Float and set or dot and dab?

kjacko · 7 Dec 2020

Hi all,
So I”m renovating my brothers house, built in 1901. It has double skin brick wall, no cavity.
I’ve been told NOT to dot’n’dab plasterboard as the moisture could travel through the bricks, through the board adhesive and into the plastered walls(?).
However, about 12 yeas ago he had the exterior of the house coated in some stuff from protectacoat. Having done a search in these forums and google it seems the company were less than well received by many.
That said, the product still seems in good condition. If so could dot’n’dab now be feasible as the outside is protected?
One wall that I have to do has been stripped back to the bricks, but it’s 2” out of plumb at the bottom. Filling 2” with hardwall could take a while.
Sadly most of the walls need stripping back to brick as the plaster is turning to sand. Plus they’ve been wallpapered so taking that off will half destroy the wall anyway.
There are 2 internal lathe and plaster walls where I’ve managed to board over 9.5mm using screws and foam adhesive.
But the internal sides of the exterior walls I need advice on.
So, please.....advise me.

markyyyyyy · 7 Dec 2020

I'd be very keen to hear also, as i'm in exactly the same position. My house built in 1902.
stripped inside (kitchen side) of exterior wall back to brick. Is sloping away (ie t get level the pboard will be 2.5cm further out from wall at top than bottom)

I concluded through posters on this forum that i was going to tackle it by injecting a dpc, tanking the wal (kitchen side internal)l with tanking slurry then using insulated plasterboard fixed to the wall. Ideally with foam, but i want to use dot and dab as its much easier to use to make the wall level.

I have researched constantly and all technical advisors at KA tanking and everbuild (febtank) advise using plasterboard foam rather than adhesive as the moisture has nowhere to go.

One website says you can dot and dab onto tanking slurry but you must have a gap at the bottom so air can circulate.

I am hoping to go with this second option as its far easier to get a wall straight, and in principle, much more straightforward.

I would also like someone to confirm if they have successfully dot and dab insulated plasterboard onto tanking slurry and had no damp issues from it?

Wayners · 7 Dec 2020

I'm not an expert but I do plaster. If you were a customer I'd advise you to investigate lime on external walls.
This explains. Also consider paints that breath

dontbelieveawordofit · 7 Dec 2020

in what capacity are you doing the work?
diyer, plasterer, ???

Notch7 · 7 Dec 2020

2" out of plumb and solid external wall with unknown damp resistance.

Batten out and fit foil backed plasterboard, stop boards off floor by an inch

I would avoid hardball if there's any risk of damp

dontbelieveawordofit · 7 Dec 2020

pretty much what he said.

kjacko · 8 Dec 2020

For me, I like to think I gauge firmly between DIYer and semi-pro.
I have “most of the gear and a good idea”.
Recently showed a friend how to plaster, gave him enough confidence and we skimmed about 40sqm of walls and ceiling together and did a really good job as we’re both fusspots when it comes to quality and attention to detail.
That said, there’s still plenty I don’t know...hence this place

I have 3 bags of hardwall, the walls are bone dry. (I had to float’n’set the entire kitchen 4.4m x 2.4m, but it wasn’t easy).
One of the walls has a rad on it, so that has to come off anyway. There’s also a window in the other wall waiting to be stripped. Only took me 20 mins to strip one wall back to brick as all the render and finish I has dried and crumbled. It’s just.....so messy

Also wondering how many would say “don’t worry about it being plum, just follow the line of the wall” and how much it would actually matter?

edit: btw this is an upstairs room sitting atop newly replaced floorboards.

markyyyyyy · 8 Dec 2020

Notch7 said:
2" out of plumb and solid external wall with unknown damp resistance.

Batten out and fit foil backed plasterboard, stop boards off floor by an inch

I would avoid hardball if there's any risk of damp

Wouldn't tanking slurry stop moisture coming through though?

My wall has one doorway, one window and a boiler next to that with pipework all over it.
Battening out isn't feasible.

I'm just going to install DPC, tank the wall (2 coats) and dot and dab insulated board.

I can't see how this is not a feasible option.

dontbelieveawordofit · 8 Dec 2020

I wouldn’t worry a huge amount about plumb unless its a kitchen or bathroom.
and being theres no kitchen hanging off it id go the batten and board route .
can’t stand hardwall in most cases ,which doesn’t help.

dontbelieveawordofit · 8 Dec 2020

markyyyyyy said:
Wouldn't tanking slurry stop moisture coming through though?

My wall has one doorway, one window and a boiler next to that with pipework all over it.
Battening out isn't feasible.

I'm just going to install DPC, tank the wall (2 coats) and dot and dab insulated board.

I can't see how this is not a feasible option.

plasterboard adhesive needs suction and a key to bond to the substrate.
tanking can end up quite smooth and definitely wont allow for any suction, in fact you could find that your adhesive will end up sliding down the wall before you get a chance to get the board up.
It could be hit and miss whether the adhesive and tanking will create enough of a bond. that’s why I suggested earlier to try a test area.
also, this time of year large adhesive blobs on the wall will have no way of drying out

markyyyyyy · 8 Dec 2020

Right OK. But if I tank the whole wall and then the adhesive doesn't stick, what do I do?

This is a view of the whole wall on question:

dontbelieveawordofit · 8 Dec 2020

https://www.toolstation.com/soudal-...MIqa_xquG-7QIVTQKLCh0-bwtGEAQYASABEgKKifD_BwE

markyyyyyy · 8 Dec 2020

Cool thanks.
In an ideal world I would use that instead of dabs but I have 30mm to make up on some parts of the wall, hence dabs more preferable form this point or view. Cheers.

kjacko · 9 Dec 2020

I thought hardwall was the “appropriate” product to use in this case.
From what I’ve taken off the walls so far it just looks like sand/cement render.
This wall will only have dado trunking stuck to it, so if dot’n’dab dab pboard will work, then it’s certainly an easier option for me.
Too many opinions that differ to make an informed choice.

markyyyyyy · 9 Dec 2020

kjacko said:
I thought hardwall was the “appropriate” product to use in this case.
From what I’ve taken off the walls so far it just looks like sand/cement render.
This wall will only have dado trunking stuck to it, so if dot’n’dab dab pboard will work, then it’s certainly an easier option for me.
Too many opinions that differ to make an informed choice.

All of the difernt options have been driving me crazy. Due to Doors, Windows and pipework all over my wal, I'm doing the folowing

1. Inject DPC.
2. Tanking slurry - 2 coats
3. Dot and dab insulated plasterboard

I'll let you know how it goes!

diynot

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Float and set or dot and dab?

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