Lightweight ceiling board options

[Hammertime]

I'm a bit concerned about weight on my ceiling and I wondered if there were lightweight alternatives to 12.5mm plasterboard?

 

The ceiling follows the line of the pitched roof (bungalow), which has heavy stone tiles on top. Rafters at 400mm centres. Originally had 12.5mm plasterboard and gypsum skim. I removed this to insulate.

 

I'd like to use clay plaster instead of gypsum, which is both a lot heavier and thicker (5-10mm). I've also got a heavy light fitting I'd like to install. I'm therefore concerned about weight pulling down on the pitched roof, so I thought about lighter boards.

 

Current options over a 15m2 pitched ceiling:

- 12.5mm plasterboard: would be 122kg over the 15m2. What was there previously, but I'll now have the added weight of clay plaster and heavier light fitting

- 9.5mm: lighter (92kg), but there are mixed opinions about whether this would sag over time?

- 15mm Cewood wood wool board: about 105kg. good base for the clay plaster

- 12.5mm xps tile backer board: would seem a good option, but issue around fire rating

 

Is there anything else I could use? Or am I overthinking this?

 

Thanks in advance.

[Russell griffiths]

Why the clay plaster ?

if you had 3 chubby roofers working on your roof would you be concerned it was going to collapse?

if it had plasterboard and skim before then all you are adding is the weight of the clay plaster, minus the original weight of the skim plaster. 
 

if you think that will overload your timbers then they should all be replaced now. 
 

how much do you think snow load is calculated at ?

far more than a light fixture 

I think you need something else to worry about. 

[Hammertime]

Thank you @Russell griffiths for the reply and the reassurance. With everything that needs to be considered in a renovation, it's easy to go down mental cul-de-sacs.

 

You're right, I wouldn't be concerned by people on the roof, but I'd been thinking about creep caused by a long term dead load. The weight difference between 5mm clay and a 3mm gypsum skim across the ceiling is 10 stone; at 10mm, it's 28 stone, so three skinny roofers permanently up there (I suppose they would be skinny if they were stuck on the roof - although it's only a bungalow, so they could jump down...). The light fitting is going to be a branch, so - depending on what turns up - might be a fourth skinny roofer.

 

3 hours ago, Russell griffiths said:

Why the clay plaster

I think it's got a nice cloudy look to it, but it's also meant to aid with moisture buffering, even at 5-10mm, as well as reduce odours.

 

3 hours ago, Russell griffiths said:

I think you need something else to worry about

Don't worry, there's the kitchen, fitting the wetroom, ventilation strategy, flooring, hot water system...I've got plenty to choose from🤣

[Russell griffiths]

Have you looked at clay plaster that is applied at thinner thickness 

a mate of mine did a couple of walls in a hotel recently and I think he said it was thin like skim coat, he troweld it up for hours and it changed colour the more he worked it. 

[Mike]

2 hours ago, Hammertime said:

it's also meant to aid with moisture buffering, even at 5-10mm, as well as reduce odours.

Clay plaster does have better moisture buffering qualities and can moderate daily variations but, if you have adequate heating and ventilation, that won't make a big difference. If you don't have adequate heating and ventilation, consider those first. Unpainted clay plaster can look good though!

 

For more, see my comment in this thread.

 

6 hours ago, Russell griffiths said:

if you think that will overload your timbers then they should all be replaced now.

^ This.

[Hammertime]

Thank you for the replies.

 

As @Mike says (that link was very useful, btw), moisture buffering is less important if ventilation and heating are decent, so perhaps I can use the clay just for aesthetics with a thinner layer as @Russell griffiths suggested. 3mm clay is only 1kg per m2 heavier than gypsum, so 15kg extra overall.

 

1 hour ago, Mike said:

8 hours ago, Russell griffiths said:

if you think that will overload your timbers then they should all be replaced now.

^ This.

So it sounds like I'm overthinking it and the 12.5mm plasterboard, clay finish and tree branch light fitting will be fine for weight and better than the 9.5mm?

 

One other thought - is it worth using wood wool board instead? Presumably it has some moisture buffering too which might help if a thinner layer of clay is used.

 

Thanks again.

Edited 13 hours ago by Hammertime

[Mike]

11 hours ago, Hammertime said:

is it worth using wood wool board instead? Presumably it has some moisture buffering too which might help if a thinner layer of clay is used.

I've not used them myself, but AFAIK they have good vapour permeability; I'm not sure about moisture buffering.

 

However, for a standard ceiling you normally wouldn't worry much about moisture so, if I had spare cash I'd choose to upgrade the insulation instead, by using either flexible wood fibre (e.g. Steico Flex) or hemp (e.g. IndiBreathe Flex). Both of these handle moisture better than synthetic insulation and also provide a much improved decrement delay (non-technically = resistance against summer heat). Hemp edges ahead of wood fibre for moisture handling, while wood fibre edges ahead for decrement delay, but not a big difference.

 

11 hours ago, Hammertime said:

the 12.5mm plasterboard, clay finish and tree branch light fitting will be fine for weight

Don't hang the light fitting from the plasterboard; you need to fix it through the plasterboard / wood wool board into a joist, or into a nogging fixed between two joists.

 

20 hours ago, Hammertime said:

9.5mm: lighter (92kg), but there are mixed opinions about whether this would sag over time?

9.5mm is OK if your joists are no more than 450mm apart, but it will be a little less robust & have a little less fire and sound resistance. I'd choose 12.5mm unless cost is a factor.

 

Edited 1 hour ago by Mike