How is a chimney flashed correctly?
The base flashing laps the covering below it, and the counterflashing sits in the masonry above, so water runs over the base and not into the joint.
A chimney is flashed in two layers. The base flashing laps the covering on the downhill side and wraps the chimney, and the counterflashing is set into a groove in the masonry above. Water running down the chimney face is shed over the base.
When the counterflashing was set in mortar only, or the groove has cracked, the joint opens. The roofer resets the counterflashing and re-seals the groove where the product allows.
- Base flashing laps the covering downhill
- Counterflashing set into the masonry above
- Groove and mortar checked for cracks
Why does flashing fail when the covering is sound?
Because flashing is a joint, and joints move, age and crack. The covering can be sound while the joint behind it leaks.
Flashing expands and contracts, and the sealant that holds it cracks where the movement is greatest. The shingles beside the flashing can look sound while the joint behind them lets water through.
Wind lifts a marginal lap and can bend a flashing out of line in minutes. The flashing is checked after a storm as well as in a routine look.
- Temperature movement cracks the sealant first
- Wind lifts a marginal lap
- A sound covering can hide a failed joint
When should flashing be replaced rather than repaired?
When the metal is split or rusted through, when the lap was set the wrong way or when it has been patched several times.
Most lifted or cracked joints can be reset and sealed. Flashing that is rusted through or split along a fold needs replacing, because each patch adds a joint that can fail.
Ask for the detail to be written down so the next roofer can see the lap.
- Metal rusted through or split
- Flashing patched more than once
- A lap set the wrong way from the start
Keeping this part of the record current
The record for this part of the roof is updated at each visit, with the date and the result, so the history stays accurate.
A homeowner who keeps this section with the roof file can compare it with the next season's check, and the change between the two is the most useful thing the record can show.
The roofer confirms each point on the day and writes the result down before leaving, so the record reflects the roof as it was found rather than as it was remembered later.
