Fixing decorative ceiling panels (10mm MDF) - substructure strength

fifthofwhisky

I need to fix approx 6 m2 of 10mm MDF decorative panels to a 2x3m ceiling. The panels are similar to tongue and groove and use a metal clip to secure the panels to the substructure. The substructure requires a timber lath at 40cm pitch to which the metal clips are affixed. 6 laths can be fixed across the 2m ceiling width with one at each edge and 4 equi-spaced in between (at 40cm pitch). I need to minimise the height loss by the substructure and ceiling panels. 
The existing ceiling is 1/2" (old money) plasterboard affixed by plasterboard nails to the roof joists (they are approx 40cm pitch running horizontal to the laths I need to fix in the other direction).

My question concerns the strength of the substructure. The supplier of the ceiling panels is recommending 22mm laths (in their example the laths are screwed directly to the timber joists - there is no plasterboard). The ceiling in question is a stairway and a vertical timber has been added to support the joists (at approx 2/3 distance in the stairway, i.e 1.4m or 0.6m from the walls lengthways) when a roof access panel and steps was installed. I think the structure can carry the additional weight of the panels. My plan is to fix the laths to the plasterboard with contact adhesive (or similar) then screw through the plasterboard at 40cm pitch into the roof joists with long screws (about 2").

Do you think the retention strength of this method will be adequate (essentially its the same 40cm x 40cm fixing grid)? If yes, what would then be what is the minimum thickness of the laths to retain sufficient rigidity and strength?  

Doozergirl

In short,  you want to hang 6 square metres of mdf to a plasterboard ceiling with contact adhesive.  And you want to secure the plasterboard with a few screws.  

Put it this way, I'd not sleep at night if I did that in a client's house.  

I know contact adhesive is supposed to be a amazing, and we use it a lot, but if something is relying heavily on defying gravity, then mechanical fixing is the way to go. If your plasterboard is plastered and painted then you're relying on the adhesion of the panel to the contact adhesive, contact adhesive to the paint,  paint to the plaster and plaster to the board, then the board to the timbers.   There's a lot to go wrong there.  

If the ceiling height was a problem, I'd remove the plasterboard and create a suitable substructure for the panels to screw directly into, as per the instructions.   Screws were invented for good reason, IMO. 

(I think you mean 'at 40cm centres' not 'pitch' by the way.  A pitch is the degree of an angle, its measurement can't be in length) 

FreeBear

Doozergirl said: In short,  you want to hang 6 square metres of mdf to a plasterboard ceiling with contact adhesive.  And you want to secure the plasterboard with a few screws. 

To be fair, the OP did say he would be trying to screw in to the joists behind the plasterboard - A bit of a crap shoot, but if he can hit the center of the joists and get at least 25mm (30mm would be better) of engagement of screw in timber, it should stay up.

But T&G is very 1970s, so no accounting for taste

fifthofwhisky

Doozergirl said:

In short,  you want to hang 6 square metres of mdf to a plasterboard ceiling with contact adhesive.  And you want to secure the plasterboard with a few screws.  

No, that is not what I said. I said I would secure the MDF to laths which are screwed into the roof joists (essentially the fixing method recommended by the manufacturer). The plasterboard is immaterial to this method - the main consideration being the weight. The contact adhesive is to hold the laths in place while I screw through to the joists.

Put it this way, I'd not sleep at night if I did that in a client's house.  
If it was done as you say - which is not what I'm proposing - neither would I. 

I know contact adhesive is supposed to be a amazing, and we use it a lot, but if something is relying heavily on defying gravity, then mechanical fixing is the way to go. If your plasterboard is plastered and painted then you're relying on the adhesion of the panel to the contact adhesive, contact adhesive to the paint,  paint to the plaster and plaster to the board, then the board to the timbers.   There's a lot to go wrong there.  
See comments above

If the ceiling height was a problem, I'd remove the plasterboard and create a suitable substructure for the panels to screw directly into, as per the instructions.   Screws were invented for good reason, IMO. 
This could be done but not really practical due to building occupancy and not wanting to do the extra work required. 

(I think you mean 'at 40cm centres' not 'pitch' by the way.  A pitch is the degree of an angle, its measurement can't be in length) 
It could be interpreted that way, though in common usage the terms are sometimes interchangeable. For example, https://konigwheels.com/wheel-info-tech/what-is-pcd/
So can you answer the question?

fifthofwhisky

FreeBear said:

Doozergirl said: In short,  you want to hang 6 square metres of mdf to a plasterboard ceiling with contact adhesive.  And you want to secure the plasterboard with a few screws. 

To be fair, the OP did say he would be trying to screw in to the joists behind the plasterboard - A bit of a crap shoot, but if he can hit the center of the joists and get at least 25mm (30mm would be better) of engagement of screw in timber, it should stay up.

But T&G is very 1970s, so no accounting for taste

Well, it's not the "T&G" you are thinking of.
Yes, I can easily get 30mm or more in through into the joist. 
So what is the minimum lath depth I can get away with?

Doozergirl

fifthofwhisky said:

Doozergirl said:

In short,  you want to hang 6 square metres of mdf to a plasterboard ceiling with contact adhesive.  And you want to secure the plasterboard with a few screws.  

No, that is not what I said. I said I would secure the MDF to laths which are screwed into the roof joists (essentially the fixing method recommended by the manufacturer). The plasterboard is immaterial to this method - the main consideration being the weight. The contact adhesive is to hold the laths in place while I screw through to the joists.

Put it this way, I'd not sleep at night if I did that in a client's house.  
If it was done as you say - which is not what I'm proposing - neither would I. 

I know contact adhesive is supposed to be a amazing, and we use it a lot, but if something is relying heavily on defying gravity, then mechanical fixing is the way to go. If your plasterboard is plastered and painted then you're relying on the adhesion of the panel to the contact adhesive, contact adhesive to the paint,  paint to the plaster and plaster to the board, then the board to the timbers.   There's a lot to go wrong there.  
See comments above

If the ceiling height was a problem, I'd remove the plasterboard and create a suitable substructure for the panels to screw directly into, as per the instructions.   Screws were invented for good reason, IMO. 
This could be done but not really practical due to building occupancy and not wanting to do the extra work required. 

(I think you mean 'at 40cm centres' not 'pitch' by the way.  A pitch is the degree of an angle, its measurement can't be in length) 
It could be interpreted that way, though in common usage the terms are sometimes interchangeable. For example, https://konigwheels.com/wheel-info-tech/what-is-pcd/
So can you answer the question?

So we're both clear that I didn't understand a word of what you wrote.  

FreeBear

fifthofwhisky said: So what is the minimum lath depth I can get away with?

I'd go for 2x1 (50mm x 25mm in modern units) battens - Cheap, readily available, and you are not limited to 2.4m lengths.

As this is a ceiling, I'd also fill the gap between the battens with Celotex/Kingspan type insulation, or at a push, expanded polystyrene. Might as well make use of the space.

-taff

Holding laths in place? Are we talking about the same type of laths that is usually meant by laths? i.e. really thin pieces of wood usually making up a lath and plaster wall....

fifthofwhisky

-taff said:

Holding laths in place? Are we talking about the same type of laths that is usually meant by laths? i.e. really thin pieces of wood usually making up a lath and plaster wall....

Laths meaning softwood (pine or spruce) 12mm, 15mm, 18mm, 22mm, or 25mm thick x 34mm or 44mm wide. The can be used for a plaster wall, also for roofing, etc.

fifthofwhisky

Doozergirl said:

fifthofwhisky said:

Doozergirl said:

In short,  you want to hang 6 square metres of mdf to a plasterboard ceiling with contact adhesive.  And you want to secure the plasterboard with a few screws.  

No, that is not what I said. I said I would secure the MDF to laths which are screwed into the roof joists (essentially the fixing method recommended by the manufacturer). The plasterboard is immaterial to this method - the main consideration being the weight. The contact adhesive is to hold the laths in place while I screw through to the joists.

Put it this way, I'd not sleep at night if I did that in a client's house.  
If it was done as you say - which is not what I'm proposing - neither would I. 

I know contact adhesive is supposed to be a amazing, and we use it a lot, but if something is relying heavily on defying gravity, then mechanical fixing is the way to go. If your plasterboard is plastered and painted then you're relying on the adhesion of the panel to the contact adhesive, contact adhesive to the paint,  paint to the plaster and plaster to the board, then the board to the timbers.   There's a lot to go wrong there.  
See comments above

If the ceiling height was a problem, I'd remove the plasterboard and create a suitable substructure for the panels to screw directly into, as per the instructions.   Screws were invented for good reason, IMO. 
This could be done but not really practical due to building occupancy and not wanting to do the extra work required. 

(I think you mean 'at 40cm centres' not 'pitch' by the way.  A pitch is the degree of an angle, its measurement can't be in length) 
It could be interpreted that way, though in common usage the terms are sometimes interchangeable. For example, https://konigwheels.com/wheel-info-tech/what-is-pcd/
So can you answer the question?

So we're both clear that I didn't understand a word of what you wrote.  

No, we're both not. If you have something helpful to contribute thanks in advance. 

fifthofwhisky

FreeBear said:

fifthofwhisky said: So what is the minimum lath depth I can get away with?

I'd go for 2x1 (50mm x 25mm in modern units) battens - Cheap, readily available, and you are not limited to 2.4m lengths.

As this is a ceiling, I'd also fill the gap between the battens with Celotex/Kingspan type insulation, or at a push, expanded polystyrene. Might as well make use of the space.

Thanks The supplier is recommending 22mm thickness. 25mm (or 1") is thicker. My question relates to what is the minimum thickness to support the structure and panels.