Need to mount TV to plasterboard but can't locate studs

[pcdj]

I'm trying to locate the studs in a plasterboard wall to mount a 75" TV but so far with no luck.

I've attempted to use a stud finder, but upon drilling into the wall where it locates a stud I'm finding no more than the 15mm plasterboard seen in the pics below.

It seems to be locating "studs" about 250mm apart, which seems too frequent.

Additionally, there's a slight vertical crack running up the wall against which I'm told there's a fairly good chance there'll be a stud behind it, but drilling through again I seem to hit about 15mm of plasterboard with nothing behind it (judging by the material left on my drill bit).

Additionally, I can't seem to find any screws joining the plasterboard to the studs using a strong magnet I bought for this purpose.

I've taken a look behind one of the power sockets situated dead centre of the wall (centered both width-wise, and height-wise) but I can't make out the construction. It looks like there's...

A solid brick/mortar wall (to which the backbox is screwed to).
A void of 10mm or so.
Insulation of 20mm or so.
15mm plasterboard.

...does anyone have any suggestions on how best to approach this?

 

[rsgaz]

If you are 100% certain that the back box is secured into masonry and it's not a 'paramount partition' (i.e. very thin walls, you would be able to tell by looking at a door frame reveal), then you must have a "dot & dabbed, insulated foam backed plasterboard and masonry wall". So no studs/battens to fix into.

You want Rigifix fixings for that type of wall. Any 'normal' (but long enough the reach the masonry) fixing can pull in the plasterboard and crack because of the gap behind it, as you tighten them up.

If you happen to drill through an "adhesive dot" behind the foam insulation, then good, that fixing will be even stronger!

 

[rsgaz]

Oh, and this better be a flat-to-the-wall style TV bracket and not a cantilever one!! What does the TV weigh?!?!

 

[pcdj]

Yes, it's a flat one! I think the TV weighs about 25kg, so I think that'll be reasonably manageable for most fixings?

You have me concerned about whether I've correctly established it's masonary behind there now - thanks for the recommendation of the Rigifix fixings; I hadn't come across them before.

Would the 'Grip It' type not be recommended in this instance, given the question on the masonary backing?

 

[rsgaz]

I think the TV weighs about 25kg

That doesn't sound much, the first 75 inch TV I found on google said 41kg!

whether I've correctly established it's masonary behind there now

Need more details first. What year roughly was the house built? Is this an internal or an external wall? If it is an internal wall, can you try and measure the wall thickness?

Would the 'Grip It' type not be recommended in this instance

I would never recommend Grip-It for a dot & dab wall. Their undercutting tool for removing the adhesive made me laugh and cringe at the same time when I first saw it. Screwing into the masonry behind is always the better and stronger way.

 

[pcdj]

That doesn't sound much, the first 75 inch TV I found on google said 41kg!

Sorry, that's a typo - it's a 65". I don't have the exact model number to hand, but Googling around for 65" LG LCDs brings me to the 25-30Kg ball park.

Need more details first. What year roughly was the house built? Is this an internal or an external wall? If it is an internal wall, can you try and measure the wall thickness?

I think it's a 1930s property, but it's had some extensive renovation in the last decade or two when this particular wall may have been touched. Unfortunately I can't see any way to measure the thickness. It doesn't terminate at a doorway at either end, but instead goes around a 90 degree corner. Behind it is the downstairs bathroom.

 

[Lucid]

Your stud finder is struggling because there's no void between the studs to help it sense the change in density. Your voids appear to be filled with insulation board, and it's probably foil lined too which won't help. The other factor is the plasterboard depth: The thicker the board the less accurate the stud finder's detection.

In situations like this I tend to go 'old skool'. Knock across the wall to see if you can get a rough position of the stud by the change in tone from the soundings. Once you've got a rough idea then use a fine drill bit - say 2mm - to make a series of test drillings to find the stud. You should be able to work out where the edges of it are so you'll then know where the centre is.

There's always a risk drilling in to walls that you might hit a power cable. Take precautions to avoid that if possible.

 

[pcdj]

I drilled six of them and had no luck with any (residue on the drill bit suggested I'd hit plasterboard in all cases). They were horizontally spaced across where the stud finder thought there was a stud (it was finding something fairly consistently vertical on this part of the wall).

You're of the opinion there are studs here then, as opposed to rsgaz above?

 

[rsgaz]

I think it's a 1930s property, but it's had some extensive renovation in the last decade or two

Then it really ought to be proper solid bricks, but looking through the knockout holes in your first picture, something just doesn't look right, even zoomed right in.

You say you've tried two test holes already and found this gap. What happens when you drill further/deeper? You're going to need 100mm drilling depth to use the Rigifix M6 size, if it's still very soft, then stop, you might end up with a hole through to the bathroom!

I can't see any way to measure the thickness.

Measure from the bathroom door jamb inside and out and calculate the difference. This really is important due to the unusual photo.

 

[pcdj]

Unfortunately my earlier description was an oversimplification. Fortunately I have a map available for illustration.

The green circle below sits on the wall in question (running horizontally, left-right). The big void behind it where the vacuum never made any tracks is where the bathroom is.

I hope you can see it's a bit more complicated than measuring the door frame.

 

[rsgaz]

oversimplification

Yowser! Fair enough!

OK, using a screwdriver (approx 4mm flat), poke into the wall hard with a twisting motion here and here....

Soft and mushy? Or brick dust/mortar and can't penetrate?

 

[pcdj]

Can't penetrate, so I think I have the answer and a few Rigifix have been ordered - thanks!

Looks like some sort of concrete brick, overlayed with a thin layer of green (painted) plaster. Whatever it is it's definitely hard to penetrate and barely chips away.

 

[rsgaz]

concrete brick, overlayed with a thin layer of green (painted) plaster.

Ah, that's much better. It was the colour that threw me, it looked roughly the right colour for thermalite blocks, but without any texture, which don't exist! So your walls will be approx 18 to 20cm overall thickness.

You might not be out of the woods yet, under that original plaster will probably be proper 'old school' bricks, extremely difficult to drill into without an SDS drill, likely the same as the bricks on the outside of the property.

Let us know how it goes.

 

[foxhole]

Looks like insulated plasterboard dot and dabbed over brickwork. Easy enough to get a secure fixing just needs longer fixing and spacer thru plasterboard .You don’t have any studwork .
If you have a large plate to fix the insulated board will not compress as the fitting will spread the load .