I have an artistic mosaic job to do, as a free hanging feature, fixed onto a plastered wall (in a barn conversion) with possibly sandstone behind.
The feature will be a combination of mosaic over a 3d inlaid casting plaster background. I would like to use plasterboard as substrate for economy, (though I have an open mind about it), but need to figure out fixings beforehand, won't be able to drill through the design afterwards and would like the fixing to be invisible anyhow. The artwork will will be 100cm square, and unframed, so will weigh a fair bit, over an interior restaurant entrance, dry conditions.

The only idea I have come up with so far is to make a wooden frame to be screwed behind the plasterboard before I even start, with strong metal hangers fixed to the wooden frame, so the restaurant owners can figure wall fixings out for themselves, (and be responsible for the safe fixing of it) but are there any better ideas?

Thanks for your time. :-)

Ah, duh, just realised I can use MDF, sorted!!! Honourable Mods, you can delete this if you like, I'm happy...

A piece of Hardi backer or similar would be better.

Hi, TB :-), good to see you!
Yes, I realized, MDF is no good either, not with the gypsum plaster, even with lots of keyneails etc... so back to plasterboard I think it is, but stuck on a thin sheet of ply to screw heavy duty fixings into, what do you think?

Hardibacker board...hm... why in your opinion would that be better? It's the safe hanging of it that worries me most...

I'd be going for a thick piece of ply Gisela, screws could pull out of mdf or backer board with all that weight (imo)

Yes, Doug, but gypsum plaster does not stick to ply... that's why I thought stick the thinnish ply on the back of the plasterboard...just for the fixings... I am paranoid about the whole thing crashing down... or maybe the wooden frame behind, maybe 1" thick, wasn't such a bad idea??

Originally Posted by Mosaic Girl

Yes, Doug, but gypsum plaster does not stick to ply... that's why I thought stick the thinnish ply on the back of the plasterboard...just for the fixings... I am paranoid about the whole thing crashing down...

Sorry Gisela, I misread your question. I thought you wanted a substrate to hang the finished piece onto as in a jig of some sort

Originally Posted by Mosaic Girl

Hi, TB :-), good to see you!
Yes, I realized, MDF is no good either, not with the gypsum plaster, even with lots of keyneails etc... so back to plasterboard I think it is, but stuck on a thin sheet of ply to screw heavy duty fixings into, what do you think?

Hardibacker board...hm... why in your opinion would that be better? It's the safe hanging of it that worries me most...

Ply is always likely to twist a little if it's not sufficiently braced.
Hardi Backer 500, the 12mm one, is sturdy enough to take a fixing and far more stable than ply or plasterboard.

It is heavy though.

I know not easy to imagine...the image is is a farmhouse animal in front of simple hills no more than a silhouette. The idea was to make the hills and sky a textural plaster affair , with some glass baubles set in as flowers for some interest and light reflection, and have the mosaiced creature in the foreground, prominent of but in between the fields and the sky - does that make sense?

could you not cast a plaster slab Gisela with fixings through, and then fix the piece to the plaster slab? ps, is it the peahens piece?

Thanks TB I 'll look into that then..

hi gisela, the problem you may find using hardi backer in this situation is the size, hardi dont make single sheets of 100cm x 100cm that i am aware of which means you will have to join it which will give double the headaches in transportatation of the piece and fitting ,is it possible to leave some tesserae out on the fixing points of the ply wood which could be stuck on after the piece is hung ?

Originally Posted by garythetiler

hi gisela, the problem you may find using hardi backer in this situation is the size, hardi dont make single sheets of 100cm x 100cm that i am aware of which means you will have to join it which will give double the headaches in transportatation of the piece and fitting ,is it possible to leave some tesserae out on the fixing points of the ply wood which could be stuck on after the piece is hung ?

Good point.
I think the HB500's are about 1200x800.

There must be a similar board that come 1m wide out there, shirley!?

can you still get fireline boards, they used to come in 8 x 4 sheets?

Thanks for all your input, guys ... at the moment I favour the plaster cast slab idea, thanks doug, actually with metal rods inside it should be strong enough for hanger loops maybe and need not be too thick? Food for thought. I really don't want to join pieces, but as the mosaic is the prone bit, Gary, fixing through that could be really awkward in this case. Eeeh, as always I have the finished image firmly in my mind, now it's a matter of figuring how to do it just right...

Originally Posted by doug boardley

can you still get fireline boards, they used to come in 8 x 4 sheets?

Never heard of them, what are they?

Fire resistant boards and slightly pink in colour, 12.5mm thick and weighs about 12kg m2