suggests, and we allow for that when advising which way the light will fall. Welsh slate is thin, and the nails on an old roof can have corroded, so the area around the opening is checked before it is disturbed. Where slates have slipped or the nail holes have enlarged, the surrounding courses are secured as part of fitting, so the new flashing is not set into a loose field. The sheet of slate above a roof window is tucked behind its upper flashing, and the lower edge is covered by an apron; the order is what keeps the water out. Fitting a unit also means supporting the cut courses on either side, so we use slate hooks or a tray at the head and avoid leaving any short lengths of slate resting only on their nails. On a terrace this is where an experienced hand shows, because the roof has to look as though the window was always there.

Dust and noise are part of the fitting day in a terrace. The party walls are thin, the houses either side are close, and we tell the neighbours before starting. Inside, the attic floor is covered, the stair is protected and the debris goes out in bags, so a lived-in house is left as clean as we found it.

Flat roofs on outriggers and rear extensions have usually had more than one covering. A kitchen wing may carry a layer of old felt, then a newer one, with a rooflight bedded in that never lasted. We look at the build-up from a ladder or by lifting an edge at the survey, and we photograph what we find so you can see it too, and where the deck needs attention we tell you before quoting. A new rooflight on a kerb then sits on something sound, with a fall for the water to run off.

The Edwardian houses in the better suburbs sit between the two extremes. Their slate is thicker and their rafters deeper than on a mill terrace, so the opening is easier to frame, but the attic often holds a water tank and old lagging, and we clear a route for both before cutting.

Access follows the street pattern. Many terrace streets in Pemberton and Newtown have only a narrow back passage or alley, so a unit often has to be carried through the house unless a scaffold can be placed in the street. In that case, we often fit roof windows from inside the attic, standing on the floor joists and working up through the slope. A long rear yard may also be shared, so a word with the neighbour about ladders and noise is courteous.

A row of identical terraces is the easy case to quote, but each roof still gets its own survey.