What causes a roof to leak only after heavy rain?

Victoria's rain is not heavy by BC standards, about 608 mm a year at Gonzales, but it arrives in concentrated southerly and south-easterly storms between October and April. A south-east facing chimney in James Bay or a west-facing wall dormer in Esquimalt takes rain sideways at speed during those storms, and a counter-flashing that shed a hundred quiet drizzles fails in one afternoon. The same roof is dry in a Vancouver-style steady rain, which is why owners say it 'only leaks sometimes'.

Tree litter is the other local trigger. Douglas fir needles and Garry oak leaves fill valleys and gutters through October, and the first big storm of November finds the valley packed and the gutter overflowing at the corner nearest the tree. The water climbs over the underlayment at the eave or pushes up the valley metal. Shaded Saanich, Highlands and Cordova Bay roofs see this almost every year; the fix is often clearing and a valley re-detail rather than a re-roof.

Note the wind direction on the day it leaked; a leak that appears only in a south-easterly points to the south-east faces and their flashings. Inside the attic, if it is safely reachable, follow the water stain uphill along the rafter or sheathing; the entry point is usually higher and further toward the ridge than the drip. From the ground with binoculars, look for a lifted ridge cap, a chimney cricket full of debris, a gutter overflowing at one corner, or a skylight with moss around its head. Give all of this to the roofer; it cuts the search time and the bill.

• Record wind direction and how hard it rained
• Trace the stain uphill in the attic
• Check gutters and valleys for overflow marks
• Compare the drip location with chimneys, vents, walls and skylights

Shingles fail by looking bad long before they leak; curling, bald patches and cracks are the warning. The places water actually gets through are the joints: where the roof meets a chimney, a wall, a pipe, a skylight, or itself in a valley. On a roof under 15 years old a heavy-rain leak is nearly always a joint and is repairable. On a roof past 20 years, one of these leaks may indicate several joints are ageing together, and a professional may reasonably suggest evaluating the whole roof rather than patching one spot in December and another in February.

Clear gutters and valleys in late October after the trees drop, and again in December. Have a roofer check chimney and wall flashings every few years on a pre-1950 home and re-seal counter-flashing before it lifts. Replace pipe boots proactively at 10 to 15 years. On flat sections, keep scuppers and drains clear before every forecast storm. None of this costs much next to a saturated ceiling and a January emergency call.