Floor prep for screed inbetween joists

[Flintstone]

Just a quick one guys, a job in the pipeline, I've been asked about the prep that will be required, here's the scenario.. 70m total area ish
New extension with new screed (50 - 60mm) and ufh.
Existing building with joists, the floor boards are coming up and the ufh is being fitted between joists and 25-30mm screed flush to top of the joists.

My question is, does the joisted side require over boarding or do you think uncoupling over the whole area will be adequate, any other concerns ?

 

[J Sid]

I priced one similar early in year and quote ditra over the lot with expansion joint. Didn't get it as didn't want to pay for uncoupling

 

[Ajax123]

Screed at that depth between joists would not constitute a structural quality floor and as such would not be be suitable for tiling without a structural support so overloading with suitable timber (probably 18mm) screwed to the joists would be appropriate. Also the weight of the screed might create a bouncy floor. Not a great one to tile so be wary.

 

[Flintstone]

Ahhh that's a very good point Alan about the weight hanging on the joists! I did ask why they were proposing to do that and they said the void was big so not viable to create a solid floor

 

[J Sid]

So screed between joists is not down to substrate then?
If not what's it sitting on?

 

[Flintstone]

No not down to substrate. You know what some of these floors are like, can be a 3 foot void below. I think they board the bottom of the joists and then sit kingspan in don't they

 

[Dumbo]

I would use spreader plates to dissipate heat , how big are the joists ,what's the span because they are chucking 42 kilos of screed a metre , I would definitely overboard . If they are thinking about heat transfer it's got to be giffa or similar

 

[Just Rizzle]

thought about over boarding in 2x12 mm cement boards laid 90 degrees on top of each other glue and screw?

 

[Ajax123]

No not down to substrate. You know what some of these floors are like, can be a 3 foot void below. I think they board the bottom of the joists and then sit kingspan in don't they

I've done loads of screed between just floors with gypsol screed. In fact did one only last week. The only one I've done with no over boarding was my own. I actually went 25mm over the top to meet the floor boards height in the adjacent floor. It was an old house with 4x2 pitched pine joists spanning 4.5m total but with a supporting honeycomb wall halfway over so about 2.2m clear spans. Had to look up loading tables and do my own defection calcs etc cos could not find a single engineer to calc it for me. Screed between joists constitutes a dead weight so all load is supported by adjacent joists which stresses the joists individually. If the screed is over the tops if the joists it becomes a composite system and works vettercwuthout heed for overboarding. I placed 45mm screed with underfloor heating 16mm pipes (20mm between and 25mm over the top of the joists) over polythene stapled and taped to 12mm OSB supported on tile batten screwed between joists and underlaid with insulation supported from the underneath with tile batten screwed upward into the bottom of the joists. I didnt tile the floor but in 6 years living on it when we replaced the laminate before we moved there wasng a single crack. I reckon that couldve been tiled.

 

[Ajax123]

thought about over boarding in 2x12 mm cement boards laid 90 degrees on top of each other glue and screw?

That would work although 1x22mm layer of Hardi would work and might be quicker?

 

[Flintstone]

I think we have a plan. Build a dwarf wall to support the joists mid way. Fit the ufh and screed to to top of joists. Board the over with 22mm giffa cement board screwed to joists, then bring new screed flush to that. Anti fracture mat the whole floor and honour the joint between the two

 

[Dumbo]

I like the sound of that, nothing like a good bit of structure .