What’s the flap over head flashings?

By - , Build 77

It may seem too much bother, but adding that extra layer of defence around your windows early on, means less chance of having to fix leaks further down the track.

Figure 2: Window head detail (low wind zone).
Figure 1: This window opening has had the underlay taped to the trimmers. This means any water running down it will go behind the flashing – not a good idea.
Figure 3: Position of underlay ‘flaps’ over window heads.
Figure 4: Window head — rigid underlay (low wind zone).
Figure 1: This window opening has had the underlay taped to the trimmers. This means any water running down it will go behind the flashing – not a good idea.
Figure 2: Window head detail (low wind zone).

Have you struggled getting the head flashing up behind the underlay lately? (By ‘underlay’ I mean any building paper or synthetic wrap or ply or fibre cement sheet – anything behind the cladding.) Lets face it, some claddings make it very difficult, it looks terrible until covered up and is another potential tear in the secondary weather defence. There’s no disputing it’s a good practice but there’s more than one way to skin a cat.

The point is to make sure any water that runs down the underlay is caught by the flashing and drained to the outside. The problem is that it’s not always easy to do (especially with a rigid underlay). If the number of calls received by the BRANZ Help Line is any indication, it sounds like one of those little jobs that gets ‘left out’.

You can see how the problem arises in the photo in Figure 1. The underlay has been turned in around the edges of the opening trimmers and taped. It doesn’t make sense to then cut into the underlay for the head flashing (we’ll deal with sill flashings another time).

One way to solve this problem is to use a flap of underlay. The underlay flap works for claddings fixed directly to the frame as well as for cavities. Lay over a strip of underlay. Instead of cutting into the underlay, leave the upturn of the head flashing on the outside of the underlay and lay another strip of underlay over it, as shown in Figure 2.

Figure 3: Position of underlay ‘flaps’ over window heads.
Figure 4: Window head — rigid underlay (low wind zone).

Extend flap vertically to a natural break. For a single-storey building it’s not far to extend the flap to the top of the wall. For a double storey (or higher), it’s simply a matter of extending the flap to the next lap in the underlay. See Figure 3.

Extend flap horizontally past opening. It’s a good idea to make sure the flap extends beyond the end of the opening by a handspan or so, just to be on the safe side.

Use a butyl strip with rigid underlay. If using a rigid underlay, it’s a good idea to fold in a ‘Z’ flashing along the horizontal joins, anyway, using a strip of butyl. A flap of underlay can then be folded in under the butyl at the next convenient horizontal line above. See Figure 4.

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Articles are correct at the time of publication but may have since become outdated.

Figure 2: Window head detail (low wind zone).
Figure 1: This window opening has had the underlay taped to the trimmers. This means any water running down it will go behind the flashing – not a good idea.
Figure 3: Position of underlay ‘flaps’ over window heads.
Figure 4: Window head — rigid underlay (low wind zone).

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