Cracks in Lounge ceiling.

[philp1863]

About 18 years ago we purchased a bungalow. We then went into the attic and added two large bedrooms and a bathroom. Over the years we have noticed that the lounge ceiling which is below the biggest bedroom upstairs has slowly developed cracks along the plasterboard joins. Also the lounge window which is nearly 8ft wide has also developed cracks going diagonally from the end of the lintel towards the ceiling. These cracks are mainly only hairline and can easily be filled/painted over.
I have asked a couple of builders for there comments and they have both said that they do not believe it is due to any subsidence. All these cracks are mainly above the window.
The room below the smaller sized bedroom does not have any cracks, but its window is a lot smaller.

My main query is regarding the main joists that have been put across the ceiling of the lounge to support the floor of the bedroom above. These joists have been placed in parallel with the original much smaller joists of the original ceiling(which supports the plasterboard). From memory, one end of these joists sits on the inner cavity wall(the 4x2 wall plate sitting on the brickwork) and the other end is supported by a steel RSJ (slotted into its side). There is a small attic area that I can squeeze into to look at the joists sitting on the wall. I remember looking at this a year or so ago and noticed that some wooden cheeses had been used to support these joists on the wall plate(probably used to level the joists?). Some of these cheeses where loose. I was not very impressed with these being loose (these seem to be a bit of a bodge). Have not tried to contact the builder, but I do know that he started concentrating as a kitchen designer/fitter not long after finishing our extension.

As a total novice and using simple logic guesswork I am assuming that the long wooded joists going across the width of the bungalow probably shrink and expand depending on the temperature of the bungalow. This I assume would probably cause sideways pressure to be put on the supporting wall that the joists are sitting on. This in turn might be causing the cracking to occur especially at the ends of the lintel of the 8ft window. To back this idea up, I believe on very rare occasions in the past I have heard a cracking or banging sound of something giving under pressure.

Ok my question is this, should the joists just be sitting on this inner wall of the cavity(the 4x2), or should some other means have been used to allow for the expansion of these joists? Could the expansion of these joists be actually causing the problem? What might be done to prevent this from happening?

At the end of the day I probably will have to employ a structured engineer to help me find a solution to this problem, but thought first that it might be useful to get any ideas or thoughts from the forum.

Thanks for your help

 

[CJRatch]

A structural engineer for hairline cracks along plasterboard joins? Don't waste your money.

The method that's been used to support the joists sounds correct. It can't be anything serious it's been standing for 18 years!

It's probably over the years the nails or screws holding the boards have worked loose. Fit a few more plasterboard screws pulling the boards tight and either reskim or fill and make good.

 

[^woody^]

A properly designed and built loft conversion floor, would not touch the existing ceiling or ceiling joists, so any movement or loading would not crack the ceiling. If the new floor is connected to the old ceiling in any way, then that's a potential cause of the cracking.

Alternatively, ceilings do crack over time for one of several natural reasons and this is common and normal.

Fine ceiling cracking is invariably difficult to correct, and may well reappear through any repair

Typically you would only be concerned if the ceiling had noticeably bowed and the floor become bouncy

It's Ok to put joists on a wall as described.

 

[philp1863]

Thanks CJRatch and woody for taking the time to give such very helpful responses. Its good to know that the joists have been added correctly(providing that I am correct in what I remember). Since the cracks in the ceiling follow the plasterboard, we will simply paint them or fill if necessary and see how long it takes for them to reappear. I think that the cracks around the top of the window could be a little more worrying, maybe??? One of the builders mentioned above did suggest that I expose the brickwork where a crack was near the lintel of the window. I think that he wanted some confirmation that the crack was on the brick joins and not through bricks. When I removed the plaster, the crack followed the end of the lintel (which was kind of what I expected). This crack is definitely the worse crack in the room and has spread further since I removed the plaster much earlier this year. I have attached a photo. I think that this lintel must be moving (maybe vibration?) and thats why I wondered about the joists expanding/contracting. Sorry, I should have added this photo yesterday. I notice that the cement on the end of the lintel is very weak(dead), which is probably a good thing to allow movement of the lintel/bricks? I was planning on repairing this joint (and maybe also the other end) with new cement. If I did this what mix of cement would you give it? Has this picture changed in any way your original thoughts on the problem?

Thanks again for your valuable input.