Leaking flat roof


We have a "flat roof" (actually a very slight pitch) which is made from lead with folded seams.

It is rectangular, with a slight slope toward one long side where there is a gutter with a downspout at either end.

We had our first proper rain in a while today, and it was torrential (and is still going, and forecast to be continuous for the next 24 hours). I quickly discovered that one of the aforementioned downspouts was blocked, and that the blockage was under the footpath between the bottom of the downspout and the outlet to the street gutter, so I detached the bottom of the downspout so that the water could freely run onto the footpath.

There are several leaks into the rooms underneath the flat roof. I think these are exacerbated by the fact that the downspout was blocked, but obviously they are not caused by that.

How can I find the cause of leaks in this roof? Some of the seams have been flattened by people walking on them over the years — would that cause leaks? There are no other obvious signs of damage, but I'm not sure what I'm looking for.

Best Answer

Lead makes a good roof, but finding leaks in one (or any flat roof) can be a real bear as the leak below may be a long way from the leak above.

If the folded seams are (as I suspect from your description) standing up, except where walked on and flattened, then yes, the walked-on parts would be prime suspects for leakage. Sometimes the the seams are all folded down and soldered, but standing seams are not usually also soldered (it's a fair bit of fiddly torch work.)

Look very carefully for cracks right next to the flattened seams. If you don't see any cracks, run a torch over the flattened area (trying to anneal the metal before reworking it) and carefully stand the seam back up - look for cracks on the lower side as well. If there are cracks you (or a lead roof specialist you hire) can clean the area and solder them.

If there is a wall above the flat roof area, also look very carefully at the flashing where the wall joins the roof - likewise look very carefully around any penetrations in the roof.

Some folks will no doubt tell you to reroof, since lead is "old-fashioned" and therefore also "old" - depending on the shape the roof is in this might be the right thing to do, but both lead and terne-plate (lead coated steel) roofs can last for hundreds of years, so it is well worth trying to fix it first, IMHO.