What Is Roof Flashing?

Overview: What Is Roof Flashing?

Roof flashing is impervious (watertight) sheet of lead, copper, zinc, rigid bitumen sheet, tin, plastic or aluminium, shaped and installed at roof junctions to prevent the penetration of rain and snow. The most commonly used material is sheet lead, and it will be on this material that this chapter will mainly concentrate.

Proprietary roof flashing (for example, roof window flashings) should not be modified, cut nor used in any way that might conflict with the manufacturer’s instructions, since there could be a risk of leaks, and the conditions of any guarantee or warranty would be broken.

To get the roof flashing right is essential because they keep the junctions watertight, so we would suggest that anything more complicated than installing soakers, simple saddles or pipe roof flashings should really be done by an experienced roofer, plumber or lead worker.

Lead Sizes and Codes

Sheet lead is sold in rolls from 150mm wide and in lengths from 3m upwards. The most common widths used in slating and tiling are 150, 225, 300, 375 and 450mm. The code of the lead is an indication of its thickness (and therefore weight). This will be shown by a strip of coloured tape stuck to the roll. The following codes are the ones most commonly used in slating and tiling:

Code 3 (green; 1.32mm minimum thickness) for soakers and other work not directly exposed to the weather. This is the thinnest and lightest of the codes that we use, it needs to be thin because bulky soakers would lift the slate or tile; it is the lightest because weight is not important for soakers since there is no exposure to wind uplift.

Code 4 (blue; 1.80mm minimum thickness) for all roof flashing such as saddles and step flashings; these flashings are exposed to the weather and so have to be more durable and heavier to resist the elements.

Code 5 (red; 2.24mm minimum thickness) used where there is expected to be a high concentration of water, as in the valleys and box gutters.

Colour-coded lead roll.

Colour-coded lead roll.

Anti-patination oil should be applied with a cloth to prevent weeping white stains (a natural discharge from the lead when exposed) wherever it is likely to run on to the tiles or slates. This should be done as soon as the new lead is installed.

At side abutments there are three types of roof flashing: soakers, step flashings and secret gutters.

Step Flashings and Soakers

Step flashing over soakers.

All double-lap materials such as plain tiles, fibre-cement slates and natural slates use soakers and step flashing. The soakers are cut to size by using the formula:

length: gauge + headlap + 25mm; width: 175mm (unless otherwise stated; 100mm on to the slate or tile and 75mm upstand)

Step flashings for double-lap materials such as slates and plain tiles are usually cut from 150mm code 4 lead. The flashing is cut so that 25mm is turned into the brickwork and a ‘water line’ of 65mm is retained, below which the lead should not be cut. The step roof flashing is installed over the soakers to form a watertight junction in lengths of no more than 1.5m, with minimal overlaps of 150mm. The flashing is secured by using lead wedges and a pointing material, usually mortar, although mastic is actually preferable as it copes better with the expansion and contraction of the lead as the temperature varies.

Step and Cover Flashings

Step and cover roof flashing for interlocking tiles.

Step flashings for interlocking tiles require the lead to be wider so that it extends on to the tiles and over the first roll. Soakers are unsuitable and unnecessary for this type of work. Widths of 300 to 375mm are normally used, depending on the required width.

Top Edges

Flashings under windows, at the top edges of porches and integral garages, for example, should be fixed in 1.5m lengths with 150mm overlap at the side. The depth of flashing and therefore the roll width is determined by the height of the relevant brickwork course (minimal upstand of 75mm) + 25mm for the turn into the joints + enough coverage on to the top course to maintain the minimum headlap. Often straps and/or folds are used for greater resistance to wind uplift.

Top edge abutment flashings.

Top edge abutment flashings.

Open valleys use code 4 or 5 lead in maximum lengths of 1.5m with 150mm minimal laps. The lead is ‘welted’ or folded back at the side edges to prevent any penetrating rain from entering the roof. The most commonly used roll widths for valleys are normally 375 or 450mm. Interlocking tiles will need an undercloak at the sides for bedding mortar on to lead valleys, otherwise the mortar will crack away as the lead expands and contracts.Valleys

Lead-lined valley.

Lead-lined valley.

Soil and Vent Pipes

Lead slates for soil and vent pipes are usually made from code 4 lead. A size of 400mm x 400mm is usually adequate and 150mm is recommended as the minimal upstand and for all sidelaps. When installing a lead slate it is important to make sure that all the lead underneath the tile or slate courses is supported so that it cannot sag. Welting (folding) the back and concealed side edges by about 25mm gives extra protection against driving rain. Once the tiles have been cut and the lead slate installed, a collar should be fitted (often with a mastic bead first) over the pipe to seal the joint.


Saddles are sections of lead sheet used to weather junctions like the tops of valleys, where the ridge comes into a wall, for instance. Simple saddles are usually formed from a single piece of lead and simply bossed or dressed into shape, but more complicated ones may need to be lead-welded.
Typical lead slate.

Typical lead slate.