Also called installation with mortar, installation with sand and cement, installation with aggregate hardener, wet-on-wet installation, it consists in the simultaneous installation in a single process of both natural stone and its bedding layer, without the preliminary construction of a screed.

These methods are well known and tested since they have been applied since the Ancient Roman times until today practically unchanged, with the only substitution according to availability of the period, of the binder and aggregate constituting the bedding mix. This results in an undoubted guarantee of durability over time, often not yet matched by the products of modern industry.

The wet installation system can be used on uneven surfaces and therefore not perfectly flat, it requires a useful installation thickness (bedding + stone) not less than 5-6 cm already considering a minimum thickness of 2 cm for the stone material used, it must have adequate physical strength, not be particularly absorbent or subject to staining or efflorescence. This system involves a double cost saving: it avoids the construction of a cement screed and, with the correct installation thickness, it allows to use stone materials with variable thickness, such as natural surfaces, which are always less expensive if compared to materials with constant thickness and to the finishing required for the walking surface.

The execution times are significantly reduced as it is not necessary to wait for the construction and subsequent complete drying (at least 28 days) of the concrete screed, even the application of the stone covering is fast if carried out by expert installers.

Particularly suitable for the construction of outdoor pavings this installation system must be carried out by specialized companies requiring a higher organizational structure as regards equipment, machinery, effective management of the construction site and continuous supply of consumables. It is also of fundamental importance that the installers have a high qualification level and experience and in addition to possessing an adequate work method must have a good knowledge of the materials used (types of sand, cement, stone) to better manage their behaviour according to the methods and climatic conditions of application.

The main wet installation methods involve the use of:

1. Cement mortar

The thickness required for the installation must include between 4 and 6 cm for the bedding, plus the thickness of the natural stone selected; this installation system is the most suitable to compensate for high thickness differences in stone elements such as for example Porphyry, Luserna Stone and in general materials at natural quarry surface.

The materials required for the formation of mortar bedding are: Portland cement 325, water, sieved natural sand or crushed quarry sand with a grain size of 0-4 mm or 0-6 mm.
The grease mix, obtained by mixing Portland cement 325 in a dose ranging from 250 to 350 kg per each cubic meter of sand, is applied in a sufficient amount for bedding a limited portion of paving, usually a pair of elements or a few more depending on their size. The stone element therefore finds its final placement with its positioning and beating with a rubber hammer.

A dense grout formed by a mixture of Portland or white cement mixed with water and a reduced part of sieved and washed sand, having a grain size of 0-2 mm, is used for the sealing operations. When the paving has reached the compactness necessary to be walked over (usually 24 hours after installation), the joints can be sealed by pouring the cement grout directly into the joints using watering cans. Once the right hardening level has been achieved, the excess material is removed with a trowel, then the surfaces are finished using a sponge and clean water or alternatively rubbing sawdust with sorghum brooms.

For more information on the use and installation of porphyry pavings, see the bibliography published by the ESPO - Ente Sviluppo Porfido (Porphyry Development Company)on these issues described in a very comprehensive way, differentiating the installation of: irregular slabs for the preparation of Opus Incertum, regular slabs for Slab Pavings, cubes for Sett paving, pebbles for Cobblestone.

2. Mortar with cement spraying

The thickness necessary for the bedding must vary between 4 and 8 cm; this installation system is used especially when it is necessary to adapt the building site dimensions to the design dimensions, even compensating for the differences in thickness of the stone materials.
Not indicated with absorbent stone materials, especially if they are of light colour and of reduced thickness.

The mortar is composed of cement mixed in doses of 300 kg each m³ of aggregate material with the addition of water until a mortar with a consistency similar to damp earth is obtained, therefore it is possible to add hydrated lime to improve the mix workability.The existing installation foundation must be levelled as much as possible and, if it located outdoors it must follow the slopes of the finished paving, the surface must also be clean and clear of dust and processing residues and must be well wetted with clean water, possibly with a jet like a "watering can" avoiding stagnation.
After checking the levels, it is necessary to create the reference mortar strips, then the bedding mortar must be placed between the strips rather quickly and precisely. The mortar must then be levelled with an aluminium support and the installation surface must be sprinkled with cement (white or Portland according to the lithotype). The lower surface of the stone element must be perfectly clean and damp to allow a perfect setting, the slabs must be positioned according to the installation drawings and geometries on the sprinkled cement still wet and must be beaten with rubber hammers until they are coplanar to the adjacent slabs.
During all processing phases, it is necessary to work carefully to ensure that the paving is clean.

A dense grout formed by a mixture of Portland or white cement mixed with water and a reduced part of sieved and washed sand, having a grain size of 0-2 mm, is used for the sealing operations; alternatively, it is possible to use pre-packaged coloured stuccos or resins, but compatibly with the width of the joints to be sealed, which must be limited to a few millimetres, and with the size and finish of the stone elements used. When the paving has reached the compactness necessary to be walked over (usually 24 hours after installation), the joints can be sealed. First the surface is moistened as required (without creating stagnation and taking into account the atmospheric conditions and the type of lithotype); then the liquid grout or putty is poured over the entire surface and spread with rubber spatulas directed at 45 degrees with respect to the joints, until complete saturation of all the empty spaces present; then an accurate final cleaning is made using rubber spatulas and sponges soaked in water.

3. Semi-dry mortar and cement grout.

This system, very similar to the previous one, is different for the directions of use: they ensure a total adhesion of the lower surface of the slabs to the bedding layer it is more suitable when in addition to having to adapt the building site dimensions to the design dimensions, it is requires to install large-sized or low-porous elements, or subjected to substantial stresses (for example those transmitted by the moving wheels of vehicles).

The installation operations are similar to those of the previous procedure as regards materials and mix formation; once juxtaposed the element to be installed on the bedding, it must be beaten with a rubber mallet until the final height is reached, then the element must be gently removed and its lower surface must be completely covered with a cement grout in the fluid state, also lattex can be added. The element must therefore be repositioned in its preformed seat and beaten uniformly to obtain a solid adhesion and perfect coplanarity and alignment.
The sealing operations are carried out in the same way as in the previous case.

4. Filling and anchoring clamps

Reserved for covering interventions involving the use of medium-large square slabs up to the height of one floor and leaning on the ground.

This method is based on the juxtaposition of individual elements, fixed one by one to the supporting structure of the masonry by means of steel (preferably stainless steel) anchoring clamps and cement-line mortar filling. The support function is therefore carried out both by the clamps and by the adhesion to the bedding filling. The stone elements, resting on each other, force the formation of joints sealed with cement grout or stucco and the use of slabs with thickness greater than three centimetres in order to ensure greater resistance to tearing.

This type of covering, for safety reasons, must not exceed the height of 3.5 m, unless a detailed design is carried out, configuring reinforced stone courses with increased thickness, increasing the number of clamps and subjecting all the possible degradation factors, such as humidity, infiltrations, temperature variations, expansions, to accurate evaluation.