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Base preparation and drainage: the part below the grass

A finished surface can look excellent while the important questions are underground. Base preparation, grading, soil, outlets, and site edges determine how the system behaves over time.

4-minute readLast reviewed July 21, 2026Installation guide

Why drainage is a system question

Water may move through the turf backing, infill, aggregate, native soil, a drain, or an approved outlet. The correct path depends on the complete site. A turf product alone cannot promise that a low point, compacted soil, blocked outlet, or surrounding grade will perform well during every storm.

Surface flowWater follows the finished grade toward a safe low point, drain, swale, or approved outlet.

Vertical flowWater passes through turf, infill, base, and—only where suitable—into native soil.

Lateral subsurface flowWater moves within a designed drainage layer or underdrain toward an outlet.

Overflow pathWhen soil, storage, or outlet capacity is exceeded, water still needs a route away from buildings and neighbours.

Ask this before approving the scope: “Where does water go from this specific area during heavy rain, and what site condition or maintenance assumption does that answer depend on?”

Infiltration is not the same as an outlet

Permeable turf and open aggregate can transmit water, but native soil may accept it slowly or become saturated. Clay, compacted fill, frozen ground, a high water table, or a low downstream connection can limit the system. A design should identify storage, infiltration, conveyance, and overflow separately rather than assuming that water disappears below the carpet.

What each base component is meant to do

Subgrade: provide stable support without trapped organics, uncontrolled soft areas, or grades that direct water toward structures.
Separation or filtration layer: limit migration between materials where needed. Geotextile is selected for a purpose; it is not automatically beneficial at every interface.
Aggregate: provide support, shaping, void space, and/or lateral flow. Gradation and fines influence both compaction and drainage.
Drain or outlet: collect and convey water only when gravity, capacity, maintenance access, and the receiving location are suitable.
Edges: restrain the system while permitting the intended water path and preserving adjacent grades.

What a thorough site review should examine

Existing grades and low points. Note where water currently collects, which surfaces drain toward the area, and whether the proposed finished elevation changes that path.
Soil and excavation. Identify topsoil, clay, fill, roots, buried debris, and any unsuitable material that may affect excavation or compaction.
Base design. Document aggregate type, depth, lifts, compaction method, finished slope, and how the base transitions to existing surfaces.
Outlets and boundaries. Review downspouts, catch basins, perimeter edges, fences, foundations, gardens, retaining walls, and neighbouring properties.
Utilities and permissions. Confirm locates, easements, municipal requirements, conservation controls, and private-property rules before digging or changing grades.

Questions to put in the proposal

Drainage and base questions for a written scope
Question	Why it matters
What is being removed and how deep?	Existing material and excavation assumptions affect stability, access, disposal, and price.
What is the base material and compaction plan?	A stable foundation needs a defined material, installation method, and finished level.
Where is the finished water path?	It clarifies whether water infiltrates, reaches an outlet, or relies on an existing drainage feature.
What happens at the edges?	Transitions can collect water or move when adjacent surfaces settle or freeze and thaw.
What is excluded?	Unknown utilities, major drainage construction, retaining work, permits, or unsuitable soil should not be hidden inside a vague allowance.

Warning signs in a vague drainage promise

A universal flow-rate number with no product, test method, soil, slope, or outlet conditions.
“No flooding” or “zero maintenance” language that ignores the site and ongoing care.
No discussion of downspouts, low points, adjacent grades, or what happens at the property boundary.
A proposal that names the visible turf but not the base, excavation, drainage, edging, or change assumptions.

Freeze-thaw and winter water

Frozen or partly frozen soil changes infiltration, snow storage adds delayed water, and repeated thawing can expose settlement or edge movement. Keep drains and overflow routes available, plan where snow will be placed, and inspect low points, seams, and transitions in spring. “All weather” is not a substitute for dimensional-stability and site-drainage evidence.

Commissioning and records

Keep pre-work rain photos, excavation and subgrade records, aggregate delivery information, hidden-drain photographs, finished elevations, product/infill identity, and completion observations. A controlled hose test can reveal local behaviour but cannot represent every intense storm, saturated soil, frozen condition, or downstream backup.

After installation

Keep the completion photos, care instructions, product information, and written scope. Follow the recommended routine for debris, spills, pet areas, snow, furniture, and infill. If water behaves differently after a major site change, record rainfall, timing, location, depth, duration, outlet condition, and surrounding flows before altering the surface.

Sources and scope

ASTM F2898 describes surface, vertical, and lateral flow concepts for synthetic sports-field systems; it does not prescribe a residential GTA design. Mississauga guidance states that property drainage is the owner’s responsibility and should direct water away from buildings and neighbouring properties.