All articles · 5 min read

Why are the walls cracking after we removed a wall?

Some cracks are normal settling in plaster and joints. Others are a sign the structure is moving — and those need an engineer to look at them right away.

The normal cracks: the plaster follows the structure

When a load-bearing wall is removed and the load is transferred onto a beam, the structure settles a little. That is expected: the beam is allowed to deflect within a limit the engineer has designed for. But plaster, filler and paint film are stiff and brittle, and they do not move without cracking.

So this is normal in the first few weeks to months:

• Hairline cracks in the filled joints between plasterboard sheets, especially in the ceiling near the new opening.
• A crack where ceiling meets wall, where two materials move differently.
• Cracks in the acrylic sealant run along architraves and skirting boards.
• A fine crack where new and old plaster meet.

What they have in common is that they are thin (under about 1 mm), stop growing after a few weeks, and sit in the surface — not in the masonry itself. They are fixed with filler and a coat of paint once the movement has stopped. That is why you should not do the final paint job the day after the temporary support comes down: wait a few weeks, so the repair only has to be done once.

The alarming cracks: five signs that cannot wait

1. The width. If the crack is wider than about 2 mm — wide enough to fit the edge of a credit card or a fingernail into — it is not a surface crack.
2. It is growing. Put a dated pencil mark at each end of the crack, and measure the width with a ruler or a strip of tape. If it keeps growing over the weeks, the structure is still moving.
3. It is diagonal and starts at a corner of the new opening. A crack running upward at an angle from the corner of a new opening is the classic sign that the load is not being spread the way it should be. It needs to be looked at.
4. It runs through masonry, not through filler. A crack that follows the mortar joints in a brick wall in a stepped pattern, or that runs straight through a brick, is a structural crack.
5. Doors bind, floors slope. If a door suddenly will not close, or a marble now visibly rolls across the floor, something has shifted vertically. It is the most tangible sign of all.

If you see one or more of these five signs, contact the structural engineer who did the calculations — and the contractor — straight away, and in writing. Do not wait for it to get worse; documenting when you first noticed it matters if this ends up being treated as a defects case.

The typical causes

• The bearing point is not carrying enough. The beam rests on masonry or a pier that is not strong enough for the concentrated load, and that spot is pressed down. That produces diagonal cracks above and beside the bearing point.
• No path further down. The load has been carried down to a floor where there is nothing beneath the bearing point — for example a basement with no column in the right place.
• The beam was sized for strength, not for deflection. It does not break, but it bends more than the structure above can tolerate.
• The temporary support was removed too early, or the bearing point had not finished curing before the load was applied.
• No engineer was ever involved. The most common cause of all. A contractor "who has done this many times before" guessed at a beam size.

How to document the cracks so someone can act on them

What an engineer needs is time, location and progression. Five minutes of work gives them the basis they need:

1. Photograph the crack with something in the shot that gives a sense of scale — a folding ruler, a coin, a credit card.
2. Measure the width at its widest point, and note the date.
3. Put dated pencil marks at each end, so you can see whether it is growing in length.
4. Mark the crack on the floor plan — which room, which wall, which height. A crack described as "in the living room by the door" cannot be found again in three months.
5. Repeat after 4 weeks using the same method. Two dated measurements are worth ten times as much as one.

If you have a measured floor plan of the home — from a LiDAR scan, or drawn on graph paper in HouseSense — you can mark each crack's location on the plan and save photos alongside it. Then you, the engineer, and any future buyer five years from now all have a coherent picture of what happened when. The app cannot assess the crack; what it can do is make sure whoever does assess it knows exactly where it is and how it has developed.

How to avoid them in the first place

• Bring in a structural engineer before the wall is touched. If the wall is load-bearing, the calculation is not a formality — it is what decides whether cracks appear. Whether a wall is load-bearing cannot be decided by you or by an app; that has to be assessed on site by a building surveyor or an engineer. Work on a home's load-bearing structure also has to be approved by the authorities before it starts.
• Ask for deflection to be part of the sizing calculation, not just the load capacity. Ask directly: "How much will the beam deflect under full load?"
• Leave the temporary support in place until it is meant to come down, and let the structure settle before the final filling and painting.
• Use reinforcing tape in every new joint between old and new plasterboard — it will not reduce the movement, but it stops the crack from telegraphing through the paint.
• Agree to a review after 3 and 12 months as part of the contract, in writing. That is when cracks show up — not at handover.
• Photograph the rooms BEFORE the renovation. Many cracks were already there. Without a "before" photo, nobody can decide what the renovation actually caused.

That last point is the cheapest advice in this whole article: walk round with your phone before the contractors arrive, and photograph every wall and ceiling in the rooms that will be affected. It takes five minutes, and it can settle an argument about liability a year later.