Why am I hitting masonry in an internal stud wall?

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Andy_L

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I'm replacing a cracked toilet cistern. Clearly this wall was tiled around the old cistern, but let's try not to get too upset about that!

The house is 15-20 years old and has internal walls of plasterboard over metal studding. The wall we're looking at is an internal wall between two bathrooms on the top floor of a 3-storey house. On the storey below there is nothing in this location, it's the middle of a room.
The hole marked in green on the photo goes through the plasterboard, into a void about 3" deep as expected of one of these stud walls, so I can use a hollow wall anchor there.
The yellow line marks the approximate position of a metal stud.
The holes marked in red all go through the plasterboard and then hit what seems to be some sort of hard masonry. There's no gap between the plasterboard and the masonry. The old cistern was basically just screwed to the plasterboard on the right hand side.

Now I have an SDS drill, so I could just plough into it and whack in a rawl plug, but I'm reluctant to do so without understanding what I'm drilling into or why it's there.
Does anyone have any ideas about why there would be masonry inside an internal wall in a location like this?

wall.jpg
 
Tile backer board maybe?
That's like a cement board.
 
That's what I was thinking but 20 years ago cement board would have been few and far between in any new builds.
 
Possibly block or brickwork put in to take the fixings for the cistern. Consider using a hybrid polymer sealant like CT 1 if you want to avoid screwing the cistern to the wall. If you want to investigate cut out some plasterboard with a multi tool.
 
Some interesting thoughts there, thanks.
Asbestos - I certainly hope not! Apparently the complete ban was in November 1999 so a house from the mid-2000's should be safe from that.
Blockwork for fixing the cistern to - that's a possibility, is that something builders do in stud walls? The plumber who installed the toilet obviously didn't know about it, since they missed it with the left hole and viewed it as an impenetrable obstruction when they hit it with the right hole, but you could easily imagine that happening on a building site.
Looks like I need to do some more digging.
 
Andy_L said:
The plumber who installed the toilet obviously didn't know about it, since they missed it with the left hole and viewed it as an impenetrable obstruction when they hit it with the right hole
Click to expand...
Silly question, but are you sure it is masonry?
A wooden stud would be pretty impenetrable to a masonry drill bit - as @foxhole suggests, I would cut out a small section and see what's going on.
 
RandomGrinch said:
Silly question, but are you sure it is masonry?
Click to expand...

A very good question! I did have a prod at it with a sharp implement, and it didn't feel like a piece of timber. (And I had tested with a magnet that it wasn't more metal studding.) But I tried an HSS drill bit on it and now I see...
wall2.jpg
...chipboard maybe?
So why would that be behind the plasterboard in a stud wall? Blup's suggestion seems a good possibility. There is also a shower cubicle against the same wall, to the right of where we're looking - maybe they put chipboard between the studs before putting the plasterboard on, to help with mounting the cistern and shower? Does that sound like something you'd do when building a bathroom?
 
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Difficult to say what material that is from the pic, you'd know more if you exposed an inch or so of the surface of the board. Its called pattressing but you wouldn't normally use chipboard (normally plywood) though house builders used to and still do all manner of crap.
 
Andy_L said:
A very good question! I did have a prod at it with a sharp implement, and it didn't feel like a piece of timber. (And I had tested with a magnet that it wasn't more metal studding.) But I tried an HSS drill bit on it and now I see...
View attachment 346098
...chipboard maybe?
So why would that be behind the plasterboard in a stud wall? Blup's suggestion seems a good possibility. There is also a shower cubicle against the same wall, to the right of where we're looking - maybe they put chipboard between the studs before putting the plasterboard on, to help with mounting the cistern and shower? Does that sound like something you'd do when building a bathroom?
Click to expand...
Common to use osb behind any area where a good fixing position is required , you can just screw into it.
 
Use your conk!

If you drill into the hole, then smell the end of the bit (taking care not to singe your snout), you should be able to tell if it's wood.

The other method would be to get a long screw and see it if bites.
 
Now we've got a better view. I lucked into hitting the edge of the board with my hole. It is 22mm thick chipboard, and looks to have some sort of white paper or plastic covering on the far face. Must surely be "pattressing" as @freddiemercurystwin said. This edge of it is roughly in line with the middle of the toilet, so presumably it was more to do with supporting the shower off to the right, than the toilet cistern.

wall3.jpg
 
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Does anyone have any ideas about why there would be masonry inside an internal wall in a location like this?
Click to expand...
It's also possible to find internal "stud walls" with studs and noggins as normal but with the spaces filled with brick, possibly "on edge". You'll possibly find them standing on a doubled up ceiling rafter with nothing below and with the verticals fixed to the ceiling rafters above, quite probably also having hangers to any purlins. In the days before plasterboard it was an alternative method to laths creating a surface to plaster on to. How do I know? this is how some internal walls in our (1902) house are built!
 
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