Homeowners keep reporting the same three failures after synthetic grass goes down straight on a slab – flooded surfaces, trapped-water odor and algae under the carpet. Most discover the exit problem only after the roll is glued, which is the worst possible moment to learn it. Every artificial grass drainage concrete patio plan therefore starts with the slab, not with the carpet. We have watched good carpet go down on bad slabs often enough to say it plainly: concrete cannot absorb water.
On soil, drainage points down; on a concrete slab it can only travel sideways. Same roll, two different hydraulics. This page maps where water can leave the system and hands over a pre-order slab checklist. The fixes sort into two families, and the story ends where the roll itself must carry the plan. Read it with your own slab in mind, because every rule here gets tested against real concrete.
What Changes When the Base Can’t Absorb Water
Rain on a patio slab behaves like rain on a road. Concrete belongs to the impervious family of surfaces – roofs, drives and sidewalks that shed water instead of taking it in. Rain hitting saturated or sealed ground flows overland downhill, exactly as water-cycle research describes for paved ground. That single fact sits at the center of every artificial grass drainage concrete patio decision made on an existing slab.
What changes is the question you ask. On soil, drainage is a materials question – which base layers let water pass. The core of artificial grass drainage on concrete is different, because sealing the ground multiplies both the volume and the speed of runoff. The slab turns drainage into a path question: where, physically, can this water leave?
The scale is smaller than a parking lot, but the physics is identical. A patio that sheds every storm contributes its full share of runoff to whatever edge receives it. That is not an argument against concrete – it is the reason the exits must be designed rather than assumed. Turf on soil inherits a drainage system; turf on a slab has to bring one along.
The three-minute site test. Pour a bucket of water on the bare slab and watch where it goes. Three minutes of observation answers half of the can-this-slab-take-turf question before anyone quotes anything.
Soil bases earn a separate conversation, because there the water can actually leave downward. Our companion guide explains how sloped soil bases manage water underground with grades and underdrains. A slab offers no such kindness, so the exits must be built on purpose.
The Three Exit Paths for Water on a Slab
Every artificial grass drainage concrete patio system leans on the same three exits, and they work as a combination rather than a menu. Water must get through th