[Q] Cracks in extension

Hiya
A single storey extension was added to my house around 15 years ago. The layout is as follows (excuse the ASCII art)
Top view of back of house (dotted line shows where the original outside wall was):
| | | | |___________ ____| | . 5| | . | | . | | 2. . 1. | | . | | . | |_______________.____________|
Back view of house:
_____________________________ | __________ 6 _______ | | | | | | | | | 4. | | | | | | | | | | | |__________| | 3. | | | | | | | | | | |_________________|_______|__|
1. The extension, which 'squares out' the back of the house. 2. Original single storey kitchen. 3. French doors to back garden. 4. Window.
The new walls are joined to the old using those metal plates which you screw onto one wall, and key into with the other (what are those things called?)
I've noticed that at point 5 there is a crack running down the length of the wall in the corner, which seems to be slightly more pronounced at the top. On the outside wall (in the new brick) these is a crack running around the bricks from the top of the wall to the french doors (6). These is a crack on the inside in more or less the same place.
I know that the extension foundations were inspected by the BCO and found to be satisfactory (same depth as house foundations). Is this kind of movement to be expected? Does it meant the foundations are inadequate? How might I find out if this is a problem?
I'm certainly haven't noticed the cracks getting any bigger (currently a few hair widths) so it might have been something that happened earlier on in its 'life'.
Any comments/suggestions?
Cheers,
-- Jon
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Movement is expected in all builds, but the amount of movement over time is what causes problems. Filling them over and redecorating will cover them up, but if they begin to open again in a short space of time, then it's a call to your builder and architect that you need to do. When a crack begins to open rapidly it's deemed as dangerous, but if small narrow cracks appear over many years it's normal. That's why polyfiller was invented. :-))
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Movement (and resultant cracking) is normal and expected. The joints at the wall should have been filled with a flexible mastic to accommodate movement with out cracking the joint.
The external ground will move depending on the seasons and the weather, the extension will move with it.
If the crack becomes progressively wider then that is the time to investigate, but hairline cracks are nothing to worry about.
Crack at '6' is normally movement in the lintel, but this again is normal and could have been caused when built. Again just monitor. But a little clear silicon externally will reseal it and wont show up like repointing will.
dg

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snipped-for-privacy@dogma.mcc.ac.uk wrote:

Thanks to dg and BigWallop for your (reassuring :-) answers.
My next question: although the BCO inspected the foundations, there is still an outstanding inspection on the final product (been on the books for ~15 years it seems). I presume it would be in my interests to fill in and decorate over the internal cracks before getting the inspection done?
Cheers,
Jon
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Possibly not. If the BCO is due a visit, then leave the cracks to show him what is happening. He'll be able to put your mind at ease if he's allowed to see the cracks and tell you if it is normal or if there is a problem. Don't try to hide it if someone in the know is on their way to inspect it. They may pick up a potential problem that we can see or don't know about in your area. That's what these guys are there for.
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Hiya

Hmm. I was worried that it looks worse than it (probably) is since it hasn't been decorated since it was built. Having to dig up the foundations would be a nightmare (probably have to go through the kitchen floor :-( ).
Cheers,
Jon
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Better to dig up the kitchen floor than dig your children out of 50 tonnes of rubble. (OTOH, it's probably OK as it is).
Christian.
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The BCO would have inspected at stages and it is common for the final one to be missed. But it is just a formality as the previous inspections are the important ones.
A properly designed foundation, properly built and inspected and approved, can still move and cause cracking. The BCO would not be really interested in your minor cracking after the works are completed.
After all - would you expect him to say that the work he has inspected and approved is defective?
dg

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