Roof leak: how to find it, what repair costs, and when it means a whole new roof

Water travels. It enters high, runs along a rafter or the underside of the deck, and drops wherever gravity finally wins — which is almost never directly under the hole. That single fact explains most repairs that fix nothing and leaks that return with the next storm.

Where leaks actually start

Shingles in the middle of a slope rarely fail first. Leaks begin where the roof isinterrupted:

• Flashing at chimneys, skylights and walls — the most common source by a wide margin
• Valleys, where two slopes meet and the water volume is highest
• Penetrations: vent pipes, whose rubber boots crack and perish years before the shingles do
• Eaves, where an ice dam pushes meltwater backwards, uphill, under the shingles
• Nail pops from an earlier installation, each one a small hole

Finding it from inside

On a dry day, go into the attic with a torch and look for the trail rather than the hole: dark staining on the underside of the deck, rusted nail shanks, damp or compressed insulation, daylight around a penetration. Follow the staining upward to its highest point. That is where the water came in.

Stay off the roof. More people are hurt inspecting a leak than by the leak, and a wet roof with unknown decking underneath is exactly the wrong place to find out where the soft spot is.

What a repair actually costs

• A cracked vent boot: low hundreds. The part is cheap and the labour is short.
• Re-flashing a chimney or skylight: several hundred to low four figures, depending on access and whether masonry work is involved.
• A valley: four figures, because shingles either side have to come off to do it properly.
• Decking replacement: priced per sheet plus labour, and it is only discovered once the roof is opened up.

The pattern is that the patch is cheap and the opening up is not. Which is why two small leaks in different places often cost more than one large repair.

The four signs it is replacement, not repair

1. Age. An asphalt roof in its final few years will keep producing leaks. Each repair buys months.
2. Leaks in more than one place. One failure is an event. Three is a roof telling you something.
3. Granules in the gutters. The mineral surface is what protects the asphalt from UV. Once it is washing off in quantity, the shingles are near the end.
4. A deck that feels soft. If the roof gives underfoot, water has been present long enough to rot the substrate, and patching the surface does not address it.

There is also the plain arithmetic: when repairs are approaching a meaningful share of a replacement, and the roof is more than about two-thirds through its expected life, repairing is usually the more expensive choice over five years.

What to do tonight, if it is actively leaking

• Put a bucket under it, and move what is underneath.
• If the ceiling is bulging, pierce the low point with a screwdriver to let the water out in a controlled place. A bulge is pounds of water, and a collapsed ceiling costs far more than the hole you made.
• Photograph everything, dated, before anyone touches it — whether or not you plan to claim.
• Call in the morning. Emergency tarping is a real service, but night rates for what a tarp achieves are rarely worth it unless water is pouring in.