Plaster cracks

[thornabylad]

Hi,

We moved into our new house last year and had the kitchen replastered. We also turned the old outhouse into part of the kitchen.

The outhouse is connected to the house; the wall between the kitchen and the outhouse was already knocked down so it was an ideal way to make a bigger kitchen. The outhouse roof is solid re-enforced concrete about 12 inches thick and comes into the house. The outhouse roof supports the bathroom wall above. (7.jpg)

Browning was used on the walls in the outhouse and the ceiling was plasterboarded and skimmed. The existing kitchen was just skimmed.

Over the last 6 months we have noticed hairline cracks appearing all over the kitchen. Especially over windows and were the walls meet the ceiling.

I have uploaded some photos of the cracks (1 to 6.jpg).

http://homepage.ntlworld.com/johnmoore/photos/

At present they seem to be getting worse. We are worried that it may be the early stages of subsidence or that the outhouse roof is strong enough or could it be just vibration or humidity causing the cracks.

Any advice will be much appreciated.

 

[masona]

The outhouse is connected to the house; the wall between the kitchen and the outhouse was already knocked down so it was an ideal way to make a bigger kitchen. The outhouse roof is solid re-enforced concrete about 12 inches thick and comes into the house. The outhouse roof supports the bathroom wall above. (7.jpg)

Are you saying the outbuilding concrete roof section is supporting the wall above ? It looks like 4" thickness to me and not 12" or are you saying it's 12" wide ? Are there any form of lintel above the roofing section ? if not I would be worried and very difficult to see from photo,I think it would be best to get in a surveror to inspect the structure of your building.Also check with your local planning dept and see if the building permission was carried out correctly.If it was done without permission,the last owner could face a heavy fines.

 

[thornabylad]

The concrete roof is exactly 7 inches thick. We have seen other houses in the road who have done the same thing. There is no lintel. We have been advised by a number of builders that the outhouse roof is as strong, if not stronger than a lintel.

So what about the plaster cracks. These cracks are actually no where near where the outhouse roof meets the main house, though they are in the outhouse, above windows and were the walls meet the ceiling.

 

[masona]

The plaster cracks is when it dried out too quickly depending of the room temperature and the absortance on the walls,just use a fine filler.

Approx what is the opening span of the 7" concrete roof ?

 

[thornabylad]

The gap is 6ft wide.

http://homepage.ntlworld.com/johnmoore/photos/8.jpg

 

[masona]

I can't see a problem but when we make a concrete lintel we have to have a analyse concrete block sample tested for stress load pressure test.
Example,supposing the concrete roof section is a weak mix ratio,it would not be acting as a lintel.Get a friendly surveror to inspect it for you for a piece of mind.
I'm afraid there are many builder's who will say "that all right mate" just by looking at it !

 

[thornabylad]

Thanks for that.

Our house is ex-council, "Built like bomb shelters those outhouse roofs", one of our neighbours as said to us. That is why some many people have knocked the walls out and extended into the outhouse.

As we bought the house just over a year ago for the son of the deceased owner, I don't know what channels were pursued regarding building regs.

 

[masona]

I don't know what channels were pursued regarding building regs.

If you pop down to your local planning and ask to have a look at your property drawing plan which is free to look at and see if the plan has been altered,if it hasn't then no building permission was applied.The choice is now up to you,either rock the boat or keep quite.What ever happens you're in the clear but you have to prove it ! and you might have trouble selling it on as they can make you convert it back to how it was and could find your household insurance invalid.I'm not trying to fighten you but rather be honest with you and believe me you're not alone but have now noticed people are now coming unstuck selling it on.

 

[GwaiLo]

Judging by the photographs and what you have said I suspect masona has got the cause of the cracks right.

Browning is used as an undercoat plaster on masonry, but some masonry surfaces have much higher rates of suction (hence they dry the plaster too fast) and in those circumstance you can use High Suction Browning instead.

masona is also correct about the concrete roof carrying the all being a bit of a worry. The thickness of the concrete is not (in itself) any indication of its strength. A beam (or roof acting as one as in this case) is fairly simple to understand with a bit of imagination.

Take a lolly stick, support it at each end, and then press on the middle. The lollystick bends. Now imagine a horizontal line drawn along the centre of the side of the lollystick. The wood above the line is being compressed (or is "in compression") the wood below is being stretched (or is "in tension").

Now concrete is very strong in compression, but very very weak in tension. So in any concrete beam you must have something in the bottom half of the beam to carry the tensile loads....and what you have is steel reinforcement.

Also the further away from the centre of the beam the greater the stresses are (this is why steel beams or RSJ's are shaped so like a letter I with much more steel at the top and bottom than in the centre).

So two things count. The amount of steel reinforcement and the position of the steel reinforcement. The problem is you can't see the reinforcement, so unless you can find out through records that the thing has been designed to take the load then you may have a problem selling in the future.

I regularly carry out pre purchase surveys and what I have seen would make me suggest that a purchaser asks if building regs consent was obtained before he bought the house.

 

[antinator]

the best way to stop the cracks is i find to seal the walls with a part pva glue part water solution and this will seal the old stuff soaking the water in also if you cant be bother to chip of the plaster which is cracked just skim it again and this time add pva solution instead of just water to make the plaster