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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.

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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.

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  • Do I need to worry at all about possibly melting the lead with a torch?
    – jbg
    Commented Mar 6, 2014 at 6:26
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    Only if you go nuts with it. So "At all" - yes; "much" - no. Don't work the point of the inner cone of the flame too close to the surface. You might take a bottle of water with you - toss a few drops on the area you've been heating - if it boils away, that's enough (in just heating before reforming.) When/if soldering the solder (which melts before the lead will) will tell you where you are at.
    – Ecnerwal
    Commented Mar 6, 2014 at 15:02
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    This was the problem! I used careful heat and pliers to gently bend all the flattened seams back to standing up straight. There have been several heavy rains since with no leaks at all. Thanks!
    – jbg
    Commented Jun 30, 2014 at 11:10
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    Great - the roof should be good for another couple of centuries now.
    – Ecnerwal
    Commented Jul 1, 2014 at 1:41
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How can I find the cause of leaks in this roof?

I use a water hose to find roof leaks. I run it slow to start, and find the lowest spot I want to test. Sometimes I get my wife to observe from inside, but you can also find something to prop up the hose and go inside yourself. Let it run for at least 5 minutes on that spot. And, wait another 5 minutes or so to allow for it to seep inside. Then move on repeating this process until you locate a leak. Fix it before attempting to locate any other leaks, and make sure it no longer leaks. Oh, one more thing, mark the exact spot of the leak or the next time you go up there you may not be able to find it.

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I'd argue the blocked downspout could have caused the leak. Roofs only work in one plane. If water has a way to go around that plane, it can defeat the roof even though the roof is working probably. In some cases, that means you are missing a flashing you should have but it can just a fact of life of gutters.

An example:

  • ice damming on a metal roof into a gutter can then lift the metal panel (whether because it isn't adhered correctly or because the ice breaks those connections) and ice on top of the sheathing. When the ice melts, it might well leak through cracks between sheathing panels. The metal panel didn't really fail
  • I have a low slope (2%) EPDM roof. I have one scupper that likes to ice dam under the right conditions. If I then get the right conditions (high sun on the roof but cold enough to keep the north facing scupper frozen), the roof can melt into the scupper faster than the ice clogged scupper can drain. Water will back up to unflashed connections that aren't designed to see water and a leak develops.

The point is just that I'd fix your gutter situation and see what happens before I spent a lot of energy on the roof.

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  • Your answer is appealing because of Occam’s Razor (the simplest explanation tends to be correct), but how far do you expect a leak at the gutter could travel uphill away from the gutter? The leaks are in a few spots throughout the area under the roof, some of which are quite far (and thus elevated) from the gutter. The slope isn’t extreme, probably only 2%, but still…
    – jbg
    Commented Mar 5, 2014 at 9:10
  • I don't know. But capillary action between two tight surfaces could move water against gravity a significant distance.
    – Keith Hoffman
    Commented Mar 12, 2014 at 3:22

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