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Screw length needed to hold in brick behind lathe & plaster

If it's lathe and plaster over brick, I assume the lathes are fixed to vertical studs attached to the external brick wall? If so, couldn't the cleats be fixed by screwing through the lathe and plaster into the studs, assuming you can locate them and they are in more or less the right position? I suppose if the cleats are mounted horizontally and not vertically this wouldn't work?
 
The cleats need to be mounted horizontally so the chances of aligning them with the studwork is very slim. Each diffuser measures 43cm square and I'm mounting them centrally on the wall (three in a row) above the TV.
 
I suggest putting some masking tape on the plaster before drilling to minimize crumbling.
 
Loving some of the replies / scope creep

It's 9kg total... definitely needs mini bored piling, and maybe some seismic isolators.
I think through the wall to a 12" spreader plate on each corner . Better make that 18" . You're the structural guy here martin, can we get away with M12 studding or should we go to M14?
 
jees, drill a couple of holes stick a rawl plug in there and use a 60mm screw. Its 9 kgs you could probably convince it to stay there with blue tac.

Just dont screw infinitely tightly it will just crush the plaster, been there done that.
 
If it's lathe and plaster (and you do need to know) then it proably won't be on timber battens. It was normally fixed to the wall with lumps of 'cement'. L and p has no strength at all to carry a load of even 2 Kg and def not 9.
Your problems are worsened by the pre drilled holes in your cleats.
If I were doing this I would mark on the wall where the holes for the screws in the cleats will be, and then attach horizontal wooden battens to the front of the lathe and p and attach the cleats to those. This will move the whole thing out by an inch, but you can secure these battens really well by first drilling through the L & P with a 1" hole cutter for each screw needed so you can easily tap in a wall plug. An inch hole will let you see to locate the batten screws into the plugs. They screws need to be in the wall, if the brick is sound, about 40 mm. 3 screws per batten should do, or 4 if the battens will be more than 75mm wide. The point of wall marking your cleat holes is ofc to establish the position of each row of cleat screw holes to get the battens lined up ready, and (obvs i know) to make sure you place the batten screws away from the where the cleat screws will eventually go.
Don't over tighten the batten screws, but just a firm fit against the L& P. The battens in place will cover your 1" cut holes. slap a bit of wall paint over them if their ends will be visible.
It's now an easy job to fit your cleats with whatever screw will just go through the whole depth of the batten. A normal woodscrew will do fine.
 
Who needs a telly when you're listening to the gently diffused sounds of a plaster wall slowly collapsing?
 


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