How do I stop a roof leak temporarily in heavy rain?

Most Greater Victoria leaks announce themselves in the same window: the first big atmospheric-river storm of late October or November, after a dry summer in which a lifted flashing, a cracked pipe boot or a moss-dammed valley went unnoticed. December is the wettest month at about 109 mm at Gonzales, and roofers are at their busiest on emergency calls then, so a same-day tarp is easier to get in the first week of a storm sequence than the third.

Where the water comes in tells you something about the house. On 1910s to 1940s homes in Fernwood, James Bay and Victoria West it is usually the brick chimney or a wall intersection under a dormer. On 1960s and 1970s Saanich and Colwood homes it is often a plumbing vent boot that has cracked, or a valley packed with fir needles from a neighbouring tree. On flat-roofed additions and carports in Gordon Head and Esquimalt, a blocked drain or scupper is the first thing a roofer checks. Photograph the drip and the ceiling before you touch anything; the insurer and the roofer both want the pictures.

Roofers themselves work under WorkSafeBC fall-protection rules once they are 3 m off the ground, and on a wet, mossy Victoria roof the surface is as slick as ice. A homeowner on a ladder in a December storm is the most common way a leak becomes an injury. A professional tarp is fastened with battens along the ridge and weighted or screwed at the edges, which is why it holds through wind; a sheet thrown over the roof from a ladder does not, and can channel water somewhere new.

Find the drip's entry point in the attic if you have a walkable path: water often runs down a rafter or the underside of the sheathing a metre or more from where it drips. Put a tray or a piece of plastic on the joists under the wet spot and a container on it. Mark the wet sheathing with tape so the roofer sees it. Then look outside from the ground: a missing shingle, a lifted ridge cap or a gutter overflowing at one corner narrows the search.

• Container under the drip, plastic over insulation
• Tape the wet sheathing so it can be found dry
• Note whether the drip stops when the rain stops
• Check whether the leak lines up with a chimney, vent, skylight or wall

A tarp buys a few weeks, not a season, and a roofer will usually schedule the proper repair for the next dry break. Once the source is found, common Victoria repairs are a flashing rebuild at a chimney or wall ($400 to $1,500 in our model), a new pipe boot ($250 to $600), a skylight re-flash ($300 to $900) or a small shingle patch ($350 to $900). If the roof is past 15 to 20 years and the leak is one of several, ask for a repair-or-replace opinion rather than a fourth patch.