Back-kitchen extension leaking where it meets the house: the flashing nobody installed

Straight answer: when a back-kitchen extension leaks along the exact line where its roof meets the original house wall, the cause is almost always the same — the joint was sealed with a cement fillet or a bead of silicone instead of proper metal flashing chased into the wall. Cement and sealant there crack within a year or two, guaranteed, because the metal-deck roof expands and shrinks with every hot afternoon while the brick wall stays put. The lasting fix is flashing let into the wall itself — and it costs less over five years than repainting the kitchen ceiling annually.

This is probably the most common leak in the terrace rows of Serdang, Seri Kembangan, Balakong and Bangi, because nearly every house has the same back extension: a metal-deck or polycarbonate roof sloping away from the original rear wall, built years after the house, by whoever quoted cheapest at the time.

Why the cement fillet always cracks

A four-metre metal-deck sheet grows and shrinks a few millimetres between a blazing 3pm and a stormy night — every single day. Rigid cement cannot move with it, so a hairline crack opens along the joint within the first year or two. Rain running down the big exposed wall above then feeds straight into that hairline. A silicone bead flexes, but UV cooks it: in two or three years it turns chalky and peels at the edges. Both are surface patches on a joint that needs a mechanical cover, not glue.

How to confirm this is your leak

• The ceiling stain runs in a line roughly parallel to the rear wall, near the joint.
• The drip starts during heavy rain — or carries on after the rain stops. Water tracks along the purlins before it falls, so the wet patch can sit a metre or two away from where water actually enters. Chasing the stain instead of the joint is how people reseal the wrong spot three times.
• From a ladder outside, you can usually see it directly: a cracked mortar line or a peeling, chalky sealant bead along the wall.
• Repainting the kitchen ceiling “fixed” it — until the next monsoon season.

The proper fix: flashing chased into the brick

Real wall flashing is an L-shaped metal strip. Its top edge sits inside a groove cut about 25 mm into the wall, sealed into the chase; its skirt dresses down over the roof sheet by a good 100–150 mm, following the ribs of the deck. Because the top edge is inside the wall, water running down the wall is thrown out over the metal and onto the roof — there is no joint facing the sky, and the metal slides freely over the roof sheet as it expands. Done once, done properly, it outlives the roof sheets themselves.

One thing flashing alone will not solve: the gutter at that joint. Many extensions drain into an undersized gutter, or a box gutter squeezed against the neighbour’s wall, and a proper monsoon downpour overflows it and pushes water back up under the sheet. Any contractor quoting for the flashing should look at the gutter fall and outlet size in the same visit — if they don’t mention it, ask.

Honest cost comparison

Is the DIY patch ever worth it?

Honestly, yes — as a stopgap. If you are steady on a ladder, scraping the old bead off, cleaning the joint dry and running a quality polyurethane (not cheap acrylic) sealant will usually buy you a dry monsoon season while you budget for the real fix. No shame in that at all. Just treat it as borrowed time, not a repair — and never do it on a wet joint or a rainy day, because sealant over damp simply lets go quietly.

If the stain is back for the second or third year running, the joint has told you everything it is going to. See our leak and waterproofing work and the price guide for what the proper job involves.