Facing the plinth with flagstone

Recommendation points

• What type of stone is better to choose for facing
• Substrate surface preparation
• What glue to use
• Laying and alignment
• Seam sealing

The most authentic look of the plinth is provided by its stone finish. For these purposes, flagstone is quite suitable: in terms of accessibility, it is inferior only to imitation of stone, however, for the durability of such a finish, a number of installation nuances must be observed. All the details on this issue are in our new instructions..

What type of stone is better to choose for facing

The concept of flagstone does not imply any certainty about the rock used, but says a lot about its format. These are plates and flakes of irregular shape, usually up to 50 mm thick. Decorating the basement with them gives the house a complete and logical look, however, the success of the entire enterprise requires sensitive consolidation in all aspects, first of all – in the right choice of stone.

The service life of the plinth finish depends on its resistance to weathering. The most detrimental effect on heated houses is exerted by changes in temperature and humidity, as well as periodic wetting and organic erosion. The influence of all these factors can be weakened by providing the masonry with solidity and uniformity of the behavior of materials. Therefore, the stone must be selected according to the type of glue used and vice versa..

One of the best combinations is frost-resistant adhesive for tiles and porcelain stoneware, on which granite or quartz stones are laid. The polymer binder is highly hydrophobic, but rather demanding on the adhesion of the glued surfaces. Therefore, the seams must be well fitted and the surfaces covered with adhesive must be thoroughly cleaned. Another option for volcanic rocks is a high-quality cement slurry on quartz sand. We will dwell on the properties of various binders in more detail later, but for now we note that in regions with a temperate climate, you can also use ordinary cement mortar for laying bricks, but only in combination with sandstone, limestone and other sedimentary rocks.

Substrate surface preparation

Since the flagstone, with all the desire, will not be able to align in an absolute plane, there are rather large curvature tolerances for it, which allow it to be laid with continuous gluing. In this version, the cladding gets rid of air pores, but becomes more sensitive to vibrations of the base. This makes finishing work on floating and prefabricated foundations more difficult..

There are several ways to improve the quality of the link between limestone and base. The first is to immobilize the glue seam, excluding laminar displacements between it and the stones. It is enough to reinforce the surface of the concrete structure with a reinforcing mesh and pre-plaster with cement mortar with a layer of 35–40 mm. Before and after applying the plaster, the surfaces must be treated with a primer, preferably with the addition of stone dust.

A slightly different cladding technique is used if there is an insulating belt on the foundation. Laying on polystyrene is possible only after applying a base reinforcing layer of slightly higher power than when installing a wet facade. Optimally, two layers of frost-resistant plaster of 20 mm, applied sequentially, are suitable. When applying each layer, the facade fiberglass mesh is recessed into it, and in this order: first, glue on the wall, then the mesh and smooth it to visual evenness.

The base must be applied with alternate priming between coats to ensure solidity. It should be remembered that expanded polystyrene does not resist ultraviolet light well, therefore, before applying the first layer, be sure to clean the surface with an abrasive trowel. Weak adhesion between layers is the cause of the most common type of destruction: due to the large weight of the stone, the tear-off effect increases especially strongly.

What glue to use

There is a significant difference in whether a cement or a polymer acts as a binder. In addition to the homogeneity factor mentioned above, it is important to be able to perceive deformations of the substrate without destruction. The latter will differ in the case of facing the basement with plaster and insulation with base reinforcement.

Insulation finishing is best done using dry adhesive mixtures, mixed with water at the place of use. Their distinctive feature is plasticity that remains after solidification. Due to this, the reinforcing layer can “float” within a few millimeters without affecting the integrity of the stone cladding. The optimal thickness of the glue is 10-12 mm, but the stones are laid with an exact fit at the seams.

Reinforced screed on the plinth is a more rigid layer that absorbs part of the deformation of the foundation and holds it together as a whole. Laying here is possible even on a mortar without additives: 3 parts of sand and one part of cement grade 400, or on Portland cement grade 500, but already in a ratio to sand of 1: 4. It is important to remember about the peculiarities of cement to maintain a large thickness of the joint – this advantage should be used if the shape of the stones is too irregular, and there is no way to cut them. But remember that the closer to the limiting thickness of the solution, the more accurately the water-cement ratio must be maintained. Of course, the addition of modifiers (water repellents, fiber) within reasonable limits is also highly welcome..

Laying and alignment

There are two ways to lay limestone. It can be used in a similar manner to tiles, glued flat to a vertical base, or stacked to form thin, sharp outer edges. It is also possible to combine both options in order to enhance the style and emphasize the architectural image. So, for example, a thicker cladding “in a stack” can favorably emphasize the porch, the convergence of the basement at the corners in the presence of rustication or under pilasters.

Laying limestone flat is the fastest way to design a plinth. It begins with soaking and scrubbing the stones with a metal brush to remove dirt and efflorescence. When the stones are slightly dry, they should be sorted, achieving a more or less uniform ratio of shades in all areas of the facing. Parallel to sorting by color, stones are divided into several groups by size. It is most convenient to align the flagstone trim along a movable dock with rings at the end that move along vertical rods. The stones are individually cut by hand and folded into a mosaic. By the way, before gluing them, it is convenient to lay them out in the same order on the blind area: the assistant collects a “puzzle” that the bricklayer glues to the wall.

The stacking method does not require precise adjustment and sorting of stones. The slabs are cut into strips 50–70 mm wide and the resulting small elements are laid over either the entire basement or its parts. The advantage here is that the limestone slice always has an almost constant shape and such elements fit well with each other. Also, the stones do not need any thorough cleaning: it is enough to rinse them immediately before laying. Well, since this type of plinth lining is considered self-supporting, you can significantly save on the binder.

Sometimes it makes sense to take out such a foundation finish a little further, so that the correct abutment under the insulation of the required thickness is formed from above. In such cases, the space between the cladding and the wall may be filled with bulk material such as foam glass or expanded clay.

Seam sealing

If the stones are fitted so tightly that the average thickness of the joints does not exceed 3-4 mm, it is better to seal them up immediately. You still cannot give such thin veins a special shape or expressiveness, but you can protect the maximum surface of the stones from the water flowing down them. Therefore, after laying the next stone, simply remove the excess solution with a glove, and after 12-14 hours, the lining is washed from the cement milk with a synthetic brush..

A more sensitive approach is required when grouting joints that are equivalent to or greater than masonry thickness. In such cases, the seams are usually embroidered, but not with the usual knife, which will interfere with the curvature. It is better and more correct to immediately scrape the mortar out of the joints to a depth of 10-15 mm and then fill them with a suitable compound. For simplicity, you can use a device like a pastry syringe – several tight bags nested inside each other with a cut corner.

It is recommended to use high-quality polymer glue as a joint filler: it has almost zero water absorption, therefore it is guaranteed not to become a weak point of the stone cladding. For a more expressive look, the grout can be made of white or gray cement, tint the composition with a color that emphasizes or contrasts with the main range of cladding.