Installing Flagstone Over an Existing Paver Patio with Drainage Issues

A homeowner in central New Jersey has a 12-year-old concrete paver patio with improper drainage pitch, resulting in standing water, staining, and moss growth between pavers. They're considering installing flagstone over the existing patio rather than tearing everything out.

Understanding the Current Setup

The patio consists of concrete pavers, but the description mentions water puddling "between pavers," suggesting a sand-set installation over a concrete base. The core problem is inadequate slope for drainage.

Overlay Options

Mortar-Set Flagstone Over Existing Pavers

If the pavers are stable and the concrete base beneath is sound, you can remove the pavers and install flagstone directly on the concrete using mortar. The mortar bed can be varied in thickness to create proper drainage slope. This avoids full demolition but requires removing the pavers first.

Thin-Set Overlay on Concrete

If there's a solid concrete slab under the pavers, a thin-set mortar or concrete topping can be applied to correct the pitch before setting flagstone. This adds minimal height and can address low spots.

Sand-Set Adjustment

If the pavers sit on a sand bed over concrete, simply lifting pavers in low areas, adding sand, and relaying them might solve minor puddling. However, this won't fix systemic slope issues across a large patio.

Key Considerations

• Height increase: Any overlay raises the patio surface, affecting door thresholds and step heights.
• Drainage path: Water must have somewhere to go. Ensure the new slope directs water away from the house.
• Cost vs. longevity: Overlays cost less upfront but may not last as long as a properly rebuilt base.

Recommendation

For a large patio with significant drainage problems, removing the pavers, correcting the base slope, and installing flagstone in mortar on a properly prepared concrete substrate offers the most durable solution. Consult a local landscape contractor for site-specific advice.