What is a thinbed screed?
A thinbed screed is a cementitious mix which can be applied to an uneven floor to provide a smooth surface to apply decorative flooring. (carpet, parquet, vinyl, tiles etc.) They can be applied (depending on the actual type) from 20mm thick down to a feather edge.
So that is another term for self-levelling floor compound?
Not Quite. (I bought a self-assembly bookshelf once, unpacked it and it just lay there. For a week. Finally I gave in and got my tools out. I never did hear from the complaints department either.) The compounds are more properly described as self-smoothing, as they do not truly self-level. Some trowelling of the compound is required, but the trowel marks smooth themself out. Putting down such a material is far easier than plastering a wall, firstly because the material tends to set more slowly, and secondly that gravity is working with, not against, you.
Why should I use one?
Because the alternative is to rip the floor up, and start again. The disruption, cost and labour of using a thinbed screed is less than the alternatives. Typically 24 hours after the screed goes down, the floor covering can go down, so that the whole project fits comfortably into a weekend.
Where should I not use one?
On top of magnesite. Magnesite is only likely in old houses (pre about 1920) on ground floors. Normally coloured coral pink or verdigris. A small lump, when held in a gas flame will glow brightly, like a gas mantle, at the edges. Magnesite needs complete removal, and replacement with 3:1 sand/cement screed. If this is not quite a good enough finish, you can use a thinbed screed on top of it. Magnesite was often made with sawdust as a filler, and when the flooring traps any water vapour this rots, the fungi resulting expanding the magnesite and ruining the flooring job.
Where do I start?
Surface preparation. Like painting and tiling, the job will only ever be as good as its surface preparation. Anything remotely loose like old adhesive, plaster, grease or laitence.
What on earth is laitence?
It appears on screeds and concrete after they are laid. The fine stuff, particularly the free lime, rises to the surface of the wet cement mixture and forms a pale, friable layer when the mixture sets. This will act as a release layer under your flooring, and the job will fail.
There seem to be a confusing number of types. What is what?
There are three basic types; Water mix, Latex, and Acrylic.
Water mix; as the name suggests this is mixed with water, and then applied to the floor. It is the least expensive of the three, but has less flexibility and adhesion, although it is a harder surface. They are now almost all protein-free, which is only really a concern if there is a possibility of damp. The older protein containing types could, under certain circumstances, give rise to fungal growth.
Latex; based upon a natural rubber latex liquid in a polythene bottle and a bag of cementitious powder. One bottle is enough liquid for one bag of powder. These have good adhesion and flexibility, but will indent under extreme point loads. These smell of ammonia during application, but the smell quickly dissipates once the compound is set.
Acrylic; based upon a synthetic acrylic emulsion in a polythene bottle and a bag of cementitious powder. One bottle is enough liquid for one bag of powder. These have good adhesion, flexibility and toughness, and with the use of the appropriate granite chippings gives a tough wearing surface suitable for (e.g) a garage. It is vital in this application that all of the grease is removed with a suitable alkaline detergent. These often smell of ammonia during application, but again the smell quickly dissipates once the compound is set.
What if I have a damp floor?
Floors with damp can have a Pitch Epoxy DPM put down. This is applied in two layers, brushed at right angles to each other, and the top layer is "blinded" with some of the powder component of the compound. Then either latex or acrylic screed is applied after the membrane cures. This membrane has a strong phenolic odour which fades with time.
The compound is going off too slowly/quickly. What can I do?
The reason is that some manufacturers adjust the formulation throughout the year, to cater for the seasonal variation in temperature. You can chill or warm the liquid component, or attempt to exchange the powder component for newer stock. Either put the liquid container into a bucket (or bath) of blood-heat water or ice water. Heating accelerates set, and cold delays it.
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John Schmitt <john49@mdx.ac.uk>
Last updated 19/12/00