GENERAL PROBLEMS AND LAYING TECHNIQUES

Before we begin analyzing the different laying techniques used for terracotta flooring, we feel it would be helpful to take a brief look at the design criteria used, or more specifically, at the problems and general aspects that should be taken into consideration and resolved effectively in outdoor flooring plans.

COMPLETING THE LAYING SUPPORT

The laying support – as a set of layers that are specialized and differentiated both materially as well as in terms of the functional layers such as the foundation surface, any drainage or waterproofing layer, the actual laying surface (i.e. the screed) and the bedding layer – is undoubtedly the first element to influence the end quality of the flooring. This is because it carries out a number of functions ranging from static functions to protecting the surface layer of the terracotta by preventing occurrences such as rising humidity, which would inevitably bring the salts contained in the soil or in the flooring layers to the surface.

The technological definition of this latter aspect must therefore be extremely precise and entails close examination with regard to the specific types that are being used (private or public flooring, areas designed for pedestrians only or for vehicular traffic) in order to take the particular operating conditions (loads) into consideration on an individual basis. Particularly when defining the terracotta floor laying support, in outdoor applications it is essential to plan for efficient drainage in order to avoid trapping moisture that would otherwise inevitably provoke surface efflorescence and/or rings. If these phenomena persist over a long period of time, they can even cause scaling/chipping of the tiling material when freezing occurs.

A further condition that must be observed – and this involves geometric and technical aspects that are nevertheless connected with the need to prevent water and moisture from getting trapped on the laying surface of the floor field (even though they come from the outside environment in this case) – is to ensure levelness and suitable slope of the back surface of the laying support.

SLOPE

Apart from the type of laying technique that is utilized, when dealing with outdoor terracotta flooring special attention must be paid during planning in order to respond adequately to rainwater run-off. This is done by dividing the floor field into different drainage surfaces if necessary and giving them an effective slope and perfect levelness (without any settling). This will avoid trapping water and moisture on the floor surface since they cause undesirable and unsightly effects due to rings, saltpeter efflorescence and dirt buildup. Moreover, trapped water is particularly dangerous in cold climates (especially in central and northern Europe), where frequent frost can cause harmful tension in the porous structure of terracotta products. The following slopes are recommended for safety:

• small flooring surfaces: gradient > 1%;
• large flooring surfaces: gradient > 2%.

EXPANSION JOINTS

Particularly when working with extensive surface areas (or when particular work environments are involved, such as extreme temperature swings), when steps are being taken for effective removal of rainwater, it is also essential to divide the overall floor field into compartments by preparing technical expansion joints in a crosswise and/or longitudinal direction (minimum 5-mm joints every five meters). This is done using top quality rubber strips or special grouting materials that can adapt to deformations in the floor field.

TREATMENT of TERRACOTTA FLOORING

For outdoor applications, we advise against doing any type of surface treatment on the terracotta. This is true above all for areas designed for intensive public use (such as squares, public pathways, landscaping, etc.) in which the considerable thickness of the terracotta elements will also ensure a high technological quality level.

When the floor is initially finished, air pollution and/or the inevitable transmission to the tile flooring field of substances contained in the materials used to complete the subfloor can generally cause a “ring” effect or surface staining that will gradually fade until they disappear, thanks to the characteristic transpiring nature of terracotta.

Every year, it is a good idea to wash the flooring with water in order to remove any build-up of dirt and deposits caused by environmental pollution. At the same time, this will also restore the terracotta floor’s “breathability” and beautiful appearance. For flooring intended to be used as an outdoor extension of living areas or public buildings (such as porticoes, covered terraces, surfaces protected by projections or roof overhangs, etc.), it is a good idea to protect the terracotta flooring as effectively as possible from moisture rising up from the ground by using adequate drainage and, if necessary, by waterproofing the screed with a damp-proof membrane or other materials.

These floors (which are limited in extension and are protected to some extent) can also be washed every so often with acid or alkaline products, which will be sufficient to maintain the beautiful look of the terracotta.

TYPES OF LAYING TECHNIQUE

Laying techniques used for outdoor terracotta flooring are the outcome of well-established experience (whose historic roots go back to the Roman floors done in opus spicatum) and it has not been until just recently, with the introduction of new laying products to the market, that they have shown any significant updating and improvement as far as execution is concerned. In general, there are three laying techniques involved in the process of executing the floor as a whole, ranging from completing the sub-floor to preparing the bedding layer of the terracotta elements to how the joints are implemented.

These laying techniques envision:

1. laying the terracotta elements on a cement-based screed (with completion of the joints using mortar);
2. laying the terracotta elements on a sand bedding layer (with subsequent sealing of the joints, again using sand);
3. laying the terracotta elements, again with a sand laying technique, but using joints that are large enough to be sealed during the final phase with liquid mortar rather than sand in order to protect the joints to some extent from water flowing through them. It is clear that of the three techniques listed above, the third one is simply a cross between the first two. As a result, below we will analyze only the first two techniques.

LAYING TERRACOTTA FLOORING ON A CEMENT-BASED SCREED

The most widely practiced solution is the one utilizing a screed made of cement mortar to form the setting support in contact with the terracotta elements. This is done in order to achieve a compact and resistant finished floor that can prevent any settling or buckling processes, particularly in demanding work conditions.

The main phases are: Making the laying screed In order to lay the terracotta elements over mortar, a screed is prepared starting with a loose stone foundation (or more rarely, it is laid directly on the ground, depending on soil characteristics). The screed, which must be thick enough for the working loads, must be well compacted and level, and it is made of cement mortar (325 cement and silica sand with a particle size of 0.3, in a proportion of 150 kg of cement for every cubic meter of sand).

When making this screed (which can also be reinforced with electro welded metal netting if necessary), as we indicated above, it is a good idea to prepare adequately sized technical expansion joints that can absorb any dimensional variations to which the overall flooring may be subjected, particularly as a result of broad fluctuations in temperature (especially when large floors are involved). These technical joints must be respected and reproduced in the finishing layer comprising the terracotta elements.

Laying the terracotta elements Once the laying screed has been completed, the terracotta elements are laid down using synthetic adhesive or a semi-wet mortar:

1. in the first case, the terracotta elements are glued to the screed using synthetic adhesive (see what has been said in reference to the criteria for contemporary laying for interiors, page 174, paragraph “Contemporary”). There are various types of synthetic adhesives available on the market which can be used, however, taking care to always follow the specific instructions given by the manufacturers.
2. In the second case in order to ensure effective execution of the bedding phase, when the mixture is semi moist, the “dusting” phase is carried out (sprinkling cement powder over the surface of the screed).

This is followed by the actual laying of the terracotta elements, which must be wetted adequately and positioned according to the design pattern along the open-joint bedding layer. (see what has been said with reference to the criteria for indoor laying, page 174, paragraph “Modern”).

Completion of the joints

The phase in which the joints are sealed – or in other words, the completion of the enclosing network structure along the perpendicular surfaces of the laying faces of the terracotta elements – is generally carried out using cement mortar (composed of three parts cement to one part silica sand with a particle size of 0.3) or cement lime mortar that must be quite fluid. The fluidity of the mortar being used is essential in order to tamp it down easily and effectively inside the network of joints, completely filling in the empty spaces without any breaks.

This operation can be carried out through various procedures:

• application with a trowel;
• casting using a jug and funnel;
• application using a “pastry bag” fitted with a metal tip.

The last two methods make it easier to control the work sequence as far as clean workmanship is concerned, but the compacting action on the grouting material is less effective and therefore, a final operation is subsequently required to finish the joints. Cleaning and finishing the joints Even when the joint-sealing operation has been done in an expert fashion, the mortar inevitably overflow from the grouting “channels”, dirtying the floor field to some extent.

A dry mixture composed of thoroughly clean fine sand mixed with a small amount (4-5%) of “325” cement is used to remove the excess cement. As soon as the grouting is done, this mixture is spread over the laid floor and is then rubbed vigorously with a medium-hard broom (broomcorn is ideal). This vigorous abrasive action removes the excess grouting on the terracotta flooring surface, while cleaning it at the same time.

LAYING TERRACOTTA FLOORING ON SAND BEDDING

An alternative solution to mortar bedding involves laying the terracotta elements on a suitably prepared sand bed laid directly over the ground or on a more stabilized laying surface. Making the sand bed In this laying technique, the sand is “compacted”, smoothed and leveled to form an even layer that is about 5 cm thick (after final compacting, the thickness will decrease to about 4 cm).

The tiles are then laid directly onto the sandy layer. Particularly when this type of laying technique is involved, it is obvious that the overall quality of the flooring will not depend solely on the characteristics of the terracotta elements, but to a great extent also on the sand and on the procedure used to make the bed that is going to act as a filter element between the tiling and the soil or between the tiling and the concrete screed beneath it.

The composition of the sand is fundamental. It is important to point out that if the sand is too fine, it will take on a pasty consistency when it is damp. If it is too coarse, there will not be enough cohesion among the grains of sand. For optimum particle size, Fuller’s curve should be respected as closely as possible. In addition, the type of compacting done on the sandy layer is also important since the degree of cohesion of the roadbed is dependent on this operation. It is essential to do this operation progressively in layers in order to achieve stable and dense compacting.

Laying the terracotta elements As a rule, the tiling elements should be laid and then thwacked with a rubber hammer, being careful to leave open joints of about 2-4 millimeters between the various elements. Completing the joints The joints are then sealed using clean fine sand (with a maximum particle size of 2 mm) until they have been fully compacted. This operation is performed by spreading the sand over the terracotta elements, making it penetrate into the joints with a broom in order to seal them. After the flooring has been completed, this operation must be repeated several times, a few weeks apart.

These joints prevent any movement between the terracotta elements while also ensuring adequate waterproofing against surface water. When working with flooring that has a considerable slope (more than 15%), you can use dry cement (5% by volume) mixed with the sand in order to improve consolidation and evenness in the grouting material. At the end of the joint-sealing phase, the flooring must be compacted. This operation is done using a machine fitted with a steel vibrating plate (with a rubber bumper underneath), going over the flooring surface at least two times.

The last phase involves wetting down the flooring by sprinkling water over it (sprinkling the water acts as a precaution against “washing” the sand out of the joints) in order to consolidate the floor as a whole. For flooring that entails more demanding conditions of use as far as working load is concerned, it is essential to make a screed that is thick enough for the planned working load. The screed is placed between the ground and the sand so the sand will abut with an already consolidated bed. For terracotta flooring that is to be placed directly on natural ground, suitable “finished flooring” arrangements must obviously be prepared with effective drainage devices.

The sub-floor in contact with the ground must be made with large stones, completing the upper layers with finer aggregates that will also help achieve the design slope of the floor field. In order to prevent the sand being used for the laying bed from gradually being washed away by rainwater and thus creating cavities (potentially causing buckling of the laid floor), a geo-fabric underlay can be placed between the sub-floor and the sand bed. Herringbone textures (and in general, all patterns with crisscrossed grouting) are particularly effective for dry laying, since separating the joints into compartments affords better protection of the sand compacted into the joints.

With this laying technique, it is extremely effective to plan adequate lateral edging, which helps resist against the pressure produced by the flooring when it is loaded. To achieve this function, concrete rolls can be set up in the subfloor, changing the laying orientation of the terracotta elements on the surface by means of a different floor texture pattern.