What size backbox do I need?

[codaamok]

I'm thinking of drilling through my external wall in downstairs living room, where my router is, to run a couple of cat6 cables up into the loft. On my internal wall, I'll fit a 2x RJ45 face plate but I'm unsure on the size of backbox I need.

Thankfully, next to where I want to drill already exists a faceplate+backbox for TV aerial. I took the faceplate off of that and found the backbox to be half heartedly fitted to the brick. Anyway, it looks like it's 25mm. Does 25mm look like the correct depth here or should I get 35mm?

My external walls are plasterboard with dot and dab. Side question for future reference, if I encounter dot and dab and want to either chase/channel, what's the best tool? What's the material like, is it like plaster?

 

[CBW]

I suppose it depends on the depths available, as with some dot and dab it can vary. Also, it depends if you can get a decent fixing, might want to consider these?

 

[Notch7]

Fairly big, not too big.

 

[JohnD]

25mm is a very common size, but 35mm gives you more room so I prefer it.

If you are cutting out hard brick or concrete the extra effort is wearisome so you might want to balance against that.

In your case you may as well use the deepest box that will fit. Put the grommets in the holes to avoid chafing the cables. The one in your pic is too small, it does not line the plasterboard and there is a shoddy gap. It looks like 35mm would have been a good fit.

 

[securespark]

It looks like a 35mm dry lining box would fit, but it's difficult to tell exactly.