Crack on wall - should I be worried?

A crack on one of the walls of my house has appeared to grow in length and slightly in width over the course of this year. Previously it's been the same for years, so am not sure why it should suddenly grow again.
I have a 1950s house with a cavity wall. The crack is on the inner wall (I can't see anything on the outside wall) and runs from one of the bottom corners of my (upstairs) landing window diagonally upwards across the flank wall for a good 3 feet.
At the moment the crack appears small in width, ranging from a fraction of a mm to 1mm - but it is so noticeable because of the position. It's just in one of those pieces of walls that you look at every time you use the stairs!
The crack also seems to increase in 'width' depending upon whether the sun is shining on that part of the house.
Does anyone think I should be worried? All of the reading I have done suggests that if it were serious - like subsidence - it would likely manifest itself on the outside walls as well and would likely be near the ground. I did wonder whether it was just the roof shifting slightly for some reason and causing the load to be re-distributed?
I don't really want to get a surveyor in because of the cost for what may be such a trivial problem - houses after all get cracks! But I am looking at moving and don't want it to cause a problem with a sale.
Anyone got any ideas or comments?
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It might be worth getting an engineer in to have a look at it. If you think it is still moving, then it's best to catch it now, or at least have it looked at to make sure.
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A 1mm crack isn't a great cause for concern IMHO, though the fact it has opened up recently after years of being stable means it's worth keeping an eye on. You could mark and date the end of the crack for instance.
My brother's house has a seasonal crack in it and it makes its presence felt by rippling the wallpaper in one corner of the loo in summer: come winter its gone again.
The things to try and look at, IMO, are whether you have a water leak under the house which is undermining things. Sometimes you can hear a hiss even when all the water is off. Have you a water meter you can inspect over a period when you use no water? The other possibilty might be a leaky drain. Depending on your drain layout it might be possible to block the drain at an inspection pit near your boundary wall and fill the drain up and see if the level goes down over a period of an hour. Easy to do if you have an outside gully etc.
Also, have you trees or shrubs or creepers planted near the house that are coming on? Some trees etc are worse than others at extracting water from the soil. You may have to cut something down to be on the safe side.
Another sort of subsidence is differential mass subsidence, where one wall of a house is heavier than the others ( fewer windows/doors ) and that will can sink relative to the rest.
A crack running from a window corner is a classic. Stress is concentrated in the corners of apertures. You may well get a crack in the outer skin eventually, not necessarily exactly in the same place as the inside crack.
Another type of crack is due to wall ties rusting, though that shouldn't affect a '50's house, and the cracks would be horizontal and periodic.
Another is due to the roof ties 'relaxing' and allowing the roof to sag under its own weight, which will spread it out, and I believe that will try and tilt the walls outwards near the top. I understand this typically gives horizontal cracks.
Yet another type of crack is due to a lack of wall ties keeping the inner and outer leaves of bricks together, and also if the first floor floorboards are not tied to the inner brick leaf in some positive way, just using the inner leaf to take the weight of the first floor. If the floorboards/joists are bolted to the inner brick leaf then the tendency for the wall to bend is much reduced.
These are just some ideas for you to mull over: some of them are unlikely causes of diagonal cracks, but it is a complicated subject and if I were you I would keep an eye on things, try and read up on the subject a bit, find a way to monitor the crack quantitatively, and try and eliminate some of the options I've listed.
It may just be that you are on clay soil in the southeast and the dry spring has taken too much water out of the soil.
I'm not an expert.
Andy.
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If you are concerned about it, multiply that concern by a factor of many in the minds, (or slimy negotiating actions), in potential buyers. The cost of a structural engineers report is insignificant in relation to the cost of not doing it.
A few hundred quid buys you a report which you can show to buyers to confirm that it is a trivial problem, or the confidence to fill the crack and paint over it.
Alternatively, it identifies a problem which you will need to deal with before you sell.
--
Richard Faulkner

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Richard Faulkner wrote:

Doing that would only set the alarm bells ringing. 1mm is not an issue. Try polyfilla. Cost 1.
NT
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Thanks for the replies so far guys.
I looked at this crack this morning and it was hardly visible, but last night after the sun was shining on the wall all day it looked like a gaping cavern!. I guess some of it is down to perspective - the minute you notice something like this, you can't help but notice it continually!
I have looked around a bit closer and can see a horizontal crack running accross the same wall in the room next door a few inches below the ceiling. It runs right accross the wall in this room. Again it's quite small in width at the moment.
Given all of this it does seem to point to being the fact that the roof has spread slightly and maybe has pulled the top few layers of bricks out. That would certainly fit in with the long horizontal crack I have found, and perhaps explains the other one.
I may well get an engineer in now to assess the roof - of course that leads to another problem! Anyone know the difference between a Chartered Surveyor(which seems to be the people advertising in the main) and a structural engineer? Would I get a better service from one than the other? or are they the same 'thing' listing under different guises?
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You want a structural engineer. www.yell.com is your friend.
Al
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The way to tell if it is actually getting bigger or moving is to araldite a strip of glass or ceramic tile across the crack. Any movement will crack the glass/tile.
Dave
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You need a Structural Engineer. A Chartered Surveyor would take your money, express some concern over the cracks, and pass the buck, suggesting that you get a Structural Engineer to inspect and comment.
--
Richard Faulkner

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Richard Faulkner wrote:

Or just inform your insurance company. I've heard they don't consider anything you can't put your fist in as a crack
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Pasta wrote:

the day you employ a struc eng to diagnose a 1mm crack is the day youve lost all perspective.
NT
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snipped-for-privacy@meeow.co.uk writes

If he wasnt thinking of selling, I would tend to suggest a "wait and see" approach. However, the day most buyers see a 1mm crack, is the day they use it to negotiate the price down. An engineers report is a small price to pay to remove the doubt, and the negotiation.
Regds
--
Richard Faulkner

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As another poster said, getting in a structural engineer to look at a 1mm crack is rather taking a sledgehammer to crack a nut. If I were you ( I have a 1/4" wide crack by the way, eeeeeh,you're lucky! ) I'd just apply a bit of logic and observation to the crack for the time being. If it widens markedly in a short amount of time ( sub one year, say ) then by all means get someone to take a look.
Given your observations so far, I would glue some thin watch glass over the crack at right angles to it ( I actually found that if I heated up some shattered light bulb glass I had with a gas burner I could stretch it flat and thin - ideal ) with epoxy and note the date. As the OP that suggested this idea says, the glass is brittle and will crack with any movement.
Second, I would see if there is some sort of correlation between sunshine and crack width, or even seasonality and crack width: that'll give you a big clue if there is a connection. I don't know that there'd be anything to see in your attic, if it is roof spreading, but it might be worth taking a gander up there and minutely inspecting the beams and their ties and seating to see if there are any clues. It may not be possible to see minute movements in a timber structure.
And of course, you could search the web to see if there is any professional ( or otherwise ) info out there on this subject.
Andy.
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wrote:

Possibly some movement in the lintel.
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Pasta wrote:

Near drought cionditions + clay soil + tree growth nearby = shrinking soil...?

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