Windows and doors: flashing, sill details and why they leak

Most window leaks are not the window's fault. They are failures at the edge, where the frame meets the wall. A frame can be tested to resist water and still leak when it is installed without a sill flashing, with the wrong sealant doing the work of a flashing, or with a gap that is not supported. Because a leak at a window runs into the wall and the framing before it shows inside, the damage tends to be found late.

The window and the opening are two different things

• The window unit is tested as a product. In Australia the standard for windows and external glazed doors is AS 2047, which covers materials, testing and installation of windows. The NCC calls it up so that windows resist wind and water penetration.
• The opening is a building detail. It is made of the wall structure, a membrane or sarking, flashings, the sealant and the cladding. Whether water reaches the inside depends on how these are put together, in what order, and whether the water has an easy way out.

A good window in a poor opening will leak. Our post on glazing and frame connections deals with the structural side, and where buildings leak lists the usual junctions.

What the NCC requires for masonry openings

For masonry veneer and cavity masonry in houses, NCC 2022 Housing Provisions Part 5.7 sets out how damp-proof courses and flashings are installed at openings (clause 5.7.4). The points that matter most are:

• A flashing is required at the bottom and top of windows and doors. There is an exception for the top of an opening protected by an eave wider than three times the height of masonry veneer above the opening.
• The sill and head flashing must extend not less than 150 mm beyond the reveals on each side of the opening.
• It must be turned up in the cavity not less than 150 mm above the opening, and be embedded not less than 30 mm into the masonry.
• It must be attached to the window or wall framing, and be continuous and visible from the outside face.
• Weepholes are not required beneath window and door sills.

These are the minimums for a standard detail. Frames in other types of wall have their own requirements, which should be set out by the designer and the cladding manufacturer.

Where leaks usually start

• No sill flashing, or one that does not turn up at the ends. Water that gets past the frame has nowhere to go but into the wall.
• Sealant used instead of flashing. Sealant ages, shrinks and cracks. It is a second line of defence, not the first. See choosing silicone sealant.
• Flashings installed in the wrong order. The sill goes on first, then the jambs, then the head, each lapping so water is directed out and over the layer below. A reversed lap draws water in.
• Gaps and unsupported frames. Large gaps filled with foam or sealant, or glazed doors installed with gaps the product was never tested for.
• Thresholds. Doors to balconies and decks are a double risk, because the water is on both sides of the sill. A threshold needs a sub-sill flashing and a sloped, waterproofed floor behind it.
• Frame and membrane not connected. The wall membrane or sarking must lap onto the flashings and the frame, so the whole opening works as one system. See waterproofing the building envelope.
• Dissimilar materials. Aluminium against some timber, mortar or treated products can corrode or stain. See material compatibility.
• Exposure. A window on an exposed or high-rise elevation sees much more driving rain than one in a sheltered street. The wind and water rating must match the site.

Before the frames go in: questions to ask

1. Which standard is each window tested to, and what rating is on the data sheet?
2. What is the sill detail? Can we see a drawing?
3. What flashing material is being used, and how far does it extend and turn up?
4. How is the wall membrane lapped onto the flashings?
5. What is the threshold detail on balcony doors?
6. Who will inspect the flashings before the cladding or render covers them?

The last question matters most. Once the cladding is on, the flashings are gone from sight. The approach we describe in the handover record post is to inspect and photograph flashings at a hold point before they are covered, so there is a record if a leak shows up later.

Renovations and replacement windows

Swapping a window in an existing wall is where flashing errors are most common, because the old flashings are often missing or damaged and the new frame is sized to the old opening. If you are replacing windows:

• ask the installer to open up enough of the surrounding wall to see what is there,
• do not accept sealant as the only barrier at the sill,
• check whether approval or notification applies (see kitchen renovation approvals for how approvals work), and
• in a strata building, check who owns the window and what the by-laws require.

If a leak does appear, see what happens when hidden defects are found and the statutory warranty rules in statutory warranties and defects after handover.

Working with us

We detail, install and inspect window and door openings on renovation and new-build work, and we investigate leaks in strata buildings through our Strata Inspections & Repairs service. To talk about your project, get in touch.

Sources

• NCC 2022 Housing Provisions, Part 5.7 Weatherproofing of masonry, Australian Building Codes Board
• AS 2047-2014 Windows and external glazed doors in buildings, Standards Australia

This article is general information, not technical, legal or engineering advice. Requirements differ by wall type, building class and exposure, so follow the designer's details, the manufacturer's installation instructions and the advice of your certifier. Full website disclaimer