Weed Barrier Placement for a Flagstone Patio: On Top, Not Underneath

A common mistake: laying weed barrier under the base soil. Weeds will grow on top of it. The correct placement is on top of the compacted base, under the joint sand or setting bed.

Correct Layer Sequence

1. Compacted subgrade
2. Geotextile (optional, for soil separation)
3. 4–6″ compacted gravel base
4. 1″ concrete sand setting bed
5. Weed barrier fabric (if desired)
6. Flagstones
7. Polymeric sand or mortar in joints

Why This Works

Weeds germinate in the joint sand or setting bed. Fabric directly under the stones blocks root penetration downward. Fabric under the gravel does nothing—roots never reach it.

For a 12×15 ft patio with garden boxes, skip fabric entirely if using polymeric sand—it hardens and resists weeds. Spend the money on quality base prep instead.