Ground supported floor advice - is screed layer necessary?

[RomanAbramovich]

Looking at the regs, there are several diagrams for solid ground supported floors, one of which illustrates ground supported floor construction being (in reverse order):

1. Compacted hardcore (max 600mm)
2. Sand blinding (min 50mm)
3. DPM
3. Rigid insulation (min ?)
4. Concrete (min 100mm)

...there is no mention of a screed layer to finish. Is the screed absolutely necessary? Or is it simply a leveling layer?

Also, what is the min thickness of rigid insulation under the floor?

Any advice much appreciated.

 

[noseall]

We always opt for a screeded floor when building extensions etc for two main reasons.

1. You can finish the concrete sub-floor within the realms of say 75mm lower than the existing without the need to be outrageously accurate. Then, once you do your knock-through you then remove the dividing masonry to below concrete level, fit some insulation and polythene and screed right up to existing, ensuring a seamless floor.

2. As well as achieving a seamless floor you can also achieve a decent finish without the worry of power-floating and bad weather, especially relevant in the Winter time. Concrete would take an age to firm up enough to power-float at this time of the year.

 

[noseall]

...Also, what is the min thickness of rigid insulation under the floor?

Usually a minimum of 80mm (Celotex, Kingspan etc).

 

[RomanAbramovich]

Many thanks for clarifying Noseall.

I've seen hardcore/concrete/insulation/sand&cement screed spec'd so many times now that I assumed the screed was part of the regs!