Rendering on plasterboard

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Woods

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How would one best prepare plasterboard for rendering?
>inb4 "you shouldn't render plasterboard"
yeah I know, but it's for soundproofing; more mass, less sound penetration

I've done some tests and though my mortar does stick to the ceiling, when trying to get it flat, it just tears off. Putting a rule to it and pretty much everything comes off. I was thinking to roughly skim coat it with with a very sticky, sloppy mix with some sand in it and let it go off to have something the render can more easily stick to.

I'd like to do a 10mm coat, rule it straight and float.

I do know that Tierrafino has a rough primer which is use to get loam plasters to bond to concrete and plasterboard. But it cost 100 euro for a bucket and I would like to avoid that.

If you have some ideas I could test out, feel free to educate me!

Cheers
 
quit being so surprised and just tell me the cheapest way to keep the render from tearing, that is, if you know of one. I'm guessing I either need a bonding agent or just skim coat it with render, scratch it, and let it dry out and then apply the float coat. I think a skim/slurry coat is going to be the cheapest. If you have better ideas, go ahead then

a little context: I've never rendered plasterboard. I made a mistake in getting gypsum boards intended for taping mud, not plastering. The boards are extremely low suction and very smooth but I use a clay/lime/sand mortar that is rather sticky so it does stick to boards pretty well. The problems just start when I try to flatten it: because the suction is so low, the render doesn't really get dry enough where it makes contact to the board: it stays wet there. So when I try to rule it off, I'm tearing everything off. It's not a timing issue because I waited a long time but no luck.

Usually I render lath and reed ceilings, hence my confusion currently. Since the people here have more experience with plasterboard, I thought maybe some of you could help me figure this out. But maybe it's too tall of an order for some of you?
 
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Just double up on acoustic board then skim it. You could also counter batten with resilient bars to the existing ceiling before you board it for belt and braces.

Do not go ahead with your idea it's shite and will fail.
 
>Do not go ahead with your idea it's shite and will fail.

No it's not. I did some test patches and once the mortar is set, the bond to the boards is extremely strong. Acoustic boards are too low in mass, too expensive for me and do not fully seal.
Jesus you folk lack imagination
 
Woods said:
>Do not go ahead with your idea it's shite and will fail.

No it's not. I did some test patches and once the mortar is set, the bond to the boards is extremely strong. Acoustic boards are too low in mass, too expensive for me and do not fully seal.
Jesus you folk lack imagination
Click to expand...
Your problem is you can’t put render onto gypsum plasterboards,and hope this helps
 
Stewie03 said:
Your problem is you can’t put render onto gypsum plasterboards,and hope this helps
Click to expand...
maybe that's the case for cement based renders, but like I said, clay-lime mortar bonds really well to it once it dries. I'll post some pictures tonight to show you. Once it's dry, it does not come off. You'd need a hammer to get it off.
 
as promised, some illustrations:

1. short video of a little test I did couple weeks ago to see if the material would bond well enough:

2. short video showing the first skim layer that acts as a primer (bonding agent) for the second coat, which you can also see in the video. The second coat rules off nicely without tearing whatsoever.

For clarity's sake: this is earth-lime mortar (10% quicklime, 30% clay, 60% sharp sand). This stuff sticks really well. It's just that these gypsum boards have no suction whatsoever, so the initial bond isn't strong enough to rule it off. But once it's dried out, it bonds extremely well. Then the second coat has a the first coat to grip on to, and that has a lot more initial suction so the initial bond is strong enough to be able to rule it off straight away.

>Are you based in USA?
No, Belgium
 
Don't understand why you asked our advice if you're just going to crack on with your own method.
You do what you want, but there's more chance of it shelling off on landing on the floor/ furniture/ family than staying put.
If the ceiling is downstairs then you also have to think of the foot traffic above also.
 
I was asking you lot specifically about how to try and solve my problem. In stead 90% of you uttered garbage and a few of you just told me that my idea wouldn't work, without much explanation as to why.
Of course I was gonna carry on with my own plan; I figure a lot of you just don't know what earth-lime mortar feels like and how well it bonds and sticks to pretty much anything. If you're used to sand cement render then yeah, I would agree: very bad idea to try and get that on to gypsum board.
I came here because I have no experience with gypsum board and to ask if any of you knew of a good bonding agent or method that could potentially help with my case. I think it's been a misunderstanding on your part, because I wasn't using sand/cement. I think, if you want to get an idea of what this stuff is like, think of bonding plaster.

>If the ceiling is downstairs then you also have to think of the foot traffic above
1. honestly, the bond is too strong for that to happen
2. this is actually an independent ceiling: there is no physical connection to the upper floor joists.

>there's more chance of it shelling off on landing on the floor/ furniture/ family than staying put
watch the first video. It's not gonna happen, the bond is far too strong. Unless there's a massive leak upstairs, maybe.
 
It's an independent ceiling and usually, with normal ceilings, I staple reed mat to the ceiling laths, but in this case, I first tried the reed mats and I attached them to the joists without using intermediary ceiling laths (don't know the proper english term but basically the wooden laths you attach to the floor joists which you then screw your gypsum boards onto).

Anyway, because the reed lath was attached to the joists directly, the joists absorbed all of the moisture from the render. Soaked wood + weight = bending. So I had to tear it all down.

here's a picture:
20240322_152923.jpg


as you can see, the reed is directly attached to the joists and in that picture, the joists are absorbing all of the moisture from the mortar.

Here is how I corrected the problem:
Rendering on plasterboard


In the picture you can see the ceiling laths that I didn't use previously.

I replaced the bent joists, used ceiling laths and then screwed gypsum boards because I was paranoid of moisture creeping into the joists again. So figured, gypsum board would be a bit safer. Problem is, the gypsum board by itself has not enough mass to reach the sound proofing levels I want. So I needed to apply another coat of render to get more mass. And that's where we're at now.

In hindsight, I should've just gone with the reed laths. My previous mistakes was that I didn't use ceiling laths on the joists and then attach the reed to that. The ceiling laths are screwed into the joists with distance screws (to level it out) and so the ceiling lath doesn't make contact with the joists. So there's no bridge for the moisture to get into the joists.

Of course, lessons learned and next time I'll just go with the reed because I do agree that would be a superior bond since it's mostly a mechanical one.

But even so, the bond that I'm getting currently is really strong; compare it to bonding gypsum plaster. And I've seen people do bonding on plasterboard so yeah

edit: in any case, I'll keep posting some updates here and if it does ever come off, I'll report it for the sake of learning. I think that's fair. Btw, this is my own property, so the risk (if there is any) is my own. I'm still figuring some things out for myself, and I wouldn't do this for a client btw.
 
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I have floated [plastered] on reed ceilings on english heritage work. I used lime mortar with animal hair added. The amount of hair that you need in the mix, lift a bucket trowel of the mix and if the hair hangs off the trowel like a mans beard then the mix is correct.
You need to either 2 coat or 3 coat the reed ceiling to get the ceiling flat. I rule it in with a derby rule.

The plasterboard lath sheets that you are using are no longer produced in UK. I have floated them years ago. The material was Thistle fibre [hair] Browning mixed 3 and 1 with sand. Apply thistle finish coat a couple of days later. This work was carried out for fire proofing not sound proofing. None of these materials are available today.
To make a building soundproof is very difficult ,as noise travels through floors walls and ceilings.
 
Gypsum is not ideal for soundproofing as it lacks mass. Lime plasters, earth plasters and sand/cement is much better. Mass alone won't do the trick though. You need a mass/spring/mass system, in physics terms. What it means in reality is a layer of mass, then a layer of air (large cavity - which acts as the spring) and then another layer of mass. This is the most efficient way to reduce sound. But there is a caveat: both layers of mass cannot be in direct contact with each other.

This is why I built an independent ceiling. The floor above is the first layer of mass. Then you have a cavity (which is filled with absorption material to stop the cavity from resonating) which is the spring. And then the independent ceiling below is another layer of mass.

I had already 2 coated this ceiling with earth-lime mortar and the soundproofing was perfect. Footsteps were gone, music and talking was gone. Before it seemed like there was just cardboard between the floors. So I know my system works. It's just a shame that I made a mistake and attached the reed directly to the joists and the joists didn't like that (even though they smiled).

In its current state, the soundproofing does not work, because my second layer of mass is currently only a 9mm gypsum board. Another 10mm of earth-lime render and then a couple mill of lime/fine sand finish should do the trick.
 
>The plasterboard lath sheets that you are using are no longer produced in UK.
I'm using regular reed mats:
www.hubo.be

Rietmat 2x5 m | Hubo

Deze rietmat leent zich perfect als wind- en zichtscherm en zorgt zo voor privacy in je tuin. De afsluitmat is vervaardigd uit gepeld riet, samengebonden
www.hubo.be www.hubo.be

There's also specialty made ones:

but in my opinion, the professional reed mats are spaced too far apart so you get fuckton of dropouts. Unironically, the regular ones from my local hardware store are much better. The reed is a bit thinner but it's much smaller space and I get pretty much no dropouts. Dropouts are a pain in the neck.

>I rule it in with a derby rule.
I use a darby as well, but only to move the mud around and get it reasonably flat. Then I take a longer straight edge on the side for the final passes. It roughens up and takes off the high spots and the low spot are then easily identified because they're shiny. Then I fill the low spots and take the long straight edge on the side for the ultimate final pass and leave the surface rough. When I come back to float it, it floats like a dream.
 
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Ive gone over wood wool boards with lime plaster at 10mm which needs render mesh inbedded as you go. Top coat with a fine lime skim coat 2mm, im guessing earth lime borne plasters would adhere too.
 
Woods said:
In its current state, the soundproofing does not work, because my second layer of mass is currently only a 9mm gypsum board. Another 10mm of earth-lime render and then a couple mill of lime/fine sand finish should do the trick.
Click to expand...

I have had a spec from an Architect for soundproofing a music room. I had to place kiln dried sand onto the plasterboard.

When plastering out a recording studio the walls and the ceiling was covered in cork. The ceiling lines and the internal angles were formed using a plastic football. This was because they did not want a music note caught in the corners.
 
Woods said:
It's an independent ceiling and usually, with normal ceilings, I staple reed mat to the ceiling laths, but in this case, I first tried the reed mats and I attached them to the joists without using intermediary ceiling laths (don't know the proper english term but basically the wooden laths you attach to the floor joists which you then screw your gypsum boards onto).

Anyway, because the reed lath was attached to the joists directly, the joists absorbed all of the moisture from the render. Soaked wood + weight = bending. So I had to tear it all down.

here's a picture:
View attachment 78775

as you can see, the reed is directly attached to the joists and in that picture, the joists are absorbing all of the moisture from the mortar.

Here is how I corrected the problem:
View attachment 78776

In the picture you can see the ceiling laths that I didn't use previously.

I replaced the bent joists, used ceiling laths and then screwed gypsum boards because I was paranoid of moisture creeping into the joists again. So figured, gypsum board would be a bit safer. Problem is, the gypsum board by itself has not enough mass to reach the sound proofing levels I want. So I needed to apply another coat of render to get more mass. And that's where we're at now.

In hindsight, I should've just gone with the reed laths. My previous mistakes was that I didn't use ceiling laths on the joists and then attach the reed to that. The ceiling laths are screwed into the joists with distance screws (to level it out) and so the ceiling lath doesn't make contact with the joists. So there's no bridge for the moisture to get into the joists.

Of course, lessons learned and next time I'll just go with the reed because I do agree that would be a superior bond since it's mostly a mechanical one.

But even so, the bond that I'm getting currently is really strong; compare it to bonding gypsum plaster. And I've seen people do bonding on plasterboard so yeah

edit: in any case, I'll keep posting some updates here and if it does ever come off, I'll report it for the sake of learning. I think that's fair. Btw, this is my own property, so the risk (if there is any) is my own. I'm still figuring some things out for myself, and I wouldn't do this for a client btw.
Click to expand...
Just scim it after throwing loads of insulation into the massive void above
 
>Just scim it after throwing loads of insulation into the massive void above
see my earlier post. Insulation goes in the cavity simply to stop the cavity from resonating. It hardly stops sound. What you need is mass. And more specifically: mass/spring/mass.

>I had to place kiln dried sand onto the plasterboard.
yeah the sand acts as a damper. I had contemplated this for my own designs but sand would've been too heavy for my independent ceiling. I am however, building a door that is filled with dried sand soon (hopefully).

>Ive gone over wood wool boards with lime plaster at 10mm which needs render mesh inbedded as you go. Top coat with a fine lime skim coat 2mm, im guessing earth lime borne plasters would adhere too.
Yeah same here. The boards we use are made by Pavatex or Steico. We used to do reveals with them and coat them with 10mm (sometimes more) loam plaster (without lime even).
In this case, I'm adding quicklime to my mix which makes the loam mortar even stickier.

Today's update:

Showing progress & some explanations

[ note: earth-lime mortar doesn't set like sand/cement. It dries and once dry, the lime take a few months to fully carbonate. So don't worry, I'll still be able to float in the morning ;) ]

View from above:

Putting a level to it:
 
Woods said:
How would one best prepare plasterboard for rendering?
>inb4 "you shouldn't render plasterboard"
yeah I know, but it's for soundproofing; more mass, less sound penetration

I've done some tests and though my mortar does stick to the ceiling, when trying to get it flat, it just tears off. Putting a rule to it and pretty much everything comes off. I was thinking to roughly skim coat it with with a very sticky, sloppy mix with some sand in it and let it go off to have something the render can more easily stick to.

I'd like to do a 10mm coat, rule it straight and float.

I do know that Tierrafino has a rough primer which is use to get loam plasters to bond to concrete and plasterboard. But it cost 100 euro for a bucket and I would like to avoid that.

If you have some ideas I could test out, feel free to educate me!

Cheers
Click to expand...
 
Woods said:
How would one best prepare plasterboard for rendering?
>inb4 "you shouldn't render plasterboard"
yeah I know, but it's for soundproofing; more mass, less sound penetration

I've done some tests and though my mortar does stick to the ceiling, when trying to get it flat, it just tears off. Putting a rule to it and pretty much everything comes off. I was thinking to roughly skim coat it with with a very sticky, sloppy mix with some sand in it and let it go off to have something the render can more easily stick to.

I'd like to do a 10mm coat, rule it straight and float.

I do know that Tierrafino has a rough primer which is use to get loam plasters to bond to concrete and plasterboard. But it cost 100 euro for a bucket and I would like to avoid that.

If you have some ideas I could test out, feel free to educate me!

Cheers
Click to expand...
Hi as you have already boarded the ceiling you could apply rib lath or chicken wire mesh apply pricking up coat or embedded render mesh in drywall adhesive to face of plasterboard just a thought ?
 
jenkstar1 said:
Hi as you have already boarded the ceiling you could apply rib lath or chicken wire mesh apply pricking up coat or embedded render mesh in drywall adhesive to face of plasterboard just a thought ?
Click to expand...
yeah I had a similar thought right after I put on the first layer. It would've definitely been a superior mechanical bond. But so far, it bonds to the ceiling very well, especially now that it has dried some more. I can hit it with my hand, fairly hard, and it feels solid. I don't think this will delaminate under normal conditions. Maybe an earthquake or a massive leak upstairs could do the trick... Thanks for the suggestion though. If ever have to do something similar for a client, I am definitely going to use a mesh.
 
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