Skylights And Velux Windows
On Flat And Pitched Roofs
Fitted, Flashed, Watertight

On a slate or tiled slope, the roof window sits in a trimmed opening and is weathered with a flashing kit made for that covering: a slate kit for slate and thin coverings, a profiled tile kit for deep concrete or clay tiles, and a combination kit where two or more windows sit side by side. Using the wrong kit is the single most common cause of a leaking Velux, because the side and head flashings simply do not sit down onto the profile.

Underneath the visible flashing, the underlay collar has to be dressed into the opening and lapped so any water getting past the covering runs out over the top of the flashing, not into the roof. That layer is invisible once the job is finished, which is exactly why it gets skipped.

Where a proprietary kit is not appropriate, a heritage or listed roof, an unusual covering, or a conservation rooflight sitting flush with the slate, the surround is formed in lead instead: soakers, side flashings and a head detail dressed by hand, which is work we carry out routinely.

A flat-roof rooflight should never sit on the deck. It sits on an insulated timber kerb, typically 150mm or so above finished level, so that the covering can be dressed up the sides of the kerb and terminated above any water that ever sits on the roof. The unit then sits on the kerb with its own upstand and seal.

The covering matters as much as the kerb. EPDM, GRP fibreglass, felt and liquid systems each have their own detail for turning up a kerb and forming a corner, and this is where cheap installations fail: an unbonded corner or a wrinkled upstand behind a lantern will leak long before the flat area does.

Falls come first. If the deck is laid level or falls towards the rooflight, water will pond against the upstand permanently. Where we find that, the falls are corrected as part of the work, using tapered insulation or firrings, rather than fitting a new unit into a roof that will kill it.

Old roof windows fail in predictable ways: misted double glazing where the seal has gone, rotten timber frames on older units, hinges and catches worn out, and flashings that have corroded or lifted. A like-for-like replacement into the existing opening is usually straightforward and can often be done in a day, with the covering around the opening renewed at the same time.

For leaks, we test rather than guess. Water is applied progressively, low to high, until the entry point shows, because a rooflight that only leaks in driving rain is usually failing at the head or the side lap, and one that leaks in any rain is usually failing at the sill or the covering below it. Once we know where it is coming in, we repair the actual detail, whether that is a re-flash, a new underlay collar, lead work or making good the covering.

The cheapest time to add rooflights is while the roof is already open. If you are having a new roof, a rebuild, a flat roof replacement or a loft conversion, adding rooflights at that stage avoids a second access charge and a second disturbance to the covering, and the openings can be planned with the structure rather than cut into it afterwards.

On loft conversions we position rooflights around head height, sight lines and the escape requirements for the room, and set them out so they line up with each other and with the windows below. A roof window fitted at the wrong height is a permanent irritation that costs nothing to avoid at the planning stage.