Screed depth minimum

I am assuming both insulation and UFH is laid on top of the poorly laid concrete slab and not below in which case you do still have the option of lifting both and then applying a self levelling laytex such as Cemploy Deep (can be applied up to 50mm ) to rectify uneven levels, this will then give you a level base to work off.. Costs of this should be passed on to whom ever laid the concrete slab as clearly it was not laid to acceptable tolerances. If this is not preferred or possible then you may wish to incorporate chicken wire as well as fibres in the sand ; cement screed especially where it is at its thinnest depths. Following good practice a screed should be a minimum of 55mm and better if it is 65 - 75mm when using UFH to avoid it breaking up. As the screed works as a thermal mass conductor for the heat from the UFH you may well experience hot / cold spots due to the difference in thickness if it is not laid to an even thickness Regards David

The use of fibres increases tensional strength, impact resistance and overall durability, whilst inhibiting plastic shrinkage cracking, settlement cracking, and thermal cracking induced by freeze / thaw cycles. But in this case you cant do nothing because 80% of the job is done so the only problem will be with this uneven floor is ,, the thicker place will be less warm then the other part of the floor