Roof flashings for metal sheets: which parts and how many?

Mark every line that needs closing or draining. An eaves flashing directs water into the gutter, while a separate fascia flashing protects the eaves face. Verge flashing protects the gable edge and a ridge cap covers the junction between slopes. Each has a different purpose even when made from similar sheet metal.

A valley occurs where two slopes meet inwardly. Wall flashing is needed where a roof meets a taller wall, and a chimney requires its own set of components. There is therefore no universal flashing pack for a 100 m² roof. Equal roof areas can have completely different edge lengths.

Distinguish straight and corner components in the schedule. A verge beside a gable differs from an abutment against a taller building, even if both are loosely called side flashing. Include a labelled sketch so the supplier can assign each item to a specific line.

Assume two rectangular slopes, each 10 m along the eaves and 5 m down the pitch. The roof totals 100 m², with two 10 m eaves, one 10 m ridge and four 5 m verge edges. The preliminary schedule therefore contains 20 linear metres of eaves, 20 of verges and 10 of ridge.

This is a geometry example without chimneys, valleys or wall junctions. It does not specify flashing widths or the full ventilation detail. If the eaves need two different strips, calculate each separately; do not order only one because both are informally described as gutter-edge flashing.

For calculation only, assume 2 m flashings and a 0.10 m lap. The first piece covers 2 m; each further piece adds 1.90 m. Five pieces therefore cover 9.60 m, not 10 m, so six are needed for a 10 m line. The lap here is an assumption: the real detail must follow the instructions and roof geometry.

Then check end details, corners and possible reuse of offcuts. A cut piece will not always fit the opposite verge: fold direction and left- or right-hand versions matter. A useful schedule lists both the line length and the number of ordered sections.

Calculate lap allowances from the number of joints. Ten components in one continuous line normally create nine joints. Do not automatically subtract a lap from every piece, including the first. Allow separately for end formations and any returns.

Profile height affects flashing position, as do board thickness, gap width and fixing method. A ready-made component for one system may not fit another. Both simple general-purpose shapes and system-specific flashings are available. Judge suitability against the actual roof cross-section.

Bespoke flashings need a section drawing showing leg lengths, angles, coated side and section length. “Anthracite verge flashing” is insufficient. For an unusual detail, a short trial piece can confirm fit before the whole batch is made.

A watertight roof also needs the designed airflow paths to remain open. Blocking the ridge with unsuitable foam can hinder drying. Protection from rain and insects must work with ventilation. The roofing instructions explain how these functions are combined.

The same applies at the eaves: a water-directing strip must not unintentionally close the air inlet. Select flashings together with the eaves batten, membrane and gutter. Changing one component on site may require changes to the others. Discuss this cross-section with the contractor before delivery.

Developed sheet width, number of folds, length, coating and order size affect cost. Flashings are priced separately from the main covering, either per piece or per linear metre of finished component. Calculating only the square metres of flat sheet does not give the price of fabricated flashing.

Include fasteners and required tapes in the order. Request labels on the bundles. At delivery, compare leg dimensions and coated sides with the drawing. Errors are easier to correct before the crew starts on the roof. Do not simply conceal coating damage beneath a lap without assessing whether repair is permitted.

No. Use edge lengths and the number of details. Area does not reveal the number of valleys, chimneys or dormers.

Not always. Allow for laps, end details and the available component lengths.