How much roof flashing should you order?

A dimensioned sketch is usually enough for a simple building. For a complex roof, use the drawings and check the measurements on site. Two pieces can look alike yet have different bend angles or a coating on opposite faces. An identical description on the invoice does not make them interchangeable.

The eaves flashing directs water from the roof into the gutter. A separate piece may protect the fascia. The detail must also show how the underlay ends and, where required, where air enters the ventilation space. Order the parts shown in that particular eaves section.

Check how the parts fit together before adding them to the order. Closing the eaves too tightly can restrict ventilation; a wrongly turned edge can send water behind the gutter. Agree the joint detail with the roofer before specifying lengths and blank widths.

For example, take 2 m lengths and assume a 0.10 m overlap solely for this calculation. The first piece covers 2 m; each additional piece adds 1.90 m. Five pieces cover 9.60 m. That is enough for a 9.40 m run before end details are taken into account.

Dividing 9.40 by 2 ignores the joints. Use the overlap specified for your own roof detail. Some locations also need folded edges, expansion joints or extra pieces. Never reduce a required overlap to make the final piece reach.

A verge flashing protects the side of the roof covering and helps secure its edge against wind. A valley carries water from two roof slopes. These functions call for different sections and fixings. Keep the designed drainage channel in the valley clear.

List each run separately and mark left- or right-handed parts where the profile is asymmetric. Check how each valley meets the eaves. Two short valleys may need more end pieces than one long valley with the same total length.

Blank width is the width of the flat strip before folding, not the visible width of the finished flashing. Dimension each leg, bend angle and hem on the sketch. On a custom order, mark which face is coated.

If you use an old flashing as a pattern, check for distortion. Do not reproduce a bend caused by removal. The replacement must fit the current roof build-up; different insulation thickness or a repositioned gutter may require a revised section even if the roof looks much as before.

The order is not complete until it includes the specified fasteners, seals and accessories. Colour alone is no guide. Select them for the supporting material, the thermal movement required and the water exposure at the joint.

Check that adjoining materials are compatible: some combinations of metals promote corrosion. Agree how cut edges will be protected. Send the approved drawing and reference list with the delivery so that the crew can sort pieces by location as they unload them.

Count the pieces, match their sections against the list and inspect the coating and long edges. Watch for similar-looking parts intended for opposite sides of the roof. Discovering a mismatch now leaves time to clarify the order or obtain the missing part.

Store the pieces so that they cannot slide against each other. Swarf and sharp edges can damage the adjacent coating. Keep the schedule with the handover documents: years later it will identify the section to reorder, while the colour alone will not.