Roof & Lot Drainage: Where Your Stormwater Goes (and What Needs a Permit)

Grading shapes the lot so water leaves the foundation; this guide is about where it goes next. The principle the engineering manuals use is a "treatment train" — deal with stormwater at the lot first, then in conveyance, and only then downstream. For a homeowner that means a simple ladder of options, from a $20 downspout extension up to engineered infiltration, and the deciding factor on most lots is the one people skip: the soil.

Know Your Soil First

Infiltration — letting water soak into the ground — only works if the ground actually drains. A quick percolation check (dig a hole, fill it, see how fast it empties) tells you most of what you need; engineered work uses a measured infiltration rate, and roughly 15 mm/hour is the threshold below which infiltration controls struggle. Sandy and loamy soils usually pass; heavy clay usually doesn't, and a water table or bedrock within about a metre of the bottom of your pit rules infiltration out too.

Clay isn't an automatic dead end, though. Soil can be amended or replaced with an engineered bioretention soil mix (sand/compost blends) so a rain garden drains — in fact, amended-soil bioretention is itself a recognized practice, sometimes designed into subdivisions. Just know that once you're importing engineered media you've moved from a weekend project toward a designed one. If your soil won't drain and you don't want to engineer it, skip to storage and conveyance.

The Options Ladder (Homeowner DIY)

From cheapest and simplest upward — most lots only need the first one or two:

• Improve the receiving soil (do this first). The cheapest, best-evidenced practice isn't a structure at all — it's the ground itself. Decompacting the subsoil and laying deeper, compost-amended topsoil (about 30 cm) across the areas that receive roof and pavement runoff measurably cuts how much runs off, and gives you a more drought-tolerant, lower-maintenance yard. Topsoil depth alone, over compacted subsoil, does little in big storms — loosen the subsoil first. In a tight subdivision, mind the grading bylaw: your lot grades are fixed by the approved plan (and often your purchase agreement), so amend by mixing compost in and re-grading back to the same approved contours — don't raise the grade, bury a rear-yard swale, or cover the minimum exposed foundation (commonly ~150 mm / 6″ above finished grade) the code requires. On large rural lots you have far more latitude.
• Downspout to grade / disconnection. Extend the downspout (or disconnect it from the sewer) and discharge onto a vegetated surface at least 3 m (about 10 ft) from the foundation, sloped away. Cheapest and most effective single step. Some cities — Toronto city-wide — require downspouts to be disconnected from the sewer.
• Rain barrel / cistern. Captures roof water for reuse on the garden. Useful and cheap, but the volume is small and the overflow still has to go somewhere safe — it complements the other options, it doesn't replace them.
• Rain garden / bioretention. A shallow planted depression that collects roof and yard runoff and lets it soak in. Works on draining soil (or amended soil), set back from the foundation, sized to the roof area it serves.
• Soakaway pit / dry well. A stone- or chamber-filled pit that takes downspout water underground and lets it infiltrate. Needs draining soil, a water table well below it, and a setback of at least 3 m from any foundation, with an overflow path. (Clear stone is the usual fill — see choosing the right gravel.)

Build in maintenance from day one. Field monitoring of residential lot-level systems in Ontario (Toronto and Region Conservation Authority's Sustainable Technologies Evaluation Program, 2013) found that infiltration practices clog and can quietly stop working — several monitored backyard infiltration trenches weren't even receiving the runoff they were built for, and one had a deck and shed built over its access. Keep the inlet and any catchbasin clear and reachable, pre-treat the inflow (a strip of grass or a swale ahead of a pit or trench traps sediment), and plan to inspect it. An infiltration system you can't reach is one you can't fix.

When You Have More Space (Acreage & Larger Lots)

With room to work and longer flow paths, the same logic scales up and a few more tools come into play:

• Grassed swales / bioswales. Shallow vegetated channels that both convey and infiltrate as water travels — ideal for moving roof and yard water across a larger property to a safe outlet.
• Rear-yard ponding / infiltration trenches. Designed low areas or stone-filled trenches that hold and soak in larger volumes; they want gentle slopes (rear-yard ponding under about 2%) and draining soil.
• Permeable pavement. For driveways, parking, and patios, permeable pavers let water through instead of shedding it — a strong option where a hard surface would otherwise dump runoff. The two common types are permeable interlocking concrete pavers (PICP) for full driveways and plastic grid pavers (gravel- or grass-filled) for light-traffic and overflow areas; both need a deeper open-graded clear-stone base and periodic vacuum-sweeping to keep draining. Price them against conventional surfaces in the Driveway Cost Calculator (it now includes permeable options) and weigh the trade-offs in asphalt vs concrete vs gravel.
• Small infiltration basins. At the upper end you're approaching engineered territory — useful on acreage but designed, not improvised.

The constraints that decide which of these is feasible are consistent, and worth checking before you dig:

General planning figures adapted from Ontario's Stormwater Management Planning & Design Manual (lot-level controls). Confirm design values locally; sizing should follow your municipality's guidance or an engineer's.

Costs & Rebates (2026)

Rough installed ballparks — they swing widely with soil, size, and access, so treat them as planning numbers and confirm locally:

Check for rebates first. Many municipalities now subsidize lot-level stormwater work — Ottawa's Rain Ready program, Edmonton (EPCOR) soakaway-pit rebates, Toronto's downspout-disconnection assistance, and GTA permeable-paving rebates (some up to a few thousand dollars), with Mississauga and Vaughan programs rolling out in 2026. A rebate can swing the math between two options, so look before you commit. For the physical earthwork and stone, the Lot Development Estimator and Fill & Gravel Calculator help you budget.

What Needs a Permit or Approval

Most simple downspout-to-grade work needs no permit — but several common moves do, and they're easy to miss:

• Anything in a regulated area. In Ontario, work near water, wetlands, floodplains, valleylands, or steep slopes generally needs a conservation authority permit under O.Reg 41/24 (in force since April 2024; development is now restricted within 30 m of a wetland, among other triggers). Other provinces have equivalent watercourse/wetland approvals.
• Connecting to — or disconnecting from — the municipal storm system. Both can require approval, and some cities (Toronto) mandate downspout disconnection by bylaw.
• Lot-grading conformance. Your drainage changes can't break the approved lot grading / subdivision drainage scheme — ponding or redirecting water against that plan is a violation.
• Septic and well separation. Adding infiltration load near a septic bed or a well brings in health-unit rules. Keep well clear.

Because the triggers are so local, confirming up front exactly what your municipality and conservation authority require — before you design — is the kind of question a focused feasibility review answers, and it's far cheaper than undoing work that needed a permit.

The Safest Path Forward

1. Get the grading right first — water has to leave the foundation before any of this matters (see grading plans).
2. Test your soil — a simple percolation check tells you whether infiltration is even on the table.
3. Start at the bottom of the ladder — downspout to grade, 3 m+ from the foundation — and only add storage/infiltration if you still have water to manage.
4. Check permits and rebates — conservation authority and municipality for approvals; the same offices often have the rebate programs.
5. Engineer the big stuff — anything beyond a rain garden or single soakaway, near a foundation, septic, or regulated area, gets a professional's sizing.

Bottom Line

Roof water has to go somewhere, and the right answer is a short ladder gated by your soil: discharge to grade well away from the foundation, then add a rain barrel, rain garden, or soakaway if the ground drains — and scale up to swales, ponding, and permeable paving when you have the space. Keep infiltration off the foundation, the septic, and the neighbour, check whether a conservation-authority or municipal approval applies, and look for a rebate before you spend. When it gets bigger than a rain garden, bring in an engineer.

Frequently Asked Questions