Is screed thick enough?

tlough

All,

Apologies if this has been covered before, but I've looked on the forum and there are varying answers.

I'm doing a ground floor extension with UFH made up as follows :
150mm subfloor (concrete + mesh)
150mm Kingspan Thermafloor TF70
40mm screed (incl UFH pipework, etc)

I've seen varying answers for how thick the screed should be, but usually in the 60-75mm range for most of the posts I've reviewed
Is 40mm too thin a screed for this base - I've beed onto the UFH company that's supplying & installing UFH & screed, and they seem happy that 40mm is enough, but I'm a bit apprehensive as to the structural strength given that it's laid on insulation....

Any thoughts, or advice?

Thanks

Slig

if you take 30mm as the diameter of your ufh pipe and if there is any sort of spring in it at all its going to be protruding above the screed. The underfloor heating suppliers leave the site after the pipes are laid so its not their responsibility if the screed isnt working. even if the pipes do sit below the surface you will still have maximum 10mm cover over them which will lead to cracking.
Even from the perspective of pouring the screed 60mm is probably the very minimum possible and 75 a more sensible option.

sydthebeat

i would specify 75mm reinforced screed as a minimum over 150mm of insulation.....

are you sure you need 150mm subfloor?

muffler

There was a thread here on this subject a few months ago. Can anyone recall it or provide a link?

sinnerboy

at 40mm you are too thin for a concrete or sand and cement screed

you will have to use a PFA screed ( pulverised fuel ash ) . it is more expensive . google and you will fine 2 or maybe 3 providers

tlough

Thanks guys for the reply.

Slig : The company installing the UFH is also putting in the screed so I presume that they are confident with the result, but I wanted to double check as 40mm doesn't seem that deep. They're laying it over 16mm pipes

SydTheBeat : The area is c50m2 open plan, and as this is the second time the floor is being laid (rogue builder), I'd prefer to over-engineer than regret it down the line

I've spoken to the company, and they say that the calcium (carbonate?) based screed which is designed to be poured to that depth, but I'll know more when they email me on the spec today

Thanks

tlough

The email brochure refers to the scree as :
"premixed blend of calcium
sulfate binder, dried sands and high performance additives"
and
"is suitable for use as a levelling
screed in residential and commercial applications in
thicknesses between 20mm and 80mm over most standard
substrates. It is ideal for use with underfloor heating systems."
and meets :
"meets the requirements of
EN 13813:2002: Screed material and floor screeds."

So I presume that it ticks all the boxes?

Thanks

sydthebeat

tlough wrote: »

The email brochure refers to the scree as :
"premixed blend of calcium
sulfate binder, dried sands and high performance additives"
and
"is suitable for use as a levelling
screed in residential and commercial applications in
thicknesses between 20mm and 80mm over most standard
substrates. It is ideal for use with underfloor heating systems."
and meets :
"meets the requirements of
EN 13813:2002: Screed material and floor screeds."

So I presume that it ticks all the boxes?

Thanks

i wouldnt,...

find out if it is suitable over 150mm insulation. That depth of insulation does not equate to a very solid substrate. These boards have a tendency to rise, warp, buckle etc.

Slig

sydthebeat wrote: »

i wouldnt,...

find out if it is suitable over 150mm insulation. That depth of insulation does not equate to a very solid substrate. These boards have a tendency to rise, warp, buckle etc.

I agree, 150mm is also probably made up of more than 1 board (ie 100mm and 50mm) further adding to the chances of it moving. If you arent too far into the build I would consider speccing up the insulation and down the thickness and adding the difference to the screed. Youll notice this at areas such as doors where the saddleboards will have to be fixed down somehow and a thicker screed means less chance of puncturing a pipe

muffler

I would agree with the above. A bit more depth to the screed and loose a bit of the insulation. More practical in any event.

RKQ

This tread might help:-

http://pie.boards.ie/vbulletin/showthread.php?t=2055081157&page=2

Thicker screed = better heat retention, thats the way to do it!:)
UFH works economically when the floor slab acts as a large storager heater.