A tar patch is one way to stop damage from a water leak across the chimney, but it isn’t the appropriate way to fix the problem. In case your immediate options are limited, however, it’s a reasonable stop-gap measure until you’re able to make the required long-term repairs.
It is Only Temporary
Putting tar across the base of a chimney can halt a water flow, but just temporarily. Over time the nest will dry out and break, letting water through again. A tar patch or seal is far better than allowing a leak to last, but consider it just a short-term remedy till you’re able to complete a correct repair, like installing new flashing around the chimney or installing a new roof along with new flashing.
Holes, gaps and rusted flashing are the actual culprits behind leaks around the chimney. Before putting a nest stain, find the problem with the flashing and repair it as much as you can by tacking down any loose borders or adding silicone caulk to gaps. Make the flashing as watertight as possible. It may have to be replaced, but tacking it down and redesigning with tar can supply a temporary fix.
The area around the chimney has to be clean and dry for the nest to form a watertight seal. Brush away any debris, like leaves and twigs, and wipe the area down with a wet rag and then let it dry. If the area is wet, rubbed up as much water as you can and then let it dry completely before adding the nest.
Spreading It Out
Spread a thin layer of tar across the base of the chimney using a plastic trowel, taking care to seal around the edges of the flashing and the adjacent roofing. Add another layer, spreading this one out further, at least 6 inches past the edge of the flashing. Press strips of cotton fabric to the nest so that it’s totally saturated. Then add a second thin layer of tar using the trowel, spreading it well beyond the borders of the fabric.