How do I get my blockwork heights to suit my TJI joists?

[aa44]

Can somebody help me sort out my blockwork heights?

In my new house that I am about to build the load bearing internal walls will be built of solid blockwork. I know the finished floor level of the first intermediate floor so I know that the top of the TJI floor joists will be approx. 22mm below this. The ffl is 99.0 m based on the site datum so the top of the joists needs to be at 98.98 m.

Am I right in thinking that if I am using masonry joist hangers then the top of the relevant row of blocks must be at 98.98m, i.e. the top of the joists must line up with the top of a course of blocks?

Some of the internal walls are two stories high and some are only one story. For the two story ones I will be using masonry joist hangers for the TJI joists for the intermediate floor. The walls for some of the rooms on the ground floor, however, are only one story high and the joist plan shows the joists resting on the top of these walls. (For example, a TJI joist is planned to run from one side of a two story load bearing internal wall across to an external wall passing across the top of the study wall.

The TJI joists are probably going to be 241 mm deep. What is confusing me is that if I set the foundation height to make the top of a course of blockwork match up exactly with the top of the TJI joists for the intermediate floor, then the top of the course of blockwork below that will be 16mm too high for the TJI joist to rest on in the study wall.

If I replace one course of standard blocks in the study wall with two courses of coursing blocks, then that would be 59mm too short. Is there a common way round this?

Hope this makes sense.

 

[Blagard]

Why not have a look here and read through some of the literature. It may help you to understand that how the industry has to work to tolerances and for the different trades.

http://www.nhbc.co.uk/NHBCPublications/LiteratureLibrary/Technical/

Edit:
I was expecting there were more photo's in there to guide you. Basically these conflicts are taken up by using cut blocks or packings to suit.

 

[aa44]

Thanks for that. I'll try not to appear in a future edition!

The NHBC good practice guide says not to mix blocks of different types in a wall. I was going to try to avoid aircrete type blocks because of their bad press for shrinkage and cracking plaster but they would make things a lot easier in situations where I need to chop a bit off a course of blocks.

If I use aircrete for one course and standard dense blocks for the rest (both high strength) am I asking for trouble?

 

[r896neo]

yes, just either cut them.

 

[aa44]

Thanks for the reply r896neo but I am not sure that I understand it.

Do you mean Yes I am asking for trouble mixing the blocks or Yes I can go ahead and use the aircrete blocks where I need to?

 

[simonjay]

Thanks for the reply r896neo but I am not sure that I understand it.

Do you mean Yes I am asking for trouble mixing the blocks or Yes I can go ahead and use the aircrete blocks where I need to?

I would not mix aircrete in with dense blocks in the same wall (except to use some dense blocks in aircrete for padstones). I would keep dense blocks internally between rooms for their better sound proofing properties. Use some bricks in with the dense blocks if needbe, or pack under joists with slate etc.
Simon.

 

[Blagard]

aa44

It is common for plaster to crack at the junction of dissimilar background materials. The classic being when bricks abut concrete. (That problem is typically resolved by using movement joints in the plaster).

If you were to use an entire course of blocks different to the rest used in the wall there may be a risk of cracking at the joint. Unless someone knows the two materials will not give rise to this type of problem then you have to ask yourself if you want to risk it. Using the odd block or brick does not usually give a problem. I am sorry, I don't know the answer to the mix you have in mind.

 

[theoldun]

You are going about everything back to front.
Note you are in Shetland, Building Regs may be slightly different than in England. Check with your building control. In England we do not use the joist hangers except for party walls. Joist hangers were introduced mainly to stop air leakage. Our building control and NHBC accept TJI built in to blockwork. To overcome air leakage, fit blocking piece each side of joist between flange, beam fill with a neat tight 10mm joint against each joist, rake back 5mm and mastic joint round joist. If you have a pain of building control, then fit air leakage caps when building in. First method is cheapest, easiest, and as said acceptable to NHBC and BC. Joist hangers are dangerous, need three course blockwork on top before you can apply any load, plus they are pigs to fit and level.
Forget about your first floor level, work up, not down. Establish your ground floor level, normally level with your outer skin DPC. Your inner skin DPC may be split level depending on type of floor construction. Normally ground floor level to top of frame is 2100 to underside joists 2400 =10 course block and 2 course of coursing blocks. We grind the last full block and 2 coursing blocks down to 2390 and that takes standard 13 up 200mm rise, 203 go flight stairs.
In your case if using 241mm joist at 600mm centres with 22mm tg chipboard, then you want storey height of 2415 = 10 course of block, last course bumped up 5 mm, and 3 course of coursing blocks bumped up by 10mm. Say 3mm a course. No problem, this will then take a standard flight stairs 13 up 203 rise and 226 go.
All block companies make their own coursing bricks to suit their blocks. We usually run our own. All you need is 300mm Sthil petrol saw. Hire one in for day , cut all you will require in couple hours.
One other thing ,we pitch our joists, beam fill and lay weatherDex tg chipboard. Besides taping decking up we also lay down poly sheeting for extra protection. Do our internal blockwork of floor, and it is not so far to fall when pitching roof.
Sketch it out, think about it, will come to you. If not come back.
Blocks .
Do not buy aerated blocks under any circumstances. Go for Fenlite, Interlyte, Aglite, Fibrolite ,Stranlite or similar. They are just as easy to cut.
oldun

 

[r896neo]

Its irrelevant now but yes i meant don't mix block types as you will likely get cracking at the joins and simply cutting some concrete blocks to suit will not take that long.

 

[noseall]

This is our method when using build-in joist hangers.

Buy a correct size (straight) joist and fix the hangers to the end of the joist.

Build up the internal blockwork up to the final full course.

Set up the joist dry upon the blockwork using packers setting it at the desired height.

Measure the gap and determine the most efficient available way to reach said height - for example: You could cut blocks in half if the gap were say 100mm or so. Or you could use coursing bricks. Or you could 'muck-up, if the gap allows. Or use slithers.