How to Install Roof Windows

Determine where the window is to be fitted. Remove the base plate from the window. In the attic, measure the height and width of the base plate with the tape measure. Mark these dimensions on the rafters where the window is to be placed, with the marker. Set up scaffolding outside, to grant safe access to the roof. Remove the roof tiles where the window will be placed, plus an additional two rows of tiles on each side.

Cut away the underfelt with a sharp knife. Cut through the exposed rafters where you marked them earlier, using a hacksaw or reciprocating saw. Nail the timber packing strips to the ends of the cut rafters to support them. Mark the position of the top and bottom of the base plate on the packing strips. Screw 4 inch by 2 inch timber pieces between these positions, to support the top and bottom of the base plate.

Measure out a strip of 1-foot wide lead sheeting equal to the width of the base plate plus 6 inches. Fold the lead sheet over the bottom timber piece and down over the exposed underfelt. Coat the surface of the lead sheet with silicone roof sealant. Slide the base plate into position in the timber frame, with the roof sealant forming a watertight seal beneath the lower portion of the base frame.

Bolt the base plate to the rafters using the coach bolts, which fasten through the fixing brackets on the base plate. Cut strips of 6-inch wide lead flashing to 3/4 of the length of the roofing tiles. Tuck a strip under each row of tiles. Nail a removed roofing tile back in place at the bottom-most row of removed tiles. Fold the lead flashing halfway across the tile. Repeat for the next row of tiles up.

Cut a strip of 1-foot wide lead sheet to the width of the base plate plus 6 inches. Place the strip over the battens above the base plate and tuck the top under the underfelt. Fold the bottom of the sheet into the base plate. Nail a row of tiles along the top of the opening, to seal the lead flashing in place. Reattach the window frame to the base plate.