Cracks Where an Old House Meets a New Extension

There is a particular crack that brings people to us more than any other. It runs down the inside corner where the original house meets the extension, usually from the ceiling, sometimes all the way to the skirting. Occasionally the floor upstairs has gone slightly uneven to go with it.

It is one of the most common things we are asked to look at, and most of the time the explanation is reassuring. But it is worth understanding, because the same crack can mean two very different things.

Why the junction is where cracks turn up

An extension is not really an extension of your house. It is a second building, joined to the first and asked to behave as though it were one.

The two halves are rarely alike. The original foundations may be shallow Victorian brick footings. The new ones will have been dug to modern depths, often considerably deeper, into ground carrying a different load. The walls are usually different too, in material, thickness and age.

Put two structures like that side by side and they will settle at different rates. They always have. The junction between them is simply where that difference shows up first.

What the crack is telling you

Width and shape matter more than length. A hairline crack that has been there since the plaster dried, has not changed, and is the same width top to bottom is usually shrinkage or thermal movement doing exactly what it always does.

A crack that is wider at one end than the other is more interesting, because tapering suggests rotation rather than simple shrinkage. One side is moving relative to the other.

A crack that reopens every time it is filled, or that is clearly wider this winter than it was last, is telling you the movement has not finished. That is the one worth acting on.

Was the extension tied in, or just butted up against it?

This is the question we get asked most often once people start reading about it, and it is a good one.

Some extensions are tied into the original wall with wall starters or toothed-in brickwork, so the two move together. Others are deliberately left to move independently, with a movement joint at the junction, sealed and often hidden behind a bead.

Both are legitimate. What causes trouble is the third option: tied together rigidly when the two halves were always going to settle differently, so the new work drags the old along with it until something gives. From the inside you usually cannot tell which you have got. That is what an investigation is for.

The ground usually matters more than the brickwork

Most movement at a junction starts below it rather than within it.

Clay soil shrinks as it dries and swells as it wets, and much of the East of England sits on it. A tree close to the extension, a long dry summer, a leaking drain, a soakaway in the wrong place, all of these change the moisture in the ground and the ground changes shape in response.

Because the new foundation and the old one are at different depths, they do not feel that change equally. One moves, the other does not, and the junction takes the difference.

When a crack is worth worrying about

Not every crack needs an engineer. These are the ones that do:

• It is wide enough to get a fingernail into.
• It tapers noticeably, wider at one end than the other.
• It passes through the brickwork outside as well as the plaster inside.
• Doors or windows nearby have started sticking, or a floor has gone out of level.
• It is getting wider over months rather than staying put.

One of those on its own is worth watching. Two or three together is worth a phone call.

What an engineer actually does about it

A structural survey starts with the crack but does not end there. We look at the pattern across the whole building, inside and out, because movement rarely shows in only one place. We look at the age and likely depth of both foundations, the soil, the drainage and the trees.

Where it is not clear from the surface, a trial pit alongside the foundation shows exactly what is down there and how deep it goes. That usually settles the question in an afternoon.

What you get back is a written report that says what is happening, whether it is still happening, and what if anything needs doing. That report is also what a lender, an insurer or a buyer’s solicitor will want to see.

The good news

The large majority of junction cracks we are called to turn out to be historic settlement that finished years ago. The fix is filling and redecorating, and the useful part of the exercise is being able to prove it rather than hope it.

Where there is live movement, catching it early is what keeps it from becoming underpinning. The cost of finding out is small. The cost of assuming is not.

Ora Labora Structural Engineers carry out structural surveys on extensions, period properties and new builds across Peterborough, London and the East of England. If you have a crack you are unsure about, send us a photo with something in it for scale and tell us how long it has been there.