Cement Board On Plastered Walls

[SteUK]

Hi all,

First post, so please be gentle!

Preping our bathroom to drag it out of the early 90s and make it a nice place to spend time in. Ripped a lot of the old tiles off to find that some moisture had got behind and blown the finish plaster coat. The wall is 50mm breeze block, 10mm bonding plaster, 3mm finish plaster.

Bought some cement boards - no more ply as they do a 6mm and said I could attach straight to the bonding coat with single part flexible adhesive and mechanical (screw and plug) fixing for added safety. I was sceptical so contacted Mapei who have advised I cannot fix straight to the bonding. Bit of a pain in the ***, but not a problem as I can finish a wall that size no problem.

Tiles I will be fixing are 300x200x9mm, and weight approx 16kg/sqm. Decided to cement board out the walls as they are a bit out of level, and the finish plaster in parts is a bit patchy (where two windows were made one and a wall removed to make separate bath and toilet room one room). Also I like the added water barrier the boards add (I will be tanking as well for belt and braces!).

So what is my question? Well, it suddenly occurred to me that I am fixing these cement boards to a finish plaster...that can usually only hold 20kg/sqm. Or is that incorrect as I am mechanically fixing them as well? Or for some other reason?

Advice very welcome and appreciated!

Thanks!

 

[Dave]

Why not just re-skim the walls and tank any wet areas. .?

 

[SteUK]

Couple of reasons...

Because the walls are a bit up and down - one patch had a 20mm football sizes protrusion where they had decided to use a lot more bonding for no reason! I smashed it out and rebonded/finished. But the whole window wall is a bit like that. The tiles are right on the 20kg/sqm limit at 3mm of adhesive, so and thicker bed will drag it over. I am quite an anxious person, don't want to risk anything falling off the wall into my daughter.

I want a decent finish, and not being a pro, a flat boarded wall will help in that quest.

It's not a massive area to be tiled, 10sqm or so, so the added cost of boards wasn't too much to stomach for the added peace of mind.

 

[Dave]

Have you the room to dry line ?

 

[SteUK]

Nope. Enough room for 6mm cement board and 3mm or so of adhesive. That will bring the room out to where it is with existing tiles. Them 12mm on top for 9mm tiles and 3mm adhesive. Have a maybe 5mm to play with on top.

 

[Just Rizzle]

were on with a bathroom this week weve completely stripped the whole room out back to the breeze block inner walls and replaced the walls with wedi boards600 x2.5mts sticking them to the walls with bal flexi addy and then plugged with the mechanical fixings then tanked and tiled walls .there are total flat and plumb plumber has chased the pipes into the breeze block and with the tiles on we are back to the original plaster line why don't you go the whole hog and strip it out totally and re board ? if you use the wedi plugs even on plaster board you could hold an elephant on the walls. would probably give you more room rather than shrinking it with what your proposing at present

 

[SteUK]

I did think about that, and think it would be the way forward but for a few things...

But don't really want to put that much dust up in the air with a 15 month old in the house (probably just paranoia but she seems to have a perminant cough as it is!). Not sure what it actually gives me, other than another 13mm or so to play with. I can get these boards in as things stand. Lastly, and this is the big one that worries me, the internal walls are only 50mm (hollow) cinder/breezeblock with bonding and finish on either side. There is not a lot it is to once you take the bonding out the equation!

 

[Just Rizzle]

I did think about that, and think it would be the way forward but for a few things...

But don't really want to put that much dust up in the air with a 15 month old in the house (probably just paranoia but she seems to have a perminant cough as it is!). Not sure what it actually gives me, other than another 13mm or so to play with. I can get these boards in as things stand. Lastly, and this is the big one that worries me, the internal walls are only 50mm (hollow) cinder/breezeblock with bonding and finish on either side. There is not a lot it is to once you take the bonding out the equation!

if you are worried about the dust get a good resperator not a dust mask go into the room have plenty of rubble sacks close the door behind you feed any extension leads under the door first with the tools u need then with some duck tape seal up the door from inside open the widow if theres one and do the business bag up as you go. I would also take in a water sprayer as this will supress any dust in the air when done take your sealed rubble sacks away that's how we did this job on Tuesday took 2 hrs but hay great fun lol

 

[nybor62]

i would strip back to the block work , dry line ,and tank wet areas . i understand about the dust , but it will be worth it in the end

 

[SJPurdy]

I agree with the above posts that re-boarding is the way to go if the walls are so uneven and the old plaster can be safely removed.
If over-boarding with thinner tile-backer boards (e.g. 6mm) then these have to be fixed on an almost solid bed anyway (adhesive spread with notched trowel), which is going to be very difficult on an uneven wall. Uneven walls can be corrected by dot and dab fixing thicker tile backer-boards and plasterboard to suitable substrates - correct quantity of dabs and end bedding; mechanical fixings; tanking as appropriate. The mechanical fixings (to the solid structural wall) give the boarding its weight bearing ability as they do if the boarding is mechanically fixed to a stud wall.

 

[SteUK]

Thanks for the replies chaps.

What constitutes an uneven wall? How many mm deviation? I recon it undulates at it's worst by 2 or 3 mm. Can I not just use a thicker bed (10mm half circle trowel maybe) to compensate?

In essence, the mechanical fixing increases the weight the finish plaster (with backer boards) can take? So more plugs/screws (to a point) will only aid the security of the board and tile weight onto the plastered wall?

Apologies for being s DIY'er with little knowledge and a lot of anxiety. Just want to do the job right as best I can, that puts no risk to my toddler daughter when she is having a bath!

 

[SJPurdy]

Earlier questions hinted at walls (window for example) being more uneven.
The mechanical fixings should be sufficient to take the weight of the board plus tiling (check with board manufacturer) so the weight is not transferred to the plaster surface.
A substrate is generally deemed to be flat enough to tile if there are only gaps of less than 3mm underneath a 2m long straight edge placed anywhere an the substrate.

 

[Dan]

Yeah 3mm over 2m. In or out it doesn't matter.