Residential floor finishes

During the 1960s cement screeds became more popular, but the relationship between the amount of water needed in the mix and single-sized coarse aggregate was not widely understood. As a result many screeds had to be made up in a consistency more akin to concrete, which made them too wet and more prone to shrinkage problems. Around 1965 it became apparent that the British Standard for sand was too fine for screeds, and the standard for fine aggregates (BS 882) was adopted. Screeds from 1970 onwards were laid much drier and are referred to as semi-dry mixes.

A defect known as 'elephant's footprints' is associated with cement screeds from the 1970s. Indentations appear under vinyl tiled floors. These can go unnoticed if other floor finishes are laid on top, and only come to notice when the original floor is exposed or the indentation becomes noticeably larger. In the 1970s the practice of laying screeds semi-dry was introduced. Still in common practice today, it was not appreciated at the time that the consequence of laying the screed too dry and over-compacting the top layer to achieve a level surface created a thin, dense crust over a crumbly base. The floor breaks down underneath the vinyl flooring, creating indents. The remedy for these floors usually requires a new screed finish.

Poorly laid floor screeds

Dense sand and cement screeds can fail for many reasons.

Key to understanding how a defect might occur is to understand what makes a good screed. The aim when laying a screed is to achieve minimum drying shrinkage. Cement screeds shrink as they dry, and the way to control this is by carefully controlling the amount of water in the mix, to keep it to a minimum. Screed mixes since the 1960s have traditionally been very dry. If the mix is squeezed in the hand, no water should escape, and the mix should form into a ball. Screeds need time to dry, and if this is compromised by laying floor finishes too quickly, the screed will not perform well. Most of the common problems relating to undulations in screeds occur in the first 28 days after laying, but may go unnoticed until the floor finish is fitted and imperfections become more pronounced.

Screed defects can slip past a snagging check, and undulations in screeds are not necessarily evidence of a problem with the floor base. Screeds can curl as they dry, because moisture evaporates quicker at the surface, causing the upper layer to shrink faster. This effect can be exacerbated if the screed is 'force dried' by applying heat in an attempt to speed up completion times. Trapping moisture within a screed by laying floor finishes like vinyl before the screed has set will cause poor bonding and sagging of the floor as loads are applied. On a domestic scale it is hard to see some defects in screeded floors, and usually most remedies centre on the replacement of poor-quality screeds with rapid-drying screeds, or by applying epoxy resins to help level out the floor. Note that if a secondary layer is being added to a floor screed to level it out, there must be no moisture left in the original screed.

Use of resistance moisture meters should be limited to identifying an initial problem. Further investigations will require more sophisticated testing to eliminate all of the possibilities. Diagnosis of screed defects needs to be carefully considered, as applying the wrong remedial repair can make matters worse.

Tolerance

The criterion for flatness of screeds is given in BS 8203 and BS 8204. It can be expected that a floor can be laid so that the deviation along a 3m straight edge is no more than 5mm.