Vertical crack in render on extension

16 Mar 2010
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United Kingdom

About three years ago we had a two storey extension on the side of our 1960's semi.

The original house is half rendered, and the extension has also been rendered (I believe it was a magnolia tyrolean) and built with Thermalite bricks. On the rear of the extension, I have noticed a vertical crack down the entire render where the extension meets the original building. Unfortunately I do not know when this crack first appeared, but the extension has only been built for about three years.

The crack is relatively narrow (approx 1mm), but I am concerned that it runs down the entire property. Is this anything to worry about or is this settlement in the extension?

Many thanks in advance...

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An update, the house is 75% rendered with brick at the bottom 25%. the crack does down the render and continues to the ground on the mortar where the extention joins the main building.

Any advise would be very grateful!

Sounds to me as if it's a movement crack between the old and new build. Most jobs nowaday would have a movement joint, filled with a coloured flexible mastic, running down the joint, from top to bottom, to allow for any slight movement between the two structures.
Excuse my ignorance, so as I have this vertical crack running the entire length of the wall suggest that an expansion joint hasn't been used?

Is this of concern or do I just need to fill the gap with mastic?

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Theres defo no movement joint in the render. Have the bricks cracked or does the crack follow the brick mortar joints? Its possible the bozo renderer (is that a word) rendered over a movement joint in the blockwork. As long as its tied in properly there's nowt to worry about. Probably wait a few years until all setlement has finished (hopefully) then repair it and repaint. Not much you can do about it now really.
Thanks for the reply.

As far as I can see there are no cracks in the bricks (only visible bricks are at the bottom of the extension). The crack appears to follow the mortar line.

So should I leave it a couple more years and then is it just a case of using a mastic to fill the gap in and paint over?

You havent got ths going on in the brickwork beneath the crack in render have you?


Rack out and fill the crack with render repair mortar but only if the movement has stopped.
Apologies for the update on my post from a few months ago, however I have been able to take some pics. Also, to add to the crack issue, I have now noticed a diagonal crack in the dry lining on the front of the house.

I would very much appreciate it if someone could take a look at the pics and let me know if I need to be worrying. For awareness, the extension has been built about three years now.

Many Thanks

You will nearly always get some movement between old and new when extensions are done. If no movement joints are provided then you end up with the cracks you have shown.

As freddymercurystwin mentioned, after a few years, any settlement should just about be done. Then you can fill and redecorate. There is little point in bothering using a mastic filler now since this is what would have hidden the intial settlement.

After redecorating you may still get a crack but I would only expect it to a hairline one.

Uneven or wide cracks would be be something to monitor, but from what your photo's show I would not be worried.
Thanks for the reply, it is very much appreciated.

The extension is only 6 foot wide, so would this be why no expansion joint appears to have been used?

Thanks again,

The extension is only 6 foot wide, so would this be why no expansion joint appears to have been used?

No, the size of the extension has little to do with it. It's just that for the very reason you have got some cracks it's often better to do a movement joint to avoid them! I did a small extension on my own home and opted not to do an movement joint. So all I got were a few minor cracks in the first year or so and then no more movement and no feature joint to show the two parts of the building were built at different times.

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