100mm hole for duct

[bigal43]

Hi everyone
I am a reasonably skilled D.I.Yer. Currently looking to fit an extractor fan in the bathroom. Main issue is the 100mm hole required to fit the vent outside is near the apex of the 'dorma'...above the ceiling in the bathroom and it looks to be lathe and some kind of masonry / plaster with a wire layer. It's about 30 mm thick with a pebble dash exterior.

House itself dates back to 1895. Not sure if this wall dates back to then but anyway. ..

I have access to the area to cut for the vent duct via the ceiling I will be replacing.

What are the best tools and methods to cut a clean 100mm hole ?

Am I better off getting a builder to replace that section and fit the vent duct...or is that overkill?

I am really worried about the whole small wall section just crumbling out at any attempt to cut this hole in there..

Any helpful and friendly advice would be very welcome.

Thanks!
Allan

 

[Himaginn]

By the sound of it, I'd go for a series of holes drilled around the circumference (stitch drilling) and an all-purpose blade in a reciprocating saw.

 

[noseall]

Have you thought about going through the roof? Piece of cake.

 

[theprinceofdarkness]

Your construction is a lot later then 1895. Then lime based plaster as used and no metal. The earliest metal I have seen is in 1930s houses. It sounds as though the dormer is timber framed (they usually are) over which galvanised mesh is nailed then is was rendered and pebbledashed. I reckon this outer layer is very brittle and fragile. A core drill will vibrate the wires and cause cracking of the render. I would avoid touching this area unless you don't mind replacing it all. If a big man with a pointy stick forced me to do it, I would cut a slightly over sized circle out by using a 6mm drill in a series of holes. Try to chip the render of the inside, then cut the wires with wire cutters. I still do not know how you can water proof the edge of the hole and stop the ends of the wires rusting. The only sure way would be epoxy resin ££££.
Frank

 

[bigal43]

Hi everyone. ..thanks for your kind advice and replies. Noseall - why didn't I think of that! ?

A vented roof tile looks like the way to go and something I could do.

Thanks again to everyone

 

[SimonH2]

How about a diamond core ? Will go through metal and rendering, not too sure about the wood laths though. I have a local small tool hire place that's not too expensive to hire one from (with the right machine to drive it) over a weekend.

 

[bigal43]

Thanks. ..but I have decided to go the roof vent option. Any kind of drilling in that section. ..I think would invite disaster!