Lead flashing pulling out of the wall

1How a flashing is meant to be held in a wall

Flashing is the lead dressed over the gap where a tiled slope ends at brickwork. Its upper edge is turned into the brickwork, not stuck to the face. A groove, called a chase or raked joint, is cut along a mortar course, the lead is folded into it, and small lead wedges pin it in place before the joint is filled.

The mortar is therefore a lock rather than a glue. It stops the wedged lead from lifting out, and it keeps rain from getting into the groove. The lead underneath is then free to move a little with the weather, which is what lets it last.

The wedges do the holding and the filling closes the joint over them. Our answer on what lead wedges are explains why a flashing without them relies on the filling alone.

2Why the joint lets go

Mortar ages. Over many winters it softens, crumbles at the face and shrinks away from the lead, especially in a chimney stack that takes the full weather. Frost, driven rain and the small movement between a roof and a wall all work on it.

Earlier repairs can add to the trouble. A cement fillet or a line of sealant laid on top of a joint hides its condition and traps water behind. When the filling cracks, the lead has nothing holding it, and the first strong gust can flick it loose.

Chimneys are the usual place, since they stand clear of the roof and the pointing is exposed on all sides, but any wall abutment can fail. Lead that has been stolen or cut back can leave the wall joint open too, and our page on lead theft covers what follows a loss.

3What loose flashing lets in

A lifted edge does not always leak at once. In light rain the lead still sheds most of the water. In a gale, rain blown sideways gets behind the lead and trickles down the inner face of the brickwork, onto the soakers and the timbers beneath. The result is a stain on a chimney breast, a damp corner of a ceiling or a patch beside the gable wall that appears only in bad weather.

The water seldom comes in where the stain is. We follow the marks up the timbers and match them to the outside, and the failed joint is often a short distance above. The hidden pieces underneath, the soakers, matter too.

A white crust at the joint may look alarming, but it is usually a surface deposit from the lead or from damp brickwork, and it is not the cause of the leak.

4The repair, in the order it is done

The sound way to fix a loose flashing is to open the joint. The tired mortar is chiselled back to a sound depth, the lead is checked for splits and straightened, and fresh wedges are tapped in at regular intervals so it cannot sag. The joint is then repointed in a mortar that suits the wall. On older brickwork we match the existing mix and do not harden it.

Sealant has a place, as a finish over a properly wedged and pointed joint where the wall is uneven. It is not a substitute for the wedges or the mortar, and a joint that depends on it will be back in a few seasons.

If the lead itself is torn, thin or badly creased, renewing that run costs a little more but is the right answer. Cement fillets, the hard beads of mortar some builders use instead of lead, are a case in point: they crack along the line where roof and wall move apart, and a lead flashing is the better detail. The rest of our lead roofing pages go further.

5When pointing alone is the wrong answer

Re-pointing a joint is only worthwhile when the lead is sound. If the flashing is the wrong weight, short in length, lifted by movement in the stack or cracked along its folds, new mortar will hide the problem for a season. Replacement of that run is the better choice, and sometimes a different metal is worth weighing; lead or zinc sets out the differences.

The survey photographs show the lead, the joint and the wall in close detail, so you can see why we are proposing a repoint or a renewal. The quote that follows is written, carries one total figure and covers the smallest job that properly shuts the water out.