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Insulating Concrete Roof and Floor Screeds using Perlite

Lightweight concretes and screed made with expanded perlite ultra-lightweight aggregate can be used to reduce loss of heat through floors. Typically ground floors are constructed with dense concrete covered with dense screed; second storey floors over garages and car parks are also usually built with dense concrete and screed giving little thermal insulation. A screed containing perlite can reduce thermal conductivity from 1.04 to between 0.15 and 0.25 w/m oC.


Standard concretes and screed have densities between 2000 and 2200 kg/m3. The density of perlite concretes and screed is from 600 to 1050 kg/m3 which improves the thermal efficiency and also reduces the weight of the final structure. It should be noted, however, that lightweight concretes and screeds are susceptible to wear and should therefore be protected with an adequate toppping to prevent indentation and dusting.


Perlite concrete and screeds are ideal for:


Typical properties of lightweight concretes and screeds:

Mix volume proportions (cement:perlite)1:31:4 1:5
Mix of Screed Consistency
Bulk density as placed (kg/m3)12001100 1000
28 day wet compressive strength (n/mm2)1312 11
Oven dried density of set screed10501000 900
Thermal conduct. at 1% (v/v) water (w/m ºC)0.250.24 0.21
Mix of Concrete Consistency
Bulk density as placed (kg/m3)1000950 900
28 day compression strength (n/mm2)86 4
Oven dried density of set screed900850 750
Thermal conduct. at 1% (v/v) water (w/m oC)0.21 0.20 0.18
Mix of Pourable Consistency
Bulk density as placed (kg/m3)900 800 700
28 day compression strength (n/mm2)2 1.5 1
Oven dried density of set screed750 700 600
Thermal conduct. at 1% (v/v) water (w/m oC)0.18 0.17 0.15

All tested in accordance with the requirements of BS EN 1015 Part 11.

Fire Protection

All concretes and screeds are non-combustible as defined in BS 475 Part 4 1970 and designed in Class 0 in accordance with the requirements of the Building Regulations.

Materials Required

Expanded perlite grades P321 and P35 are suitable for concrete and grade P15 and P321 for screed. All grades are supplied in heavy duty economy plastic sacks holding nominally 100L (3.5 ft3) and weighing 8 to 10 kg depending on the grade. Cement should beordinary Portland and conform with BS EN 197 part 1. In cold weather, or when a quicker than normal set is required rapid hardening Portland cement may be used. An air-entraining admixture, conforming with the requirement of BS EN 934 part 2 is advisable.

Site Mixing

It is essential that the expanded perlite, cement and air-entraining admixture are accurately proportioned and the mixing time carefully controlled. The strength figures given in this data sheet are those obtained when the screed is tested by the methods of BS EN 1015 PART 11. In practice dry strengths are likely to be higher.

A free-fall drum-type concrete mixer is satisfactory for mixing. First add most of the required water to the mixer, then the air-entraining admixture followed by the cement. Mix until a slurry is formed, then add the perlite. The mixing time after the addition of the perlite should be as short as possible. Additional water can be added to obtain the required consistency.

Site Placing

The concrete or screed should be laid, levelled and then covered with a polythene membrane and allowed to cure for at least 18 hours. Power compaction should not be used. The surface will require topping with either a traditional cement and sand screed or mastic asphalt or proprietary brands of resinous screed. On external flat roof decks, it is advisable to make provision for breather vents especially where the topping is built up felt or mastic asphalt.


It is recommended that the placing of the concrete mix and topping or screed should be the responsibility of one trade only and the specification should read:

"For concrete (or screed) be mixed by volume with Ordinary Portland Cement plus air-entraining Admixture to a.....consistency and placed to a thickness depth. After setting and curing for.....days it shall be topped a thickness"