Does weed membrane under paving actually work?

The name causes the confusion. These fabrics are geotextiles, and their primary engineering job is separation: keeping a stone layer and a soil layer from mixing. Weed suppression is a secondary property that works well in some situations and not at all in others, and paving is one where it mostly does not.

Step by step

1. Work out where the weeds actually come from.In a jointed hard surface, they come from above. Seed lands in the joint, organic debris collects and rots down into a growing medium, and the plant roots into the joint itself. A membrane 200mm below the surface is irrelevant to that entire process.
2. Use membrane under gravel, without hesitation.Here it does two useful jobs: it stops gravel migrating down into the soil and soil pumping up into the gravel, and it does genuinely reduce weed growth from perennial roots below. A gravel path without a separator loses stone into the ground for years.
3. Use a separator geotextile under a sub-base on soft ground.On clay, silty or soft ground, a heavy geotextile between the subgrade and the MOT type 1 stops the stone punching into the soft material below. That is a structural benefit and it is why it appears on properly specified drives — nothing to do with weeds.
4. Do not put membrane between the sub-base and the laying course.It traps water at that level, prevents drainage through the pavement and provides a slip plane the laying course can move on. Membrane belongs at the bottom of the build-up, on the subgrade, not in the middle of it.
5. Choose the right fabric.Woven geotextiles are strong and are what you want as a separator under a sub-base. Non-woven needle-punched fabrics drain better and suit gravel and drainage applications. Thin plastic landscape sheeting sold as weed control does none of these jobs well and clogs.
6. Never use polythene.Solid plastic sheet under a hard surface stops water draining, holds it against the underside and makes both frost damage and pumping considerably worse. Anything under paving must be permeable.
7. Deal with the joints if weeds are the actual complaint.Keep the surface swept so debris does not build up, keep the joints full — sand, mortar or a resin compound — and clear moss before it becomes a seedbed. That is what stops weeds in paving.

Common mistakes

• Expecting membrane to stop weeds in paving joints — The weeds are growing in the joint, above the membrane, in debris that landed there. The membrane is not in the way of anything.
• Laying membrane on top of the sub-base — It stops water draining down through the pavement, holds a wet layer immediately under the paving and creates a plane the bedding can slide on.
• Using polythene or damp proof membrane — It is impermeable. Under a hard surface it holds water where you least want it and makes frost heave and rocking much more likely.
• Buying the cheapest fabric — Light landscape fabric tears during compaction and does not have the strength to act as a separator. If it is worth putting in, it is worth buying at a grade that survives a plate compactor.

Alternatives

• Woven geotextile separator: The right choice at the bottom of a drive or patio build-up on soft ground. High tensile strength, keeps the sub-base out of the subgrade, permeable. Buy by weight or grade rather than by the words on the packet.
• Non-woven needle-punched geotextile: Better drainage characteristics, softer, easier to handle. The right fabric for wrapping a land drain, lining a soakaway or under gravel.
• Gravel stabilising grid with integral fabric: For gravel paths and drives, a cellular grid with a fabric backing does the separation, the weed suppression and the stone retention in one product. Considerably more effective than membrane alone.
• Polymeric or resin jointing: If the real objective is fewer weeds in a patio, this addresses the actual cause. A sealed joint gives seed nowhere to germinate.

Questions people ask