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Screw length needed to hold in brick behind lathe & plaster

ToTo Man

ToTo Man

the band not the dog
I'm attempting to hang some 9kg wooden diffuser wall panels that have been supplied with 15cm long french cleats that have 4 screw holes. I drilled a pilot hole to test the wall construction. It's 40mm from the front of the lathe & plaster to the brick wall behind, the air cavity between the two is probably 20mm-25mm.

I want to use screws and rawl plugs that are long enough to go through into the brickwork but I don't know how far into the brickwork they should go for a secure fit?. What would be sufficient if each screw is supporting 2.25kg?

EDIT - The size of the screw holes in the french cleats limits the diameter of screw I can use. The supplied screws and plugs just stop short of reaching the brick.

preview-lightbox-French-Cleats-Europe-nobg.jpg
 
ToTo Man said:
The size of the screw holes in the french cleats limits the diameter of screw I can use
Click to expand...

Which is ?

You could always drill them to a greater diameter, I would suggest a minimum 5mm round head screw 120mm long.
Assuming the lath and plaster is 15/20mm thick then 40 mm void ( studs ? ).

The problem will be getting the plugs into the hole in the brick.
Are you sure the the 40mm is a void and not the thickness of timber studs.
 
misterdog said:
Which is ?

You could always drill them to a greater diameter, I would suggest a minimum 5mm round head screw 120mm long.
Assuming the lath and plaster is 15/20mm thick then 40 mm void ( studs ? ).

The problem will be getting the plugs into the hole in the brick.
Are you sure the the 40mm is a void and not the thickness of timber studs.
Click to expand...
Apologies, I mis-typed. 40mm is the total depth from the front of the plaster to the start of the brick, so the void between is probably 20mm-25mm.

The inner thickness of the supplied screws, according to my calipers, is 2.4mm. The cleat holes are just over 4mm in diameter so there's definitely scope to use a thicker screw but I don't think there's enough metal there to enlarge the holes to accept a 5mm screw as the holes are close to the edge.

If I am unable to seat the rawl plug into the brick is it sufficient to use a plug only in the lathe & plaster section?
 
If it is lath and plaster you might be able to hit one of the studs and get one really good fix.
If you can’t then all you need is 30mm in the brick so 40+30=70mm screws.


Pete
 
Wire sizes for screws are quoted as gauges - you need at least 12 gauge screws because the bit in the plaster will be essentially unsupported. If that means drilling out the holes in the cleats, so be it.

So something like 3"x12. The best bet would be to buy some continuous wall plug as well and cut to size - that too comes in gauges, usually colour-coded, usually in lengths of something like 25/30cm.

You are never going to mount the cleats so that the load is evenly spread.
 
Ian G said:
Do you mean plasterboard ,?
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I'm assuming it's lathe & plaster because it's a 130-yr-old sandstone house and no construction work has been done on this wall for at least 50 years (which is when my grandparents bought it!). It could be plasterboard if the previous owners had done work, I don't know. Does it make much difference?
 
ToTo Man said:
I'm assuming it's lathe & plaster because it's a 130-yr-old sandstone house and no construction work has been done on this wall for at least 50 years (which is when my grandparents bought it!). It could be plasterboard if the previous owners had done work, I don't know. Does it make much difference?
Click to expand...
If it’s plasterboard then plasterboard fixing can be used, you can test by pushing a flat screwdriver in and see if you hit wood.

Screws come in metric sizes these days.


Pete
 
I’ve really no knowledge here as my DIY skills only extend to a soldering iron, but I suspect you need hollow wall fixings such as this (Amazon). I’m pretty sure drilling into the brick or sandstone from that distance is a non-starter. A combination of screws into the wood batons if you can find them and hollow-wall fixing where you can’t is the logical approach.
 
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Your biggest issue here is getting the plug into the wall without damaging the lath/plaster, your second biggest issue is when you tighten the screw is you could well crack the plaster due to it flexing so do not over tighten. To answer you question though you only need 40-50mm into the brickwork. 4mm x 80mm should suffice but make sure you use the right sized plugs and masonry drill bit.
 
As above, Corefix with the steel core that bridges the gap are excellent for plasterboard with a void to the wall.
 
If it genuinely is lath and plaster, it will be unbelievably crumbly and take far, far less of a load than plasterboard, in fact, essentially no load at all, especially if of any age.

Dot and dab is just a few mm gap between back of plasterboard and the wall.
 
Vinny said:
Generally speaking, not so in the trade, not for the gauge, length yes. I suspect that almost anything confuses Joe Average, so they reason that metric mm is possibly least so.
Go to pretty much any specialist fixings supplier and they quote screw gauges.

For instance - A4 Marine Grade Stainless Steel No.8 Countersunk Slotted Self Tapping Screws 4.2 - Spalding Fasteners
Click to expand...

These all seem to be metric.

https://www.ironmongerydirect.co.uk...4fZYQnsxXukmvmMAKHHiGXWrappkvgTMaAjYEEALw_wcB


As are all my Reisser screws in my set

https://www.screwfix.com/p/reisser-...XEr5mvHeS6hNQOX1qHAaAsS7EALw_wcB&gclsrc=aw.ds


I can’t remember the last time I bought screws by gauge.


Pete
 
Tony L said:
I’ve really no knowledge here as my DIY skills only extend to a soldering iron, but I suspect you need hollow wall fixings such as this (Amazon). I’m pretty sure drilling into the brick or sandstone from that distance is a non-starter. A combination of screws into the wood batons if you can find them and hollow-wall fixing where you can’t is the logical approach.
Click to expand...
When the joiner came up to wall-mount my TV he basically took the brute force approach of powering through the plaster with his drill into the brickwork and asking questions later. There was one fixing he wasn't happy with so he moved that one over a bit but all the others were fine. He didn't even check for live cables or pipework (though it was highly unlikely as it's an external wall with a chimney), just straight in there with his power tool! The noise when the drill reached the brickwork was horrendous but he grinned and retorted "that's the sound ye want tae hear, means yer telly willnae faw aff the wall anytime soon!". Job was done in 10 minutes! :D
 
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Vinny said:
If it genuinely is lath and plaster, it will be unbelievably crumbly and take far, far less of a load than plasterboard, in fact, essentially no load at all, especially if of any age.
Click to expand...
It must take some load as I've had 6kg bass traps hanging off this wall for the past decade on nothing more than picture hooks into the wall with pins. Also used to have very heavy framed mirror hanging off a couple of the same style of hook. Maybe I was lucky nothing came crashing down?!

picture-hook.jpg
 
Some modern adhesives which come in gun tubes are ridiculously strong. If possible I'd glue a large flat piece of ply to the wall and screw your brackets to that (assuming the whole thing will be hidden).
I swear that the wall would come down before that wood comes off. Eg Gorilla extra strength grab adhesive, Sticks like sht.
 
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