When evaluating options for upgrading a driveway or garden pathway, the question of whether you can block pave over concrete often arises. The short answer is yes, it is entirely possible, but doing so requires careful planning and specific preparation to ensure long-term stability and to prevent future surface failures.
Understanding the Existing Concrete Base
Before you block pave over concrete, it is essential to assess the condition and purpose of the existing slab. Concrete is a rigid material that does not move, whereas block paving is designed to flex slightly with ground movement. If the concrete is sound, clean, and level, it provides an excellent foundation. However, if the concrete is cracked, uneven, or suffering from weed growth through its joints, these issues must be addressed before laying the blocks, as imperfections will telegraph through the new surface.
The Critical Role of Preparation
Successful installation hinges entirely on preparation. You cannot simply sweep off debris and start laying blocks on a dirty concrete surface. The concrete must be thoroughly cleaned using a pressure washer to remove moss, algae, and ingrained dirt. Following this, the surface must be completely dry. Any dust or moisture trapped between the concrete and the bedding sand will compromise the adhesive bond and create an unstable base for the blocks.
Installing the Sand Layer
A crucial step in the process is the addition of a layer of sharp sand. This layer acts as a slip layer, allowing the rigid concrete to move slightly without cracking the block above. It also provides a comfortable and level working surface for final adjustments. Typically, a 30 to 50-millimeter layer of damp sand is spread and screeded to ensure it is perfectly level. This step is non-negotiable for a professional finish when you block pave over concrete.
Addressing Level Transitions
One of the primary challenges when you block pave over concrete involves managing the height transition. Block paving sits above the existing ground level, which means it will sit on top of the old concrete. This creates a potential tripping hazard at door thresholds or where the new surface meets lower adjacent areas. To mitigate this, you may need to remove a thin section of the concrete at entry points or carefully plan the cutting line to ensure a smooth transition between surfaces.
Edge Restraint and Stability
Unlike laying block paving on a prepared earth base, where edges can be anchored with trenches and kerbs, block paving on concrete relies heavily on the correct laying pattern. Running bond or herringbone patterns are highly effective because the blocks lock into one another, distributing weight evenly across the surface. This interlocking structure prevents the slabs from shifting. Without this stable base or proper edge restraints, the blocks risk shifting or breaking due to the rigid concrete underneath.
Legal and Structural Considerations
Lastly, it is vital to consider the implications of building on top of concrete regarding drainage and regulations. If the concrete slab acted as a damp proof membrane (DPM) for a garden room or shed, sealing it might trap moisture. Furthermore, while block paving is generally considered a permeable surface, concrete underneath prevents water from soaking into the ground. You should verify local regulations regarding ground coverage and stormwater runoff to ensure your project complies with environmental guidelines.
