Foundation Masonry with tight, and frustrating, tolerance

[DeanAlan]

Good morning all.

 

Due to a shallow public sewer I have to build over and a planning restriction on height of our first story I have a need for a relatively low foundation masonry build up over the mass concrete that has been poured and I'm concerned about how to get this both level and close enough to the required height.

 

Build regs drawings show 285mm to bottom of beam bearing for block and beam floor. That is based on a perfect concrete level and I've established that the concrete is 10mm to 29mm lower than specified so I need to build up between 295mm and 314mm.

 

What is the right way to build up to that height? A trench-block is 215mm high. A brick is 65mm high. Excluding 2 mortar courses (based and intermediate) that is 280mm. Splitting 15mm over 2 courses (to reach the min of 295mm) seems fine. Getting to 314mm would need the two mortar courses to have 34mm of depth between them which is too much - isn't it?

 

I don't plan on doing this brickwork myself but the bricklayer I am talking to keeps sucking in air between his teeth so looking for advice on how best to do this. 

 

Should, for example, we cut the trench block in half (height wise) to create to trench-blocks that are roughly 215/2 (107.5)mm high to introduce another mortar course and that would mean that the 34mm would be split over three so 11mm (ish) each which seems ok.

 

And/or - can you get trench blocks that are less that 215mm high - haven't found any yet.

 

Appreciate your help.

 

cheers,

-Dean

 

[Russell griffiths]

Forget the trench blocks and just use a standard concrete block laid flat. 

Also your floor beams DO NOT. Sit on a mortar bed, they sit directly onto the last course of blocks. 

[Declan52]

As above 3 courses of blocks on their flat will give you 3* 100mm plus 3 motar beds. You can just bed them normally as 15mm of a height increase is nothing to worry about.

[Brickie]

As above-I’ve been doing footings exclusively for around a year now & laying blocks flat is what’s done to get over dips in the concrete. We would lay centre of block to centre of wall so when the wall steps back in to 100mm it’s balanced nicely. 

[DeanAlan]

Thanks @Declan52 and @Russell griffiths 

 

Noted re beams not bearing on mortar.

 

So something like https://www.forterra.co.uk/product/thermalite-aircrete-hi-strength-7/ blocks which come in a variety of widths could be laid on their side making that width their height in that orientation.

 

One example would be a 100mm width block, 3 of them laid on their side with 3 x5m mortar beds comes in at 315mm. Where the concrete is at its highest (10mm below plan) that would put the top at 305mm against 285 so 20mm higher than plan. Where the concrete is lowest we we need to make up about 19mm height difference which would take the mortar beds in those areas to around 11mm - is that too much?

 

The 20mm above plan might cause some other problems though given steels and other things that are already under fabrication.

 

Thermalite blocks are quite easy to cut aren't they? Would it not be better - although I can see there being quite a bit of waste - to put these block in their normal orientation and cut blocks to the size required to hone in on the planned heigh of 285mm and not call for too much or too little mortar?

 

If there are 2 courses that would be 10mm in total of mortar leaving 295-10 (as min) to 314-10 (as max) to make out of 2 blocks. That would be a combination of a normal block of 215 + (70 to 98) cut block. Is it reasonable to cut these blocks to 70mm or better to cut 2 blocks at 142mm (for min build up) and 156 (for max).

 

Am I making this harder than it needs to be?

 

Thanks for your patience ?

 

cheers,

- Dean

[Declan52]

Yes too much over thinking. The brickie will take the height measurements from the lowest point. Using a level transfer a mark to each corner and all these marks will be dead level. If he has to bed up a bit or tighten down a bit on any corner its no big deal. 

When they are built if they are 2/3mm out it's no issue. 

[Russell griffiths]

My opinion obviously. 

If you use an aircrete block below ground you need a swift kick in the meat and two veg. 

Concrete blocks only for me. 

[DeanAlan]

Thanks all - I think I have been over thinking it. This is my first (I think I will recover and it hopefully will not be my last) time doing this and I'm taking the role of main contractor and principle designer between my architect / engineer and trades. We do have a particular complicated build in the foundation area due to this public sewer we are building over.

 

Anyway, think it is as simple as 215mm high blocks (aircrete or concrete) and 65mm bricks and the 2 mortar beds will allow me to my bricklayer to make up the level to 290 min / 314 max.

 

cheers,

-Dean

[Brickie]

Exactly,and the concrete brick course works a treat for your lintols over pipe runs & air bricks.