Aquapanel on top of plasterboard?

[TheRealRobG]

I'm replacing the double-skinned plasterboard stud wall surface in my bathroom, which is also a party wall with my neighbour, hence the double-thickness plasterboard for fire regs.

As expected, once the tiles were removed, both layers of plasterboard behind the shower are damp and full of black mould, and the vapour barrier behind is covered in droplets of moisture. The front later of plasterboard was extremely soft and the tiles fell off with practically no encouragement.

The studs are at 600mm centres, so I'm adding extras to make them 300mm centres, and the plan is to fix a skin of plasterboard, then a poly vapor barrier followed by an outer skin of Aquapanel and finally tile & grout with 17kg/sqm floor/wall tiles. The idea of the poly barrier is to prevent moisture penetrating the Aquapanel from damaging the plasterboard behind.

However, my concern is that the only Aquapanel screws I can buy are 40mm, but Knauf's website recommends using longer 55mm screws for a double layer board construction which seem not to be available anywhere in the UK.

Do you think if I double the count of screws to one every 100mm rather than 200mm that this would sufficiently compensate for having only 15mm of screw penetration in the wooden studs?

Failing this, is it possible to use drywall screws with Aquapanel, or is this a non-starter? (lack of corrosion / strength / penetration ability?)

[Dave]

Hello and welcome...

Have you not thought about just re-doing the double skin plasterboard and then tanking to prevent any chance of moisture ingression..?

[mikethetile]

too much weight for 15mm penetration, you can get sheridised screws in all sizes

alternatively follow daves advice

[TheRealRobG]

Hello and welcome...
Have you not thought about just re-doing the double skin plasterboard and then tanking to prevent any chance of moisture ingression..?

I hadn't as it happens. I'd come to the conclusion that Aquapanel would be the best possible surface to hold my relatively heavy floor/wall tiles up in the wet areas. It's tempting to just fit a single layer and be done, but I know that's the wrong thing to do.
When you say tanking - what specific kind of treatment did you have in mind for the plasterboard, and are we talking standard p/b or vapourproof?

[Dave]

Just standard 12.5mm plasterboard and then a tanking membrane..

But first.. what tiles are they..?

[TheRealRobG]

Do you happen to know where I can get suitable self-drilling/countersinking corrosion resistant screws? The only ones that spring to mind which might be at all suitable are decking screws, but will they penetrate cement board?

I've seen the Screwfix A2 decking screws, but it sounds like they are brittle and prone to fracturing, which would be a big problem in this scenario...

Can you countersink Aquapanel with a standard C/S bit?

Thanks!

[TheRealRobG]

Just standard 12.5mm plasterboard and then a tanking membrane..

But first.. what tiles are they..?

They're 9.5mm thick 300mm sq from Topps - 17kg/sqm. They're called Padova 10. Rated for floor or wall.

[Dave]

They are fine for the plasterboard way then.....

The choice is yours..

[TheRealRobG]

They are fine for the plasterboard way then.....

The choice is yours..

Thanks for the advice Dave! It seems that replacing the p/b and tanking it is the way to go. There's no point in flogging a dead horse with the Aquapanel if I can't source the correct screws. It'll probably work out a lot cheaper too.

Can you recommend a good tanking agent for plasterboard that would be compatible with Bal PTB (flexible white single part adhesive)?

I don't want to use PVA. I've heard too many bad rumours...

Also, should I be using moisture resistant p/b too?

Thanks!

[Dave]

You can either use a tanking solution kit... BAL do one or as i prefer to use , a tanking membrane... Either Schluters kerdi membrane or Durals Wp..

[TheRealRobG]

You can either use a tanking solution kit... BAL do one or as i prefer to use , a tanking membrane... Either Schluters kerdi membrane or Durals Wp..

I already have some Bal APD which I was going to use to prime the non-Aquapanel areas.

I think I'll get some Bal WP1 to tank the wet areas and return the Aquapanel and associated gubbins to Wickes.

Thanks for all your help!

Rob.

[Dave]

No probs Rob.. please do let us know how you get on..

[TheRealRobG]

OK, so I decided on an experiment. I cut a small square of plasterboard and a small square of Aquapanel, and I drove them both into a bare stud with a single 40mm Aquapanel screw with just 15mm penetration into the stud.

I was able to hang all my weight (80 kilos) from that piece of Aquapanel without any sign of it giving way.

So I think I'm going to trust those screws to hold up what probably amounts to about 2kg of combined board and tiles each without having nightmares...

[pjc]

PTB ? no good for walls :Pete

[TheRealRobG]

PTB ? no good for walls :Pete

Sorry - PTB?

[whitebeam]

Pourable thick bed

[mikethetile]

OK, so I decided on an experiment. I cut a small square of plasterboard and a small square of Aquapanel, and I drove them both into a bare stud with a single 40mm Aquapanel screw with just 15mm penetration into the stud.

I was able to hang all my weight (80 kilos) from that piece of Aquapanel without any sign of it giving way.

So I think I'm going to trust those screws to hold up what probably amounts to about 2kg of combined board and tiles each without having nightmares...

Hi Rob
the rule of thumb is a minimum 50% of screw length penetration into the substrate

knauf reccomend 55mm screws for double width plasterboard

knauf tech dept will advise you on this and give you the details of your nearest stockist

we do our best to give you the best advice we can, we want your job to succeed as much as you do

[TheRealRobG]

Yes, the PTB was sold to me by the numpty store manager of Topps. I since realised he'd boobed when I read the data sheet. Will get it swapped for Single Part Flexible before I start hanging. Thanks for the tip though.

[TheRealRobG]

Hi Rob
the rule of thumb is a minimum 50% of screw length penetration into the substrate
knauf reccomend 55mm screws for double width plasterboard
knauf tech dept will advise you on this and give you the details of your nearest stockist
we do our best to give you the best advice we can, we want your job to succeed as much as you do

Thanks for the comment. Beyond my unscientific stress test, I'm OK with the shorter screws as I figure the recommended screws need to be able to support the max 50kg/m tile loading. My tiles weigh a third of this. Also, the back layer of plasterboard will be supported by it's own screws. Call me crazy, but I'm going to go with my instincts on this one. I emailed Knauf tech a few days ago, but they never got back to me, so I gave up on the official route. Even besides all that, the bottom row of boards will be resting on the floor, and the next row will be sitting directly on them, so what will the screws actually be supporting? After much worry and concern, having carried out my test I finally have a good feeling about this.

[mikethetile]

thats up to you Rob

its your job and your responsibility

[pjc]

Yes, the PTB was sold to me by the numpty store manager of Topps. I since realised he'd boobed when I read the data sheet. Will get it swapped for Single Part Flexible before I start hanging. Thanks for the tip though.

take it back and get your money refunded look at your local small tile shop you will find it cheaper than topps for materials topps charge way above SRP :Pete

[TheRealRobG]

Out of interest, Knauf finally responded:

Dear Robert,

With regards to your recent email, in this instance we would recommend using proprietary moisture resistant screws from another manufacturer to ensure that you have at least 25mm penetration into the timber as per British Standards.

Please do not hesitate to contact us should you require further information.

Kind Regards,
Royston Lorriman
Technical Support Officer - Knauf UK GmbH

I hadn't realised 25mm was a British Standards requirement, but there you go. In this instance, I think 15mm is plenty personally.

[TheRealRobG]

Wanting to explore all avenues - while picking up some plumbing bits from Screwfix today, I also decided to grab a box of Turbo Ultra 4.5 x 50mm. They're slightly less expensive than the genuine Aquapanel items, and have the benefit of being made from A2 stainless, so effectively rustproof rather than just coated.

They're very nicely made (and a lovely colour) and seem to go in very cleanly indeed, with even less cracking around the head than the genuine items when driven fully flush. The head is slightly smaller than the official A/P screws, but less cracking probably means they'll hold better anyway. I think this might be the solution, so I'm going to go with those and return the Aquapanel screws to Wickes.

[mikethetile]

Hi Rob

Im pleased you picked up some 50mm, I dont use screwfix so dont know their prices but i know stainless dont come cheap

as you say they are a pukka job, fit and forget

what price can you put on peace of mind, you clearly want to do a proper job which is why you have come on here seeking advice from the proffessionals

bs are the minimum standards we we work to, we go beyond that in most cases, with all these jobs the devil is in the detail, every part of the job from ripout to finish is done to the highest standard using correct methods and materials

thats why we spend so mmuch time here, we learn from each other and are more than happy to passs on the benifit of our experiance to others wether they are experianced or new to tiling

we wont give you the wrong info, if 40mm screws were adequate for double tacking we would say so but I have seen failures over the years when overbording has been done with 40mm screws and the only weight has been the boards and skim and some had been up a few years before failing

good luck with the job, keep us posted as you go, post pics we love pics on here and stay and have the craic with us

mike

[trevortine]

Wanting to explore all avenues - while picking up some plumbing bits from Screwfix today, I also decided to grab a box of Turbo Ultra 4.5 x 50mm. They're slightly less expensive than the genuine Aquapanel items, and have the benefit of being made from A2 stainless, so effectively rustproof rather than just coated.

They're very nicely made (and a lovely colour) and seem to go in very cleanly indeed, with even less cracking around the head than the genuine items when driven fully flush. The head is slightly smaller than the official A/P screws, but less cracking probably means they'll hold better anyway. I think this might be the solution, so I'm going to go with those and return the Aquapanel screws to Wickes.

Was gonna say you could use stainless steel screws (available from screwfix) but you bet me too it! Is the inner plasterboard sheet of the pink variety, ie, fireboard. If so make sure you replace like for like

[TheRealRobG]

Was gonna say you could use stainless steel screws (available from screwfix) but you bet me too it! Is the inner plasterboard sheet of the pink variety, ie, fireboard. If so make sure you replace like for like

No, it was standard white as far as I could tell, though very slightly thicker than standard. I replaced it with 12.5mm as that's what I had, but I figure the Aquapanel is probably better fire protection than standard plasterboard anyway. I'm not taking it all down now though, so that's how it's going to stay.

The Screwfix Turbo Ultra screws worked great. Better than the Aquapanel screws if anything, as they countersink without 'blow-out' unlike the A/P screw heads. I did have a couple of heads shear off the Ultra screws when I ran into knots in the studs (A2 stainless isn't as forgiving as standard screw steel), but I simply added an extra screw adjacent to the broken one. Two out of 300 breaking isn't a bad ratio. All in all, I think I'd definitely choose these over the Aquapanel screws if I tackle another similar job.

[TheRealRobG]

Here are some photos of my work-in-progress as requested. Note that I backed the Aquapanel behind the shower and along the bath wall with a vapour barrier which is just stapled along the top and the A/P screwed over it. I'll trim it so that it hangs slightly over the upstand on the shower and is covered by the tile bottom edge.