UFH trays without DPM sheet?


New Member
Looking at installing profix UFH panels.
Floor build up is paramount, and I understand if a screed is unbounded, the screed has a deeper minimum depth.
For this reason, I'm looking at putting down a liquid DPM, followed by a slim (5mm at the thinnest point) screed, with the trays on top.
My question is, whenever I see people pouring liquid screed into trays, they always have a DPM sheet underneath, will I need one of those in this instance?
Also, because of the thin depth of the screed, I don't particularly want to want to screw the trays into the floor, any ideas what I could use to stop it floating when I pour the screed?