Gypsum (Knauf GifaFloor) Flooring Substrate and Joins

[Mmark]

We are installing Knauf GifaFloor FHB18 as a flooring substrate. We also have between joists spreader plate UFH.

After the plumber installed the spreader plates and pipes, we found the joists the builder did were approximate 400 centres and not exact. The result, in a 4x3 bedroom, not all joins would be on joists.

Do I need to make them take up all the UFH and put down some support timbers, at where those supported joins would lay? Or am I worrying too much about this?

 

[Deleted member 174758]

Providing the tongue and groove joints have all been glued with the appropriate adhesive there is no need for them to land exactly on the joists. This video from the manufacturer (in German) shows just that at about 00:55 onwards (sorry about the link - for some reason it won't embed). The overshoot of joists shouldn't exceed 300mm, but if you are double-skinning and glueing the top layer to the bottom one I doubt that even that would matter so long as the joists between the layers were staggered. BTW good luck trying to lift this stuff after it has been installed - the glue in the joints makes it very hard to lift without doing a lot of damage

 

[Mmark]

Providing the tongue and groove joints have all been glued with the appropriate adhesive there is no need for them to land exactly on the joists. This video from the manufacturer (in German) shows just that at about 00:55 onwards (sorry about the link - for some reason it won't embed). The overshoot of joists shouldn't exceed 300mm, but if you are double-skinning and glueing the top layer to the bottom one I doubt that even that would matter so long as the joists between the layers were staggered. BTW good luck trying to lift this stuff after it has been installed - the glue in the joints makes it very hard to lift without doing a lot of damage

There is no second layer, in our setup. spreader platers between joists, GifaFloor FHB18 then Carpet Underlay.

I see what you mean from the Video i will ask the Knauf. I just hope that works for the thinner FHB18, and doesn’t require FHB25. our centres are 400, at worst it looked like joins were mid way, so 200 proud or less.

 

[Mmark]

Supplier came back and said we should be ok, but it has a reduced load of 50%, they only had values for FHB25 not FHB18.

“The values have to be reduced by 50% if there are joints in the zone between the bearing structure. If you were using FHB 25, the attached document outlines the loadings. The FHB 18 is a product for domestic loadings only. This would be around 2Kn but we do not have any data for unbacked joints.”

So it should be OK, they just don’t have data to show it for FHB18, so at my own risk.

I have no idea what sort of loads half of 2Kn would be. Or what sort of load is 2Kn. I want it stay well below 50%. So for a bedroom what sort of things would take the load would go over 40% of 2Kn.

 

[Mmark]

I opened a different thread, with a specific Q on how to mitigate the 1kn load risk. I did some calculations and it’s borderline.

 

[Mmark]

I have provided a much more detailed update in this thread:
https://www.diynot.com/diy/threads/how-to-strengthen-gipsum-gifafloor-flooring-joins.535869

It includes a detailed stress test report, with stress setup and results, for FHB18. Seems I will not be at risk of them breaking, just making creaking a little.....