Patio slabs sinking on gravel or chippings: what to do?
For patio slabs sinking on gravel or chippings, four repair routes are available, ranging from re-laying 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 can sink, rock or move in relation to one another. Before repairs begin, it must be decided whether the existing slabs are to be re-used, covered over or removed.
Re-laying the existing slabs involves the least work. The sunken or rocking slabs are lifted, the loose bed beneath is re-levelled, and the slabs are replaced flush and with full support. To stabilise a loose gravel or chippings bed over time, suitable plastic honeycomb grids can be installed. These grids retain the fill material in individual cells and largely prevent it from shifting sideways under load.
Where the existing concrete or stone paving slabs remain in place, they can form the sub-base for new patio tiles. This requires the existing covering to remain permanently load-bearing and sufficiently level, without rocking slabs or major differences in height. If 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 WARCO range. The tiles are water-permeable and have drainage channels on the underside, allowing rainwater to drain through the covering. Falls, outlets and existing drainage routes must be checked and kept permanently clear.
If the concrete or stone paving slabs are removed while the gravel or chippings bed is retained, suitable plastic honeycomb grids are installed in that bed. The grids are filled to their upper edge with the existing material or with suitable new material. This arrangement requires sufficient installation depth; WARCO requires a depth of at least 6 cm when refurbishing an existing loose bed. The fill is screeded 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. A further loose layer above the honeycomb grids is not appropriate, as the material could shift again above the grids.
The fourth route removes both the existing paving slab covering and the entire 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 synthetic membranes containing plasticisers are present, a suitable separation layer must be installed between the waterproofing and the rubber granules. Damaged waterproofing must be professionally repaired or renewed before laying the tiles.
Where the existing build-up height is to be retained, height adjustment can be made with suitable polystyrene boards of adequate compressive strength. The insulation boards must be suitable for the loads involved, be fully and stably supported, and must not obstruct water drainage. Whether this build-up is possible depends on the existing roof construction.
For every repair route, falls, drainage routes and outlets must remain functional. On balconies and roof terraces, the permissible load per unit area, required upstand heights, compatibility of the materials used and, where relevant, protection against wind uplift must also be considered. The appropriate approach depends on the condition of the existing covering and waterproofing, as well as on the existing roof build-up.