Laying Paving Slabs on Existing Concrete Base

A natural stone terrace in Mediterranean style or modern straight lines and classic colors: a terrace should correspond to personal taste and create a special ambience. However, before you as a garden owner can relax on the terrace in the summer, the first thing to do is to lay the patio slabs. You want to lay the patio slabs, whether natural stone or porcelain stoneware, on a concrete substrate? This is how it’s done: Laying slabs on concrete and avoiding typical mistakes!

When laying on concrete (bonded substrate), in contrast to laying on a gravel base course (unbonded substrate), the slope to be maintained is particularly important, as here the water cannot seep through the substrate, i.e. the concrete slabs. The necessary water drainage can be done, for example, by a drainage mat. Terrace slabs can be laid on a concrete slab in three different ways. Read more about the different types of laying, their advantages and laying tips in this article.

Preparing the concrete slab for laying

As a preliminary work before laying should be checked whether the existing concrete slab has a sufficient slope. Terrace slabs are laid on a slope of 2%, sometimes even 3%. Here, the surface condition of the terrace slabs also plays a role, e.g. in the case of materials with a split surface, a little more slope is recommended so that the water drains off better.

If the concrete slab has no slope, the required slope can be created, for example, with a screed. To prevent water from penetrating the concrete slab and also the adjacent structure, i.e. the house, you should still apply a waterproofing layer before applying this leveling layer. The surface of the leveling compound must also be sealed.

laying patio slabs on concrete

Prevent discoloration and frost damage with drainage mat

Drainage systems are used when patio slabs are laid on a concrete slab.

A drainage mat is applied directly to the concrete slabs, which are sealed and have sufficient slope. The main benefit of the later invisible drainage mat is the very good drainage of seepage water towards a gully or drain. In addition, the drainage mat prevents water from rising up into the pavement. Discoloration due to stagnant water can thus be avoided.

The use of a drainage mat on the concrete slabs thus helps to permanently avert water and frost damage.

lay patio slabs on concrete

Laying the terrace slabs:

After ensuring the drainage of rainwater from the concrete slab, now to the laying of the patio slabs. There are different laying variants, each with different materials as a paving bed:

Laying on a bed of chippings

Loose laying on chippings is relatively simple and inexpensive. It also has the advantage that slabs can be easily replaced should this become necessary at a later date.

Natural stone patio slabs should be laid on high-grade chippings. Other chippings, pebbles or gravel contain easily soluble minerals/iron compounds that can reach the top of the slabs via capillary action and cause discoloration (efflorescence) of the natural stone.

The chippings are applied 1.2-2 in high on the concrete slab with drainage mat and levelled. Then place the patio slabs and tap them into place with a white rubber mallet. Use a spirit level to check that the slabs are level with each other and that the recommended slope is maintained. In addition, a taut mason’s string can help to check the straightness of the patio slabs and joints.

Note: When laying in a bed of chippings, it is advisable to edge the terrace surface, as otherwise the chippings could slip away in the outer area of the surface. For this purpose, edge stones are used or the outer row of the terrace slabs is firmly glued.


Partially bonded laying

Partially bonded laying is similarly easy to perform as loose laying. Here, too, a layer of high-grade chippings is laid over the drainage mat. Then apply tile adhesive to the back of the patio slabs using the buttering method and lay them in the chippings bed. Tap lightly with a rubber mallet and fix in place. Do not forget to include an expansion joint at the edge of the laying surface.

Then grout the slabs with viscoplastic grouting material by applying the grouting mixture to the joints using a water hose and a rubber scraper. Then clean the surface of any dirt and, in the case of natural stone, apply an impregnation.

Advantages of partially bonded laying: Partially bonded installation combines the advantages of loose and permanent installation. It is easy to perform, just like the loose laying and scores like the fixed laying in the mortar bed with a water-permeable joint that does not allow weeds or ants.

Lay in drainage mortar

Drainage mortar as a paving bed is somewhat more expensive, but allows the craftsman to lay the patio slabs very firmly and with a solid paving joint. The special natural stone adhesive (also: drainage mortar or single-grain mortar) is water-permeable and commercially available as ready-mixed bagged material. It can also be mixed by the customer himself from high-grade chippings and trass binder. The correct mixing ratio (6:1) is important, otherwise the laying cannot be carried out without errors.

When making your purchase, make sure that the drainage mortar is suitable for natural stone. The drainage mortar to be used consists of high-grade chippings, grain size 0.08-0.2 or 0.08-0.3 in and an aggregate consisting of trass binder. Drainage mortar or single-grain mortar is mixed in a concrete mixer or with a stirrer according to the mixing ratio on the package instructions. The laying of the terrace slabs on concrete slab is then carried out in at least 1.18 -1.96 in thick drainage mortar “wet in wet”. This means that the flexible adhesive is pressed into the wet drainage mortar bed on the back of the natural stone slab. In this case, the drainage mortar should be protected from drying out or only mixed gradually so that it can be applied when wet. Use a white rubber mallet here, as when laying in a bed of chippings, to tap the terrace slabs coated on the back with flex adhesive into the mortar bed, and a spirit level to lay the slabs level with each other.

Laying on stilts

Laying on stilts

This laying variant, in which you can use stilt bearings, mortar bags or slab bearings, is particularly suitable for laying terrace slabs on balconies or roof terraces. However, this method of laying is not recommended for all natural stone, but mainly for hard stone such as granite, quartzite or basalt. Not only does it save on weight, but it is also not necessary to lay the slabs with a slope when laying stilts, as rainwater can run off between the approx. 0.16 in wide cracks and under the slabs. However, the substrate, in this case the concrete slab, should have the recommended slope. The terrace slabs are placed on the stilt bearings and laid with a cross joint. How convenient: the slabs on the concrete surface can be quickly and easily replaced in the future. More information about laying on stilt bearings, mortar bags or slab bearings can be found in another magazine article.

Typical mistakes – better to avoid!

  • Laying on a concrete slab without a slope or without waterproofing → Water can penetrate the concrete slab or the adjacent structure and cause damage. In addition, frost damage and discoloration/ efflorescence of natural stone are possible if water cannot drain off properly.
  • Insufficient connection and expansion joints planned → stress damage may occur.
  • Inexpensive chippings are chosen instead of high-grade chippings → colored efflorescence or rust stains may occur on the patio slabs.
  • Incorrect laying mortar used → frost damage, water damage, and discoloration of the terrace slabs cannot be ruled out.

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