Rendering over timber section in block wall

[Mr Blobby]

For reasons that make me use bad language, our block build house has a couple of window openings that need to be made smaller, but only on the outer leaf of a block cavity build.

Window height must be reduced by about 300mm.  The obvious thing to do is to insert a intel and a course of blocks, but that may be a little too destructive. 

Builder and joiners are suggesting a timber stud, to be built with ply across the face and then steel mesh across the front to render the timber stud to the blockwork.  My concern here is the timber getting wet and rotting/warping and then render cracking.  Breather membrane and bitumin paint on the ply has been suggested. 

 

What's the best approach here?  Does the timber need to be ventilated?  Would render board at the front be better than ply?  Can mesh be nailed to render board?

 

[nod]

Mesh directly over the timber then render if sand and vent 

Breathable  renders Cement board fibre mesh and base coat 

[dpmiller]

A lintel would't be too hard to set in surely, its not structural after all...

[Mr Blobby]

10 hours ago, dpmiller said:

A lintel would't be too hard to set in surely, its not structural after all...

 

The problem is the windows are actually in place so cutting the outer leaf risks damaging the windows. 

I reckon I could cut a hole carefully with a grinder but perhaps the builder knows the bricklayer won't have my patience and will hack it with a stihl saw without much care for my windows.

 

Also the windows are on first floor with rubber roofed porch underneath so more scope for damage from falling masonry.  (I know, its a crap place to end up in but I can't turn the clock back😕)

I'm also told that if we did insert a lintel and a course of block then the render would still be at risk of cracking anyway because its new block against old block (although I'm pretty sure this would be more stable than a timber stud bolted into the hole).

 

20 hours ago, nod said:

Mesh directly over the timber then render if sand and vent 

Breathable  renders Cement board fibre mesh and base coat 

 

Thank you @nod for your sage advice. 

So for a silicone based breathable waterproof render like sto, we should cover the front and underside of the timber stud with render board.  Then do block wall render buildup as below, with reinforcement mesh across the join from timber to blockwork.

 

Should I do anything else with the timber stud, like bitumin paint, or wrap in breather membrane?  Would I need to leave a ventilation gap underneath like a timber frame build? 

 

Edited Saturday at 19:08 by Mr Blobby