Lead Flashing vs Mortar Fillets

What a mortar fillet is

A fillet is a triangular bead of sand and cement mortar laid in the angle between a roof covering and a wall. It is often found on terraces and extensions built in a hurry, or added by a builder as a cheap repair where a flashing was missing. The idea is simple: the mortar fills the gap and makes a slope that sheds water. In the first few months, it looks tidy and does the job. The trouble starts later, because the materials on each side of it behave differently from the mortar and from each other.

Why fillets crack

Cement mortar is hard and brittle, and it does not stretch. Roofs, however, move. Timbers shrink and swell with moisture, tiles expand in the sun and contract in the frost, and a new extension may settle a little against an older house. A fillet is stuck rigidly to both the wall and the roof, so any relative movement between them tears it. The crack is often a hairline at first, running along the edge where mortar meets brick, and it lets in water that runs down behind the fillet into the roof. Frost then works into the crack and enlarges it. Within a few winters, pieces of the fillet drop away. Because the leak is behind a solid-looking bead, it can be hard to spot until the ceiling below is stained.

Why lead does it better

Lead flashing is a metal strip held in a groove cut in the wall, which is called a chase, and dressed down over the roof covering with a lap. It is not stuck to the roof at all, so the roof is free to move underneath it. The groove holds the top edge mechanically, with wedges, and the lap sheds water onto the roof. Lead is soft enough to take up minor movement without cracking, and it can be shaped to follow the courses of a stepped wall or the angle round a chimney. Our page on how lead flashing is fixed into brickwork sets out the steps, and the question of sealant is explored in whether lead flashing can be repaired with sealant.

The best practice position is that mortar alone is not relied on to keep a joint watertight; the same thinking lies behind the move to mechanical fixing of ridge and hip tiles. A fillet may be reasonable as a temporary cover, but lead, with a proper chase, is the lasting answer.

When a fillet is acceptable

There are a few cases where a mortar fillet has been traditional, such as in some old stone-built cottages and on certain old roofs where the original builder used lime mortar fillets, sometimes called cement fillets in later repairs. On a listed building, the conservation officer may ask for like-for-like work, and a lime fillet may then be right. Elsewhere, a fillet is mainly a patch. It is not a good option on a modern extension, where the movement is larger and there is usually a damp-proof course or cavity tray to deal with, and our page on lead flashing on an extension shows the proper detail.

What a failing fillet looks like

You can often spot trouble from the ground or from an upstairs window. Look for a grey bead along the roof edge with visible cracks, a gap between the mortar and the brickwork, loose crumbs of mortar in the gutter, or green algae streaking below it where water has been running out. Inside, a damp patch on the wall or ceiling next to the roof edge is the signal that water is getting behind. Tapping a fillet that has lost its grip will often sound hollow. If you see any of these signs, have it looked at before the next wet season, as repairs get dearer once timber has been wetted.

Replacing a fillet with lead

The job is straightforward in principle. The old fillet is broken off carefully so as not to damage the tiles or bricks, the wall is cleaned, a chase is cut in a mortar joint above the roof line, and lead is wedged and dressed. Stepped flashing or soakers follow the slope of the roof where it meets a gable or side wall. The brick or tile is made good where the old fillet pulled away. Allow for some hidden repairs, such as rotten timber or damp insulation behind the fillet, which only show once it has gone.

We photograph the existing fillet and the joints around it, and we say plainly whether the lead needs doing now or whether the fillet has years left. More on the subject sits in the lead roofing and leadwork hub, and our main page covers everything else. To arrange a survey, call 01256 213956.