I having been having a lot of issues with the bricklayer who I hired to build my extension. one of them is that he put 4by2 treated timber between the two level of my extension for the first floor joist to sit on and built the second level on top of the wood, see attached pics. I did question it at the time but he assured me that it make the structure much better and it is a normal practise and nothing to worry about. however, a plasterer came around for quote today and he was rather surprise of what he saw. he said you can not mix masonry with wood. is that true? do I need to take the wood out and insert thermalite block instead.
can you build on wood
- Thread starter Zehroni
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Never seen it done like that before, but then you get wooden lintels, wall plates take the load of roofs... Can imagine it shrinking/moving a bit with moisture/temp so I assume the wall will be dot and dabbed
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Removing it should be fun, especially if he’s skewed a few nails into it to hold the joists. I know it’s wierd, but what’s the issue leaving it as is? Compressive strength? Thermal? Movement? Slip plain?
Block-to-block joint with mortar would have some tensile strength, and a lot of shear strength. Block-to-timber joint presumably has neither, so your inner leaf would contribute nothing to prevent the wall from falling outwards. Or am I mis-interpreting the photographs?
As an aside, why are there two courses of half-height blocks above the level of the joists?
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mortar isn't for tensile strength, you want reinforced concrete for that. Mortar just fills the gaps between the blocks, gravity does the rest.
timber is not stable under changes in humidity and modern building materials are not very flexible so I'd imagine thats the reason.
Those (plus fire) are relevant, but are not major issues in themselves.
Timber is still built in to walls – wallplates and joist bearings come to mind, plus timber lintels in the past which are still performing. It will be stable enough once built in, and will take relatively high compressive loads of up to three storeys.
The mayor issue is of risk – risk of dampness and rot and risk of fungus and insect attack. “Ah” you might say, “what about timber framed houses?”. Well in that case the risk of rot and attack is known and mitigated against via the vapour-check and breather membrane layers, so yes it’s still possible to damage these and this could impact the frame, but in general use the frame is protected.
With masonry, the risk is not managed to the same extent. The timber in the wall may suffer persistent penetrating or condensation moisture, and it could decay. Or it could be a good environment for insect attack. So the timber is not protected as it would be with timber frames.
And then there is the issue of disproportionate damage, access, and extent of repair if timber got damaged – the wall (and roof) above settling unevenly, and accessing the work. Ok, it’s all repairable and it wont happen suddenly, but it will be a costly, awkward and disruptive job. So why risk it?
So generally it’s a no-no to build in timber to walls, and it should only be done if the risks are managed and precautions taken.
In the OP’s case it looks like it’s not even treated timber, and the bricklayer’s blasé comment and inaccurate statement that it’s common and makes the structure much better is just wrong and gives away his ineptitude.
Nice one, good points. I reckon the whole course of blocks is gonna have to come out to get the timber out, if he’s nailed it 
Your bricklayer is a total muppet.....did he come from checkatrade or similar online
Assuming you have metric gauge brickwork externally, that timber will have made the brick ties out of alignment.
What has been used as a cavity closure at the door opening?
I hate to ask what else he has done wrong
I beg to differ. Gap-filling + gravity is an antiquated method of construction that only works if walls are made very thick at the base, like a mediaeval castle. I think you will find that any good mortar does have some tensile strength, and that is why a mortared wall cannot be pushed over as easily as one made from dry bricks. I seem to recall that OPC mortar has tensile strength around 10% of its compressive strength, but possibly memory is playing tricks with me on that point.
A common exam question for engineering students in my era was to calculate the angle at which a masonry tower could lean before collapsing (a) if built with mortar having zero tensile strength and (b) if built taking advantage of the tensile strength of OPC mortar.
Mortar has a tensile strength of around 2.5MPa according to https://warwick.ac.uk/fac/sci/eng/r...tu/pubs/rn/build/still_2004/still_11-2-04.pdfWTF? A leaning wall or tower of bricks is held together by mortar due to adhesion and friction, not tensile strength.
Mortar has less tensile strength than a Crawford's wafer of the same thickness.
I have no idea what the tensile strength of Crawfords wafers are
Edit- mistyped 2.5
Brickwork is usually most efficient when subject to compression,eg vertical gravity loads, but far less reliable in tension.
Usually tension is only an issue in tall, thin brickwork panels subject to horizontal wind load. The panel will bow slightly, causing
some tensile stress in the mortar joints in the leeward side of the panel. How much you can allow for this depends mainly on the
water-absorption properties of the brick, low-absorption bricks giving slightly higher values of tensile resistance in the mortar than
very absorbant bricks.
In most traditional brick low-rise domestic buildings, the tensile strength of mortar is of no significance.
Usually tension is only an issue in tall, thin brickwork panels subject to horizontal wind load. The panel will bow slightly, causing
some tensile stress in the mortar joints in the leeward side of the panel. How much you can allow for this depends mainly on the
water-absorption properties of the brick, low-absorption bricks giving slightly higher values of tensile resistance in the mortar than
very absorbant bricks.
In most traditional brick low-rise domestic buildings, the tensile strength of mortar is of no significance.
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