morning folks, i've been asked to screed a floor and was wondering if anyone could give me some advice to make sure i approach this correctly. the screed will be 75mm laid on top of ufh and 75mm cellotex floor insulation. the floor construction is block and beam. the screed will be delivered ready mixed. should i find a company that will put fibres in the screed or add extra reinforcement, ie chicken wire or similar product?? i could swear i read a similar thread saying to keep the screed covered for a week to reduce moisture loss after laying?? the area is 6x4m do you have to think about expansion joints with screed. up to now i've just done a couple of kitchen floors and hallways so this is my first large job. i will have a lad helping me who's more experienced but would like to get as much input as poss to make sure it goes spot on. the job is for clients of an architect friend so could lead on to me doing every extension of his if all goes well. thanks for the input. David
this method of construction is termed Floating as the screed is on a layer of insulation. TO do it correctly the screed must be at least 65mm deep and must be reinforced with either D49 steel mesh or more commonly nowadays with Polypropylene fibres. The easiest way to do it is to order it in from a screed supplier complete with fibres. If you have not done much screeding before you would be wise to go for a mix retarded up to 8 hours to give you plenty of time to work with it and get it flat. One thing to consider is the workability of the screed. Too dry and you will struggle to compact it ending up with the "apple-pie" effect and too wet and you will struggle to get a flat finish without trowel marks. Take a hand full of the screed and squeeze it in your hand. It should be wet enough to hold itself together, comapcting without deforming through your fingers.
Assuming there is only 1 heating zone there is no reason to put a joint in the screed bay at these dimensions - typical maximum would be 36 to 40m2 with an aspect ratio no more than 2:1. You have 24m2 at 3:2. NHBC would say smaller bays but I would be happy with this in one. You will need a minimum of 5mm compressible material around the perimeter to allow for expansion. The screed may well crack randomly as is the nature of these screeds.
Whilst I won't go as far as to say block and beam does not deflect provided it is constructed correctly and is designed with the necessary soft body impact and live loading criteria then deflection is not normally an isssue. Again assuming that thedirection of the beams is horizontally perpendicular to the short side of the bay then I would not think deflection would cause any issues whatsoever.
I realise it is now an insitu concrete base rather than block and beam - that will only help things. The method of construion and floor make up from top to bottom should be as follows
Screed on top
UFH pipes
Slip Membrane (minimum 500g polythene)
Insulation
Damp Proof Membrane (minimum 1200g polythene)
Concrete Substrate
When you have laid the screed it should be cured for a minimum of 7 days following installation by covering with polythene. This will help minimise shrinkage cracking and curling.
Do not comission the underfloor heating for a minimum of 28 days following installation and then do so slowly.
After this has been completed (and only after) sand the screed to clean it up if necessary prior to priming and tiling.
