First floor screed for UFH

[Elshanko]

Hi, I'm installing wet under floor heating to a first floor bathroom but have a problem with floor levels. The top of the joists are 100mm below the adjacent floor levels - ie the bathroom floor joists are 100mm below the level of the hallway.
My first thought is to screw ply (22mm?) onto the joists then lay the UFH pipes and screed (75mm) onto this.
What do you guys think?

 

[thereisaplan]

We just installed a NuHeat wet system downstairs on scree, but upstairs used WarmUp electric upstairs because of the expense and complexity of wet on a wooden floor. Have you considered http://www.nu-heat.co.uk/products/underfloor-heating/floor-constructions/joisted.html?

Home owner not a trade BTW.

 

[Elshanko]

Thanks for your suggestion thereisaplan.
I definitely need to make up 100mm to get to my adjacent floor levels though. Other suggestions or comments on my proposed solution gratefully received!