Why does the kerb usually fail before the glass?
The kerb is the join between the window and the roof covering, and it takes the flow of water that runs down the slope above the opening. Metal that is lapped the wrong way round or sealant that has aged lets water run behind the flashing.
A kerb flashing that was fixed with its laps facing the wrong way sends water into the joint rather than over it. Once the water is behind the metal, it runs down the frame and shows as a stain on the ceiling some distance from the opening.
A cracked dome or a failed seal lets water in more directly, and the stain sits immediately below the unit. The technician looks at both, and the position of the stain decides which one is the source.
Skylights fitted in the decades when many houses were built often have a kerb of a size and material that current flashings do not match. A replacement flashing is cut to fit the existing kerb, not forced onto it, and the difference is noted on the scope so the owner understands why the flashing is not a standard part.
What does condensation between the panes tell the owner?
Condensation between the panes means the seal has failed and moist air has entered the unit. It does not usually mean the roof is leaking, but it does mean the glazing has lost its insulating value and the unit should be replaced or re-sealed by its maker.
A seal that has failed cannot be dried out. The maker's replacement unit is the usual fix, and the technician records the failure so the owner knows the reason for the change.
Condensation can also come from the room below. Steam from a kitchen or a bathroom that does not vent outside can condense on the glass, and the technician checks the room before blaming the unit.
Why is the old kerb checked before a new unit goes in?
An older kerb may be out of square, rotted at the corners or fixed with an old strip that will not hold a new flashing. A new unit fitted to a poor kerb can leak at the first join, so the kerb is corrected first.
The technician measures the opening against the new unit's specification. A taller unit needs a deeper kerb and a longer flashing, and a shorter one needs a different lap. The difference is written into the scope so the flashing is made to fit.
Where a kerb has rotted, the rotted wood is cut out and replaced before the new flashing goes on. The owner is told when the kerb must be renewed as well as the glazing.
The new flashing is also checked against the roof covering around it. A flashing that laps correctly over the frame but sits too low under the covering lets water run back under the lap. The technician checks each lap from both sides before the covering is fixed back, so the water runs over every joint.
How is the ceiling below checked after the work?
The ceiling below the opening is checked for staining, soft plasterboard and moisture in the joists after the flashing has been re-made. Any wet material is dried and recorded before the room is closed.
A stain that has dried shows the water entered before. The technician notes its size and position, and the owner is asked to report any new mark after the next rain so a second route can be found if one exists.
The record of the test and the drying is kept with the scope, so the owner can show the repair was made and the ceiling was dried before it was closed.
Is a skylight with a lifted kerb always a full replacement?
No. A lifted kerb is often corrected by re-making the flashing and re-sealing the frame, and the glass can stay. The technician decides after checking the frame, the seal and the covering around the kerb.
A kerb that is square, sound and only lifted at one corner can take a new flashing and a fresh seal. A kerb that is rotted, out of square or fixed with an old strip usually needs more, and the owner is told the difference on the scope.
The decision is recorded with the reasons. A repair that keeps the glass is the cheaper route when the frame is sound, and the scope names what was kept and what was replaced.
