By Michael Hailwood, Director, Laticrete UK
During a recent conversation with some experienced tilers I was asked whether they could tile onto a screed that had not substantially cured (green screed). Typically, their inquiry was about screeds that are three to four days old, as opposed to next day tiling.
My initial answer was … technically … ‘Yes!” However, this answer came with a series of questions and some discussion to see if they understood all the issues. For instance, I asked, ‘What do you mean by green screed?’ ‘Are you aware of the ramifications of tiling on green or young screeds?’ ‘Do you have to tile so early, or can it be delayed?’
There have been many successful installations over green screeds with the thin-bed adhesive method and the thick-bed methods of days gone by. However, things have changed and are still changing. Tiling backgrounds are becoming more impervious, tile density is generally increasing, tiles have dramatically increased in size, adhesives are becoming more highly modified, grouts are less porous and, amongst other things, there is a resultant slower dissipation of construction moisture in the tiling system, so it may be time to rethink this whole issue.
First, for the purpose of this article, I will better define ‘what is meant by ‘green screeds.’ It was not that easy to find a definitive answer. So, in the absence of any official definition, suffice to say, it is a screed that has set and can withstand tiler’s traffic without damage, but has not reached any nominal dimensional stability; as well as one that has not reached its designed strength and still contains high levels of its mixing water: typically still dark in colour due to its moisture content. All of which are variable depending mix design and site conditions. For this article, the screed is non-proprietary i.e. not pre-bagged and without additives; 3:1 – 4:1 sand: cement and nominally 50mm thick.
So, you may ask, what are the issues with tiling over screeds that are young, dimensionally unstable, weak and still wet or very damp? Here are some of the prime considerations that need to be taken into account prior to tiling onto green screeds.
The most obvious consideration is the effects of the expected and ongoing drying/shrinkage on the tile as the young screed cures and dries. Shrinkage continues after the tiles are adhered with adhesive, and the magnitude of which, will be determined by factors such as the screed make up, thickness of the screed, substrate absorption and prevailing site conditions, amongst other things.
This shrinkage can create stress within the tile that can result in damage, the magnitude of which is dependent on such things as how early in the life of the screed it is tiled, the size of the tile and site conditions to name but a few. Ideally, extraordinary steps are required to control the additional stresses that the tiling system will be exposed to, like more frequent movement joints in the screed, through to the tiled surface finish and use of an anti-fracture or de-coupling membrane.
Photograph 1 depicts tiles that have reflected cracks, which appeared following installation over a young and subsequently shrinking screed.
In normal tiling practice, depending on favourable site conditions, screeds should be allowed to cure and dry for a period of time to allow for initial and drying shrinkage to take place. BS 5385: 3- 2014 suggests a minimum of 28 days, depending on the thickness.
However, to enable swifter tiling, various adhesive companies have produced anti-fracture, highly modified adhesives that can cope with a certain amount of shrinkage movement. These are adhesives that absorb a nominal amount of movement created by the shrinking screed during its cure period without affecting the bond.
However, the key questions are; how much shrinkage can these adhesives negate, even after the inclusion of the obligatory extra movement joints, and how can tilers be sure these counter measures are sufficient? Are they likely to end up with tiling systems that can be prematurely and permanently stressed even before they go into service? How does one calculate the ideal time to wait with a given adhesive before a young screed can be tiled and will the use of large format tiles exacerbate the situation? All these questions need to be considered and answered, preferably with the help of an adhesive company before the work commences.
Next are the effects of prolonged exposure of moisture on tile installation materials and the tile as the work progresses. For instance, the slow drying of the highly modified adhesive results in greater waiting times before grouting can commence which may result in the grout being subject to efflorescence.
There is also the risk of staining and dimensional instability with such humidity on natural stone tiles. Photograph 2 depicts moisture vapour from a damp screed ‘bubbling’ unset grout during an installation.
We also must consider the effects of the moisture that is locked into the system once the tiling has been completed. Is it going to cause high pH levels that affect and damage adhesive layers? Is this going to continue to cause efflorescence? Is it going to cause delay and possible loss of revenue by preventing immediate pedestrian access? For instance; laying large format porcelain tile on a green screed on a waterproof membrane with a highly-modified adhesive will as indicated previously produce conditions that will excessively prolong the setting, curing and drying of the system, especially in colder conditions.
This will mean extended periods of protection for the floor until the installation system is strong enough for traffic or risk damage; a scenario where it is a good case for waiting a few more days so we do not have to worry about the prolonged protection.
As so often is the case these days, stone and tile fixers, due to no fault of their own, are sometimes pressured into speeding up the process of laying tiles to meet unrealistic construction schedules or simply to make up for lost time by other trades, but at what cost, and who wins or loses in these situations?
It has been my experience that a lot of the risk can be taken out of this practice by at least waiting until the screed has lost a significant amount of moisture, commencing tiling a few days or so after it has gone from green to grey, installing additional movement joints and using a highly polymer modified adhesive with exceptional bond strengths such as Laticrete 254 Platinum (C2 TE S1) to encourage the correct outcome.