Which flashings keep water out of a roof, and when should they be replaced?

When replacing a gutter, inspect the eaves flashings too. Moving the gutter up, down or away from the edge can change where water lands. Even a rust-free eaves detail needs checking: water must drain from both layers and any ventilation inlet must remain open.

Verge flashings cover the side edges. Their fixings must resist wind, especially near corners. A loose flashing may rattle and bend; a rigid improvised joint can restrain movement. Select the shape and fixing positions to suit the covering and its supporting structure.

A ridge cap protects the joint between the two slopes. On a ventilated roof the air still needs an outlet. The metal cap alone does not make the junction complete: seals, protection from wind-driven rain and connections to the sheets all matter. Closing every gap can defeat the ventilation design.

A valley collects water from two slopes. An abutment flashing channels it along a higher wall. A chimney needs an upslope back gutter or flashing, side flashings and a downslope apron. These junctions differ in shape and water load. A colour-matched angle is no substitute for a properly detailed junction.

A hatch, roof window, ventilation duct or new wall changes the path of runoff. Check that it does not send water into an existing lap. Long pockets and narrow outlets trap leaves and slow drainage. The junction should remain accessible for inspection and cleaning.

Draw the roof and label each edge by function. For example, a simple gable roof with two 9.40 m eaves, a 9.40 m ridge and four 5.10 m verge edges has 18.80 m of eaves and 20.40 m of verges. These are measured lengths before laps or cutting allowances.

Count pieces using their supplied length and the overlap at each joint. In a worked example, five 2 m lengths with four overlaps of 0.10 m cover 9.60 m. The 0.10 m is an assumption for this calculation only. Use the lap and joint detail specified for the actual system.

Look for corrosion holes, cracked folds, permanent bends and enlarged fixing holes. Check areas patched repeatedly with sealant. Even with little rust, a flashing that traps water may need replacing. Minor damage to a finish does not necessarily justify replacing a whole run.

A higher profile, thicker roof build-up or repositioned gutter may make the old flashing unusable. Do not force it into place. Measure the completed substrate and allow for the selected system. For an unusual junction, trial-fit a piece before making the full run.

After fitting, remove swarf and offcuts, especially in valleys and behind chimneys where they can be hard to spot. During use, check the fixings, finish and drainage. Thin flashings are not general-purpose brackets for cables or equipment; these need their own suitable fixings.

An inspection after high winds can catch a loose part before it causes a major leak. Photograph the condition from a safe position. When cleaning, avoid directing a strong jet under laps. Before touching up a coating, remove the cause of damage, prepare the surface and choose a compatible product.