What is a roof window flashing kit?

What is in it

An apron at the bottom, carrying water from the window onto the covering below.

Soakers up each side, interleaved with the covering course by course so water running down the roof is directed around the window.

A head flashing at the top, taking water from the covering above and directing it to the sides.

An underlay collar, connecting the roofing membrane to the frame, so anything that gets past the tiles is carried around the opening and back onto the membrane below. This is the part nobody sees and the part that saves you in driving rain.

Why the profile matters

Kits are made for the covering they have to lap.

Flat profile for slate and plain tile, which lie flat.

Deep profile for concrete interlocking tiles and pantiles, which have a roll. The sections are shaped to bridge that roll.

Fit a flat kit to a profiled tile and there is a gap under every roll. It will not leak on the day. It leaks in the first driving rain from the wrong direction, and it is the most common defect we are called to correct.

Combination kits

For two or more windows side by side, weathering the space between them. Two single kits butted together is not the same thing and will leak in the gap.

Low pitch kits

For roofs below about 15 degrees, with deeper laps, usually down to around 10. Below that a roof window is the wrong product.

The parts that weather the opening

A flashing kit is the set of metal components that seal the junction between a roof window and the surrounding covering. It is supplied separately from the window, it is size specific and covering specific, and it is where most rooflight problems originate.

A typical kit has an apron at the sill, side sections or soakers running up each side, a head flashing at the top, and on some systems a drainage gutter above the window that carries water around it.

Matching the kit to the covering

Kits come matched to the covering, and they are not interchangeable.

Flat profile for slate and other flat coverings, where the flashing lies directly against a smooth surface.

Deep profile for concrete and clay interlocking tiles, where the tile has a raised profile with a channel running down it. The flashing has to be formed to dress into and over those channels.

Specific patterns for pantiles and other shaped coverings.

A flat kit fitted to a profiled tile leaves open voids under the tile channels where it crosses them. Water gets under within a season, and because it enters below the flashing it appears inside as a leak that seems to come from nowhere.

This is the most common installation fault we find on other people’s work, and it is exactly where a cheap quotation economises, because the kit is a separate order line and nobody can see which one was used once the job is finished.

What it is not

It is not the underlay collar. That is a separate component, and its job is to carry water running down the roof membrane around the opening rather than into it. A perfect flashing kit over a badly lapped underlay collar still leaks.

It is not the insulation collar either, which sits inside and closes the gap between frame and structure.

All three are needed and all three are separate lines.

The sequence

Underlay collar first, dressed and lapped. Then the flashing kit. Then the covering reinstated over the flashing, never under it.

Reversing that order is a guaranteed leak, and none of it is visible once the work is finished.

What to ask

Which kit is specified for your covering, by name and part number, and whether it is included in the price. A quotation that cannot answer that has not been written from a survey.

On a replacement, a new kit should be included as a matter of course. Reusing a flashing that has been on the roof for the same twenty five years as the window that just failed is a false economy, and on an exposed roof it is where the next leak comes from.

What it does

It weathers the junction between the window frame and the roof covering, turning water away from the frame and back onto the roof.

It is a set of pressed metal components made for a specific window size: side sections that interleave with the covering, an apron at the sill, and a head flashing at the top with the covering above lapping over it.

The profile error that causes leaks

Kits come in profiles matched to the covering type, and this is the most common specification error on a rooflight job.

Flat profile for slate and plain tile, where the covering lies flat and the flashing can sit tight against it.

Deep profile for pantiles and profiled interlocking tiles, where the flashing has to be dressed down into the troughs and over the rolls.

A flat kit on a deep profile roof cannot follow the tile form. It bridges the rolls and leaves open channels in the troughs, running straight under the flashing. Driven rain finds them.

Kits for two or more units

For two or more windows fitted side by side or one above the other, a combination kit gives a single continuous weathering system with purpose made sections between the units.

It specifies a fixed gap between the openings, which means the structural opening positions are determined before any carpentry starts.

The recessed option

Several manufacturers offer a recessed kit that sets the window lower into the roof plane than a standard one. On a designated building it can be what turns a marginal proposal into an acceptable one, and it is worth naming specifically in a pre application conversation.

What the flashing does not cover

The whole weathering system. The underlay collar below it does the secondary work, and without that dressed and lapped correctly, the flashing is working alone.