Patio slabs sinking on gravel or chippings: what to do?
Four refurbishment approaches are available when patio slabs sink on a bed of gravel or chippings. They range from relaying the existing slabs to removing the entire existing build-up.
On balconies and flat roofs, concrete or stone paving slabs are often laid on a loose bed of gravel or chippings. If this material shifts under load, individual slabs may sink, rock or move against one another. Before refurbishment begins, it must be decided whether the existing slabs are to be reused, overlaid or removed.
Relaying the existing paving slabs involves the least work. Sunken or rocking slabs are lifted, the bedding beneath them is re-levelled, and the slabs are replaced level with one another and with full support. Suitable plastic honeycomb grids can be installed to stabilise a loose gravel or chippings bed over the long term. They retain the fill material in separate cells and largely prevent it from shifting sideways under load.
Under the second approach, the existing concrete or stone paving slabs remain in place and serve as a sub-base for a new patio covering. This requires the existing covering to remain permanently load-bearing and sufficiently level, without rocking slabs or substantial changes in level. Where necessary, it must first be re-levelled. Patio tiles made from rubber granules can then be laid over the existing covering. Their exact thickness depends on the selected range. The tiles are water-permeable and incorporate drainage channels on the underside, allowing rainwater to drain through the covering. Falls, outlets and existing drainage paths must be checked and kept permanently clear.
With the third approach, the concrete or stone paving slabs are removed while the existing gravel or chippings bed is retained. Suitable plastic honeycomb grids are installed in the bed and filled to their upper edge with the existing material or suitable new material. Sufficient installation depth is required. Refurbishment of an existing bed requires at least 6 cm. The fill is levelled flush with the top of the grids, creating a level, stable sub-base on which patio tiles made from rubber granules can be laid directly. An additional loose layer above the honeycomb grids is not useful, as the material could shift again above the grids.
The fourth approach removes both the existing slab covering and the complete gravel or chippings bed. Patio tiles made from rubber granules can be laid directly on an intact, load-bearing and sufficiently level roof or balcony waterproofing layer. Where waterproofing membranes contain plasticisers, a suitable separation layer must be installed between the waterproofing and the rubber granules. Damaged waterproofing must be professionally repaired or renewed before installation. If the existing build-up height is to be retained, suitable polystyrene boards with sufficient compressive strength may be used for levelling. The insulation boards must be suitable for the loads involved, fully and stably supported, and must not obstruct water drainage. Whether this build-up is possible depends on the existing roof construction.
For every refurbishment approach, falls, drainage routes and outlets must remain functional. On balconies and roof terraces, the permissible area load, required connection heights, compatibility of the materials used and, where applicable, protection against wind uplift must be considered. The appropriate approach depends on the condition of the existing covering and waterproofing, as well as the existing roof build-up.