Courtyard Drainage

The one paved space with no downhill side

Courtyards are a wildly varied set of places that share exactly one hydraulic problem. A hotel motor court, a hospital healing garden enclosed on four sides, a university quad with a single arched exit, an apartment podium deck sitting on a parking structure, a Charleston house built around its own patio, an office building wrapped around an open-air room at its center. All of them are bowls.

That is why this is not patio and sidewalk drainage with a different noun on it. A driveway, a terrace, a parking lot — each has a lower edge, so water that beats the drain still leaves, just rudely. A courtyard has no lower edge except the doors. When the drain is undersized, blocked, or frozen, the surplus does not run off: it stacks up, deepens, and finds the lowest threshold in the enclosure, and that threshold opens into the building. The failure mode here is interior, and the mistake we see most often is treating courtyard drainage as landscape hardware when the job is keeping water out of a lobby.

Courtyards also collect from above, because the walls pitch toward the space — roof leaders at the base of a wall, scuppers off a low parapet, balcony deck drains stacked five floors up and piped into one corner. The contributing area is routinely two to three times the paving anyone thought to measure. Then the type-specific details: a court enclosed on four sides may see two hours of low winter sun, so puddles sit for days and refreeze nightly — and water expands about 9 percent when it freezes, jacking a grate frame out of its bedding. One mature tree drops its whole canopy into a room with a single drain in it. And on a podium deck the “ground” is a waterproofed structural slab, so whatever you cut into is somebody’s ceiling.

What an enclosed court needs, and where our catalog stops

Roughly half of what a courtyard needs is not something we sell. We would rather say so than sell you a channel for a job that needs a pump.

Surface collection — ours

Channels, catch basins, grates, risers, outlet adapters, solid pipe and fittings. If it is standing on or sheeting across the courtyard floor, it is our layer.

The route out — by others, and it has to exist first

Every courtyard drain ends at a pipe leaving under or through the building: a cored penetration, a run under slab, and a tie-in to a storm riser or site main at an invert low enough for gravity to work. That is the plumbing contractor’s and civil engineer’s scope, and on commercial work it is permitted. Settle it before you shop — a beautiful channel that terminates at a wall is landscape sculpture.

Subsurface and french drains — contractor territory, not ours

Courtyards do genuinely use subsurface systems, so be precise about what they are. A french drain is a trench of washed stone around perforated pipe, separated from soil by filter fabric; its job is relieving water already in the ground, not catching water on a surface. In a court on natural grade it belongs under the planting beds, so the root zone drains and the bed stops bleeding saturated soil across the paving, and behind any raised bed’s retaining wall, where it relieves hydrostatic pressure and is structural rather than optional. All of that is dug and piped by a contractor: we sell no perforated pipe, washed stone, or geotextile, and nothing in our catalog is a french drain. On a podium deck the equivalent layer is a drainage mat over the waterproofing membrane, which belongs to the waterproofing trade.

Pumps — not ours, and sometimes the only answer

Sunken courts, basement lightwells, and areaways at the foot of an exterior stair are where gravity is simply unavailable: if the floor sits below the invert of the pipe you would tie into, no channel on earth drains it. Those need a sump, a pump, and a high-water alarm — we sell no pumps, sumps, floats, or controls. Find out which case you are in before you spend a dollar on channel.

Separators and interceptors — downstream, by others

A restaurant court with a wash-down hose, or a service court where containers get rinsed, can trigger a separator or interceptor requirement. That sits downstream of our drain, engineered by the plumbing designer, and we do not supply it.

Running the numbers for a room with no exit

Courtyard paving is laid as flat as the trade dares — furniture has to sit level, and accessible routes cap cross slope at 1:48, about 2 percent. The working range for shedding water is 1/8″ to 1/4″ per foot, so a court lives at the bottom of that band with no margin for a settled slab. Water concentrates in four predictable places: where a leader or scupper lands, along the wall facing the driven rain, inside every door threshold, and wherever the mason found it convenient to be low. Plan a second collection point too — on flat paving there always is one.

How much water is actually in there

Two figures cover most of it: 1,000 sq ft of roof or paving produces about 623 gallons per inch of rain, and every 100 sq ft of hard surface sheds roughly 1 GPM for each inch per hour of intensity. Work a real one. A 40′ × 60′ court is 2,400 sq ft of paving; add 3,000 sq ft of roof and balcony pitching in — conservative for a two-story building wrapped on four sides — and you have 5,400 sq ft. That court takes on about 3,360 gallons per inch of rain, so a 2″ storm delivers close to 6,700 gallons into a space with no door out. By rate it is 54 GPM per inch per hour: 108 GPM in a 2 in./hr storm, about 216 GPM in a 4 in./hr cell. Run your site through the Runoff Calculator and read the method in Sizing for Storms — before you pick a channel width, not after.

The bottleneck is the pipe, and here you get one shot at it

Grates and channels look like the capacity question and almost never are; the outlet is. It matters more here than anywhere else we sell into, because that pipe runs under a building: on a driveway an undersized outlet is a Saturday with a shovel, in a court it is a saw-cut slab, a shored trench beside a footing, and a closed lobby.

Hold that against the worked example: one 4″ line out of that 5,400 sq ft court carries a 2 in./hr storm with nothing to spare and roughly half of a 4 in./hr cell. A single 6″ line, or two 4″ lines on separate routes, is the honest answer — and doubling the fall to 1/4″ per foot lifts every number above by about 40 percent, often cheaper than a bigger pipe if the invert allows it. Size the buried pipe first, then choose channel and grates to match.

Load class, from a cloister to a delivery court

Courtyards run the full ladder, sometimes inside one enclosure. Under EN 1433, Class A (A 15) is a 15 kN test load, published by ACO and NDS as 3,372 lbs — pedestrian paving, cloisters, garden courts. Class B (B 125) is 125 kN, about 28,101 lbs, and it is where you belong the moment maintenance carts, scissor lifts, or event trucks cross the grate — in a hotel or campus court, weekly. Class C (C 250) is 250 kN, about 56,202 lbs: service courts and fire lanes. Class D (D 400) is 400 kN and is not exotic — a delivery or trash court where a forklift wheels pallets off a box truck belongs there, on a ductile iron frame. As a trade heuristic the AASHTO ratings sit loosely in the middle of that ladder (H-20 is a 16,000-lb wheel load, HS-25 is 20,000 lb), but they are different test regimes, not equivalents. Full breakdown in the Load Class Recommendation Guide; on an accessible route read Understanding the ADA first.

Why a pop-up emitter has no job inside these walls

This is the wrong answer we correct most often, so it gets its own heading. A pop-up emitter is purely a gravity device: water backs up in the discharge line until the head lifts a hinged lid, spills at grade, and the lid drops flush when flow stops. For that, the emitter must sit lower than the basin invert and on open ground where the water is allowed to go. A courtyard offers neither — nothing inside the walls is lower than the courtyard floor, and the pipe is heading under a building toward a storm connection, not out to a lawn. Nothing lifts the lid; and if anything ever did, it would be spilling inside the space you were trying to drain.

The same logic disqualifies the other daylight-dependent options. A dry well can occasionally work in a court on natural grade with permeable soil and a low water table, but that is uncommon inside a building footprint and impossible on a podium deck, where the chamber would sit on a waterproofed roof. On structure, or below the storm invert, the honest answer is the pumped one, and it is not ours.

Where each piece lands inside the walls

Perimeter lines, and the sump they run to

The highest-value run in most courts is a channel tight against the building, catching what the wall sheds and what the leaders deliver before it reaches the middle of the space — under the scupper, across the door threshold, along the wall that takes the driven rain. Intercept at the wall rather than asking dead-flat paving to carry water to a center drain, and treat any run fed by a roof leader as a different animal from the run beside it: that one needs the outlet, not just the width (see downspout drainage). Land it in a real catch basin, where silt drops out, a plumber can rod the line, and pipe steps up in size — the depth of the box is sump volume below the outlet, the only place leaf litter and grit can gather without entering a pipe that runs under a building.

Planted beds, tree pits, and the leaf problem

A flat grate in a mulched bed seals over in one autumn. Dome grates earn their keep here: the raised profile sits above the mulch line so bark and leaves slide off the sides instead of matting across the openings, and the open area is on the vertical faces where it stays clear. The 18″ dome grate carries 89.40 sq in. of open area and a published 386.77 GPM at 1″ of head through 1/2″ openings; the 12″ version 154.79 GPM; the 9″ square 96.36 GPM. They are cataloged under atrium grates. Our rule for a planted court: dome grates in the beds, flat grates on the paving, never a flat grate under a tree.

The grate is the finish — and it needs to stay in its frame

Nowhere else we sell into is the drain viewed from four sides at conversation distance and from the windows above, so treat it as a specified finish. Brass covers satin and polished in 3″ through 6″ round and square collars plus 9″ and 12″ basin tops; chrome gives the same idea in a cooler key; decorative grates hold the botanical and wave patterns for Spee-D, Mini and the 12″ basin, plus uncoated raw iron Dura-Slope grates in weave, diamond and tile pattern (24″ × 6″) that take on a rust patina architects either love on sight or reject on sight. Cast iron and ductile iron are the workhorses as the class climbs; the material comparison lives in the grates section. And because courts are unattended rooms where loose grates walk, specify the lock hardware: Pro Series security clips (#849 on the 12″), the Dura-Slope ProFit grate lock (DS-122), or stainless screws through the grate ears.

What we’d spec for a courtyard, and why