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Cracks Where An Extension Joins The House
A vertical crack has opened along the line where a new extension meets the original wall.
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What's happening?
This is the most common crack in a house that has been extended, and in most cases it is not a defect. An extension is a new, lightly loaded structure on new foundations, sitting against an old, settled one on shallower footings. The two are almost never the same depth, they carry different loads, and they respond differently to seasonal ground movement. Something has to give, and it gives along the junction. A fine, straight vertical crack on that line that stabilises within the first year or two is behaving as expected. What is not expected is a crack that keeps widening, steps into the brickwork away from the junction, or comes with the extension floor or roof dropping.
What usually causes it?
- Differential settlement between new foundations and the original shallower footings
- No movement joint designed into the junction, so the two structures crack instead of sliding past each other
- New masonry drying and shrinking in its first heating season
- Extension foundations taken to a different depth from the main house, so seasonal clay movement affects them differently
- The new roof or floor loading the old wall in a way it was not built for
- Poor tie-in at the junction — starter ties omitted, or the old wall not properly toothed and bonded
- A tree or drain near the junction affecting one foundation and not the other
Is it dangerous?
Usually low. A hairline vertical crack on the junction line that has settled down is cosmetic, and it is common enough that surveyors expect to see it. Raise your assessment if the crack is over about 3mm and still growing, if it steps diagonally into the brickwork away from the junction, if the extension has visibly dropped relative to the house, if the roof junction has opened, or if there is now a gap you can see daylight through. Those say the two structures are moving apart rather than simply settling against each other.
Can I fix it myself?
Monitoring and patience are the right first response, and you can do both. Mark and photograph the crack with a dated ruler and check it every month for a year through a full cycle of seasons. Internally, filling a stabilised junction crack with a flexible filler rather than a rigid one is sensible, because the joint will always move a little. Do not fill it rigidly in the first year and do not repoint the outside face solid while it is still moving — it will simply crack again in the same place.
When should you call a builder?
Call the builder who did the work first if the extension is recent and still within any warranty, and get it recorded. Call a builder or engineer independently if the crack is growing, is wider than 3mm, steps into the brickwork, or the extension has dropped. If the extension was built under building control, ask for the completion certificate and raise the crack with the building control body while the file is open.
What will the builder actually do?
- Establish whether the crack is on the junction line only, or extends into the masonry of either structure
- Check the extension's foundation depth and design against the original, opening a trial pit if necessary
- Monitor the crack over a full seasonal cycle before proposing a permanent repair
- Form a proper movement joint at the junction, with a compressible filler and a sealed external face
- Stitch the masonry with helical bars where the two structures genuinely need tying together
- Make good the render, plaster and external pointing in a way that allows the joint to keep moving
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