Building round floor joists and block walls on top of joists

[craig1234]

I am building an extension and am thinking of building in the floor joists rather than using joist hangers. In one section the joists would have to be cut into the existing brick wall (the other end will be block). I am looking for some advice on the following:
1.On what do I bed the joists i.e. just on a layer of sand/cement - if so how
thick
2What do I need to do around the edge and top of the joists i.e. leave a
slight gap, fill with sand/cement etc.
In another section of the extension I will have joists running from opposing directions which will end on top of a partition wall (block). i.e. there will be the ends of two joists adjacent to each other with one running in one ditrection and the other in the other direction. Should I bolt the two ends together or not?
Finally, I will construct a block wall on the first floor under which I will have 4 joists bolted together (easier to handle than using a steel beam I thought). Do I treat these 4 joists the same as a single joist with respect to building them in and how do I start the block wall i.e. do I literally bed the blocks on mortar straight onto the joists (the floor boards would then abut the wall)

 

[^woody^]

You need to lay the joists on a level course of blocks, not mortar

The joists need to be surrounded with a flexible filler so that when they shrink, there are no gaps. Not mortar

Not sure about the bolting joist question. t depends what they are loaded with or if just meeting by each other.

You can't build a masonry wall off timber joists.

 

[craig1234]

Thanks Woody.
The joists on the partition wall just meet - the wall will continue above them -my thoughts are that they don't need to be bolted together.
I showed a block wall on top of joists on my building regs drawings and it wasn't questioned although there were other things that were shown that were missed and then queried when I started to build!!. If I can't build directly onto joists (only light weight blocks would be used) then what do I need to add to the joists to be able to build off of?

 

[^woody^]

Timber will move and swell/contract seasonally, which leads to movement in any masonry wall built off it ... and I'd be surprised is any BCO would approve it on site

Ask your BCO first, as otherwise you will need a steel beam or lintel, or change the masonry wall to a timber stud

 

[craig1234]

Woody, I have checked my building regs submittal and there is a separate sheet produced by my structural engineer where it clearly states that I intend to use 4 x 195mm x 175mm C24 timbers bolted together with 75mm light weight blocks for the wall (non load bearing). The span is to be 4.8m with a height of 2.4m. I remember now that this sheet was submitted following my initial submittal after which the building control officer requested calculations for the joists underneath the wall. I assumed that I could build directly onto the timber as that is how it was done in another house that I owned although that is much older. I guess I need to query things with the council!

 

[noseall]

Yes a masonry wall can be built of a timber bearer but it has to be underwritten by a structural engineer.

We did something similar in Chiswick a few years back and there was a lot of arguing between the client and the chief BCO.

The client happened to be an S.E but unfortunately could not read his own illegible scrawl.

It is a very fussy arrangement and involves a lot of prickly dog tooth washers and really should only be considered whereby using a steel beam is prohibitive.

 

[craig1234]

Thanks Noseall.
I have been undertaking a little more research and I found an article (from Celcon's web site I think) which said that 3" Celcon blocks could be used. My design is underwritten by a structural engineer (calculations were sent to Building Control).
So, I presume that I can just bed the blocks straight on top of the joists - is any specific mix required?
Also, is there any benefit in keying in the blocks rather than using wall starters. The reason I ask is that on a previous project where I constructed a block partion wall (onto a solid concrete foundation) using wall starters, at the top it cracked (vertically where the two walls met) i.e. the partion wall appeared to have pulled away - the bottom was fine. I am confident that the wall starters were nice and tight - perhaps it was just shrinkage!

 

[noseall]

Use wall starters for the key. They will behave better. The problems you had with a previous job are nowt to do with wall starters.

Fix some expanded metal roll on top of the timber before bedding the first course.

 

[garyo]

Re: Craig's first question about bedding in joists, I also need to do this in a couple of months time, and was intending to build them in with 1cm of mortar all round then when it's green scratch out 20mm of the mortar depth and then fill with expanding foam at a later date. Is this acceptable?

Cheers.

 

[craig1234]

Garyo,
I understand that you cannot use expanding foam but must use mastic around the edges.
Craig

 

[^woody^]

The issue with timber beams is not one of loading (a timber beam can support a block wall) but one of movement.

The potential for a timber beam to move naturally and deflect over time is so much greater, and this will lead to cracking in any masonry wall - which then leads to cracking of finishes and also potential stability issues

So whilst it is possible, it is not good practice at all when an alternative beam (or alternative design) is available