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 with the temperature, and the sealant that holds it moves with the metal and cracks where the movement is greatest. Over time the joint opens, and water follows the opening under the covering. The shingles beside the flashing can look sound while the joint behind them lets water through.
Wind adds to the problem. It lifts a marginal lap, and a windstorm can bend a flashing out of line in minutes. The flashing is checked after a windstorm 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
How is a chimney flashed correctly?
The base flashing laps the covering below it, and the counterflashing is set into 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 sits in a groove in the masonry above. Water running down the chimney face is shed over the base and out over the covering.
When the counterflashing has been set in mortar only, or the groove has cracked, the joint opens. The roofer resets the counterflashing and checks the base lap.
- Base flashing laps the covering downhill
- Counterflashing set into the masonry above
- Groove and mortar checked for cracks
When should flashing be replaced rather than repaired?
When the metal is split or rusted through, when the lap was set wrong from the start or when it has been patched several times.
Most lifted or cracked joints can be reset and sealed. Flashing that is rusted through, split along a fold or patched more than once needs replacing, because each patch adds a joint that can fail. A roofer who replaces the flashing says so in the scope and names the material.
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
Where wet weather finds the flashing first
At the lower edge of the step flashing, at the back of a chimney and at vent collars, where water sits longest after rain.
Water on a roof runs down and to the edges. It stops longest at the lower edge of step flashing, at the back of a chimney where the counterflashing meets the masonry, and at the base of a vent collar. Those are the places a wet season tests first, and they are the places the roofer checks from the attic before the rains.
A stain on the wall line below the flashing, or a damp patch on the chimney breast in the attic, points to the joint that is failing. The roofer marks it on the plan and tests it with water.
- The lower edge of step flashing
- The back of a chimney at the counterflashing
- The base of each vent collar
Choosing a flashing material for a wet site
The material is chosen for its resistance to moisture and to the cleaning the roof needs, and the choice is written on the record.
Flashing in a wet, mossy setting must resist moisture and the cleaning it gets. The roofer names the material on the scope, and the record says why it was chosen for the site, so a later repair uses a compatible product.
A material that is incompatible with the covering can corrode at the joint, so the compatibility is checked before any part is fixed.
- Choose for moisture and cleaning resistance
- Name the material in the scope
- Check compatibility with the covering
