I recently visited a client’s backyard, which was almost entirely paved with paving stones.
The homeowner didn’t know when the paving stones were installed. They had moved in about three years ago, and the stones were already there. It seems the previous owner had renovated the garden before selling the house. From my observation, the stones had been in place for roughly that amount of time.
They called me to ask if the sunken paving stones could be repaired and how much it would cost.
The backyard likely had a slight slope initially. Allan block retaining walls were built along the edges to create a flat surface. The retaining walls appear to have been installed at the same time as the paving stones.
Sand was filled in, and the paving stones were installed, probably well-compacted. Yet, over time, they sank.
So why do paving stones sink? Can’t it be done properly from the start?
The answer is: even with proper installation, sometimes stones may sink, and sometimes they won’t. Let’s explore the main reasons paving stones sink and how to prevent it.
- Strong base soil is essential.
Base soil refers to the condition of the ground before any new construction. It can be divided into fill (raised soil) and cut (excavated soil). Cut soil has been compacted naturally over a long period by gravity and rain, so it is stable. The problem usually occurs in fill soil. In the example above, the retaining wall was built and soil was raised, which means it’s fill soil. A professional landscaper would normally use gravel or road base instead of soil and compact it thoroughly to reduce settling. However, fill soil will settle over time. If paving stones must be installed on fill soil, the homeowner should expect at least one repair in the future. This must be communicated upfront. Without this warning, clients may think the initial installation was faulty after a few years. - Adequate base material is required and must be compacted.
Gravel, gravel-sand mix, or sand is used as base material. Proper thickness prevents settling and allows water to drain quickly. For foot traffic, at least 3 inches of base material is needed. For heavy loads like vehicles, at least 6 inches is recommended. Thickness depends on soil type, usage, and purpose. - Compaction of the base is critical.
Sprinkling water while compacting creates a denser and more even base than just using mechanical compactors alone. Water is more effective than physical force. - Structural work requires expert guidance.
Retaining walls, paving stones, and other structural installations need professional advice. Cutting corners or installing without proper understanding may lead to higher costs and more problems in the long run.
Written by Andy Lee
Owner of Andy’s Landscape Inc. – Landscape Contractor in Maple Ridge, BC