Best Adhesive? - Wood to Concrete/plastic Wetroom Tray

[Andehh]

I have a Wetroom tray that was incorrectly fitted, causing it to flex ever so slightly - repeatedly cracking the grout. (It should be on 400mm joists, they installed it on 600mm joists)

To rectify it I am taking out the ceiling below it, and bracing it up from beneath.

In short, I am looking at some scaffold boards, turned 90 degrees, notch them out for the underfloor heating pipes then screw them to the joists and secure them to the underside of the wetroom tray.

As a result I am looking for the best bonding/grip adhesive that will create a very solid bond between wood and the concrete/plastic wetroom tray. The wetroom tray is slightly trellised underneath so this bond will need to fill the trellised holes slightly - no more then 10-20mm.

As a result, can anyone recommend a suitably strong adhesive that can be used in fair quantities to create a very solid bond? Ideally one that be applied in large quantities as I will over engineer this as much as I can.

I want to try and prevent any movement in the tray itself, so flexible adhesives have me somewhat uncertain in their suitability?

Many thanks in advance

 

[gregers]

poly glue,foams up and is waterproof.

 

[Andehh]

Thanks mate, welcome other recommendations. This adhesive is as much to take up that extra mm or two between the brace & the wetroom tray as it is to stick them together.