How a new roof ties into an existing one

The junction where a new extension roof meets the existing house is where extensions leak. It is not a roofing problem so much as a flashing problem, and the correct detail depends on three things: whether the new roof meets a wall or another roof, whether the pitches match, and whether the materials are the same. Get those three right and the junction is unremarkable. Get any of them wrong and it will leak in the first serious storm.

Almost all the guidance available online describes asphalt shingles, roof sheathing, ice-and-water shield and attic ventilation. None of that describes an Australian extension, which is typically concrete tile or steel sheet on battens, with sarking and no attic. The principles transfer; the details do not.

Why this junction is the weak point

Everywhere else on a roof, water travels downhill across overlapping surfaces and off the edge. At the junction with an existing building, three things change at once.

Water is concentrated — a whole roof plane's worth arrives at one line. The junction is usually at or near the low point of the new roof, so it receives everything. And it is the one place where two independent structures meet, each of which moves slightly and independently of the other.

Add that the existing building was never designed with this junction in mind, and you have the highest-risk detail on the project. It is also, from the ground, completely invisible.

The four situations

New roof meets a wall. The most common case, where a lower extension roof runs into the side of the existing house. Water running down the wall has to be directed out onto the new roof rather than behind it.

New roof meets an existing roof plane. The two roofs form a valley, or the new one abuts partway up the old slope. This creates a concentrated drainage line that must be sized for both catchments.

Different pitches. A new roof at a shallower pitch than the existing one, which is extremely common because extensions are often built low to preserve window heights and setbacks.

Different materials. A metal extension roof on a tiled house, or the reverse. Increasingly common and the trickiest of the four.

Flashings, in principle

You do not need to specify these yourself, but knowing what they are makes it possible to tell whether the job has been done properly.

Apron flashing runs along the low edge where a roof meets a wall, turning up the wall and out over the roof surface.

Step flashing is used where a sloping roof meets a wall running up the slope. Individual pieces are lapped, each one stepped up with the courses, so water is always directed outward.

Back gutter or saddle flashing sits at the top edge where a roof runs into a wall, catching everything coming down the wall and diverting it sideways to both sides.

Valley flashing carries the concentrated flow where two roof planes meet.

The single most important characteristic of all of them is the upstand — how far the flashing turns up the wall or under the covering. Too short an upstand is the most common reason a correctly shaped flashing still leaks in wind-driven rain, and it is why an extension can be watertight for two years and then fail in one storm.

Where the flashing has to go

The rule that matters: a flashing must be built into the wall, not just stuck onto it.

Water running down a masonry wall does not only run down the face. It travels within and behind the surface. A flashing sealed to the face of the brickwork intercepts the surface water and lets everything behind it run straight past into the junction.

Correctly done, the flashing is chased into a mortar joint, or turned into the cavity, or lapped behind the cladding, so that water arriving from above is caught rather than bypassed. On a brick veneer wall this also means the cavity flashing and weep holes above the junction need to work, otherwise water inside the cavity is delivered neatly into the new roof space.

This is the detail that most often separates a junction that lasts from one that does not, and it is buried by the time anyone can inspect it.

The pitch problem

Roofing products have minimum pitches. Below that pitch, they no longer shed water reliably, because wind and capillary action overcome the overlap.

Extensions are frequently built low — to sit under an existing window, to stay within a height limit, or simply because it looked better on the drawing. If the resulting pitch is below the minimum for the chosen roof covering, the roof will leak in heavy rain and no amount of flashing will fix it, because the problem is the field of the roof rather than its edges.

Minimum pitches vary by product and profile, and manufacturers publish them. It is worth confirming that the intended covering suits the intended pitch before the roof is framed, not after.

Tile meeting metal

Mixing materials at a junction introduces problems that neither material has alone.

The profiles do not match. Tiles are modular and deeply profiled; sheet roofing is continuous with a shallow rib. Where they meet, the flashing has to be dressed into the tile profile on one side and lapped over the sheet on the other, and the gap under each tile has to be closed.

Dissimilar metals corrode. Certain metals in contact, or where water runs from one onto the other, corrode galvanically. This is a genuine and well-documented failure mode in Australian roofing, and the compatible combinations are specific. It is worth asking the roof plumber to confirm compatibility rather than assuming.

Movement differs. Steel expands and contracts with temperature considerably more than tile does. A rigid junction between them will work loose.

Sarking has to be continuous

Sarking is the second line of defence, and at a junction it needs to be lapped so that water landing on the new roof's sarking is carried out into the gutter, and water arriving from the existing roof is carried onto the new sarking rather than under it.

Where a new roof is tied into an old one that has no sarking at all — common on pre-1990s tile roofs — there is a decision to make about whether to open up and extend it. Leaving the junction with sarking on one side only is a frequent cause of leaks that appear to come from nowhere.

Box gutters

If the design puts a box gutter at the junction, treat it as the highest-risk element in the whole project.

A box gutter sits within the building envelope. When a conventional gutter overflows, water goes over the edge and onto the garden. When a box gutter overflows, it goes into the house.

Three things make one safe: adequate capacity for the catchment it serves, an outlet sized and positioned to clear that volume, and — critically — a genuine overflow provision that discharges outside the building if the primary outlet blocks. Leaves block outlets. A box gutter without an overflow is relying on nothing ever going wrong.

Box gutter design is specialised. It is reasonable to ask who designed it and to what standard.

What to ask, and what to check

1. Ask what the junction detail is before work starts, and ask for it to be drawn rather than described. A junction that cannot be drawn has not been thought through.
2. Confirm the roof pitch suits the covering, against the manufacturer's published minimum, before framing.
3. Ask how the flashing enters the wall — chased, cavity-turned, or lapped behind cladding. If the answer is that it will be sealed to the face, ask why.
4. Ask about metal compatibility wherever two different materials meet or water runs from one to the other.
5. Ask whether a box gutter is involved, and if so, what the overflow provision is.
6. Photograph the junction before the tiles or sheets go on. This is the only opportunity anyone will ever have to see it, and it takes two minutes.
7. Inspect after the first heavy storm, from inside the roof space with a torch, rather than waiting for a stain to appear on the ceiling.

Common questions

My extension only leaks in storms. Is that the junction?

Very likely. Wind-driven rain is exactly the condition that exposes a short upstand or a face-sealed flashing. See why your roof only leaks in heavy rain for how to trace it.

Can I extend with a flat roof against a pitched one?

You can, but a genuinely flat roof needs a membrane rather than a sheet or tile covering, and the junction detail changes accordingly. What you cannot do is use a low-pitch sheet product below its minimum pitch and expect it to behave like a membrane.

Does the existing roof need work too?

Often, yes. Tying a new roof into an old one usually means lifting tiles, extending sarking, and reworking flashings on the existing side. A quote that includes no work on the existing roof at all is worth asking about.

Who is responsible if it leaks later?

Ordinarily the builder, within the defects liability period and the statutory warranty period that applies in your state. Photographs of the junction before it was covered are the most useful evidence you can have in that conversation.

Should an engineer be involved?

For the structure, potentially — see whether you need a structural engineer. For the flashing and drainage detail, the relevant expertise is a roof plumber, and on anything involving box gutters, a hydraulic designer.

Where to go next

The junction is covered permanently within a few days of being built, which makes the window for getting it right very short.

Read next: what lock-up stage means — the junction between new and existing is precisely what determines whether an extension is genuinely weathertight — or what order trades come in on a renovation.

Two minutes with a phone camera before the roof goes on is worth more than any amount of investigation afterwards.