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Eavestrough Size Calculator

Calculate Canadian eavestrough size from roof area, pitch and design rainfall per NBC 2020 / CSA B406. K-style, half-round and box, with downspout sizing in CAD.

Eavestrough Size Calculator

Calculate eavestrough size from roof area, pitch and design rainfall per NBC 2020 / CSA B406.

Recommended eavestrough size
5" K-style
Peak flow: 77.8 gpm · Flow per downspout: 38.9 gpm
Effective drainage area: 1,870 sq ft (Pitch factor: ×1.1)
Minimum acceptable
5" K-style
Downspout cross-section
3 × 4 in (12 sq in)
Suggested downspout count
1
Reference standard
NBC 2020 9.26 / CSA B406 / CRCA Roofing Manual

What this calculator does

This calculator sizes Canadian eavestroughs from three inputs: the projected roof area that drains into the eavestrough, the roof pitch, and the design rainfall intensity for your region (NBC 2020 Appendix C provides per-city values). It applies the rational method, accounts for wind-driven rain on steeper pitches via a pitch factor, then matches the resulting peak flow against published hydraulic capacities for K-style, half-round and box profiles to recommend a nominal size.

It also sizes the downspouts and tells you the minimum number needed to handle the total peak flow — the question that catches out about a third of homeowners doing first-time replacements.

How to use it

  1. Enter the projected roof area in sq ft. Plan-view footprint, not on-slope area. For a Canadian project home, length × width minus garage if separately drained.
  2. Pick the pitch. This sets the wind-correction factor that converts projected area to effective drainage area. Most Canadian residential roofs sit between 4/12 and 8/12.
  3. Set the design rainfall intensity. Default is 100 mm/hr (NBC 2020 Appendix C value for Toronto/Montreal). Use 60 mm/hr for Vancouver (longer-duration but lower-intensity Pacific rainfall), 110 mm/hr for Halifax, 90 mm/hr for Calgary, 75 mm/hr for Edmonton, 95 mm/hr for Quebec City.
  4. Choose the profile. K-style for standard residential, half-round for heritage replacement, box for commercial or built-in fascia detail.
  5. Set the number of downspouts. Two is typical for sub-2,000 sq ft single-storey, three for 2,000–3,500 sq ft, four for larger or for split-pitch front eavestroughs.
  6. Read the result. The big number is the recommended nominal size. The minimum-acceptable line is the smallest size that just handles the load.

The rational method (what the math does)

Peak flow into an eavestrough is calculated by:

Q (gpm) = effective drainage area (sq ft) × rainfall intensity (in/hr) × 0.0104

Effective drainage area is the projected (plan-view) area multiplied by a pitch factor that accounts for wind-driven rain:

PitchPitch factor
Flat (≤ 4°)1.00
4/12 (18°)1.05
5/12 to 6/12 (22.6° – 26.6°)1.10
8/12 (33.7°)1.20
12/12 (45°)1.30

The pitch factor is calibrated against Environment Canada climate data and CSA B406 corrections for windward elevations. Coastal BC and Atlantic Canada sites can add another 10% if the eavestrough is on the windward side of prevailing storms.

Per-downspout capacity tables

For K-style aluminum at standard ¼-in-per-10-ft slope:

Nominal sizeCapacity per downspout
4-inch K-style33 gpm (2.1 L/s)
5-inch K-style80 gpm (5.0 L/s)
6-inch K-style130 gpm (8.2 L/s)
7-inch K-style195 gpm (12.3 L/s)

For half-round at the same slope:

Nominal sizeCapacity per downspout
5-inch half-round50 gpm (3.2 L/s)
6-inch half-round90 gpm (5.7 L/s)
7-inch half-round140 gpm (8.8 L/s)

The recommended size is the smallest profile that handles the per-downspout flow with at least 15% reserve. The reserve covers leaf-clogging in fall, ice formation in shoulder seasons, and rainfall events above the 10-year design storm.

When to step up a size

Step up from the recommended size if any of these apply:

  • Ice-dam-prone region. Manitoba, northern Ontario, northern Quebec, Maritimes, interior BC: ice-dam meltwater fills the eavestrough faster than the design rainfall flow, especially during January thaws. Step up one nominal size.
  • Long single-pitch runs. Beyond 40 linear feet of single-pitch eavestrough, the high end sees stagnant water during heavy rain. Add a midpoint downspout or step up one size.
  • Steep mountain pitches. Roofs above 9/12 (common in Whistler, Banff, Mont-Tremblant chalet country) catch more wind-driven rain than the standard tables assume. Add 10–15% to calculated flow.
  • Mature tree canopy. Lawrence Park (Toronto), Outremont (Montreal), Rockcliffe Park (Ottawa), Westmount: dense deciduous canopy means partial leaf-clogging is the norm. Step up one nominal size and consider leaf-screen installation.
  • Pacific coastal. Vancouver, Victoria: while design rainfall intensity is lower, sustained-duration rainfall (12+ hours of moderate rain) can push the eavestrough to capacity for longer than the design storm assumes. Use 6-inch K-style as the default rather than 5-inch.

Downspout sizing — the CRCA rule of thumb

The Canadian Roofing Contractors Association rule is 1 sq inch of downspout cross-section per 100 sq ft of effective drainage area. Standard pairings:

DownspoutCross-sectionDrains up to
2 × 3 in6 sq in600 sq ft
3 × 4 in12 sq in1,200 sq ft
4 × 4 in16 sq in1,600 sq ft

Underspouting is the most common Canadian eavestrough failure. A correctly sized 6-inch K-style on undersized 2 × 3 downspouts will overflow at the high end during a heavy summer thunderstorm because the downspout chokes the flow before the trough fills. Always pair downspout size to eavestrough size: 5-inch K-style with 2 × 3, 6-inch K-style with 3 × 4, 7-inch K-style with 4 × 5.

Common Canadian edge cases

Ice-dam-prone overhang. Where the eaves overhang a heated soffit, snow melts at the warm portion and refreezes at the cold eave edge, building an ice dam. The eavestrough should be sized for liquid water during peak rainfall (this calculator), and the ice-dam problem solved separately by attic insulation to R-60 (per NBC 2020 Section 9.36) and ice-and-water-shield underlayment.

Heritage Quebec City / Old Halifax. Listed properties typically require half-round profile in copper or zinc, matching original 19th-century specification. The calculator’s hydraulic recommendations still apply for capacity-checking, but heritage variances may not allow upsizing without conservation officer approval.

Combined sewer connection in Toronto / Montreal. Where downspouts feed into a combined sewer, City of Toronto’s Mandatory Downspout Disconnection Program (in effect since 2007) requires disconnection from the sewer for residential properties. Plan for splash-block or rain-barrel connection at downspout discharge.

Mountain chalet, steep pitch, snow-shedding metal roof. Standing-seam metal sheds water (and snow) faster than asphalt, concentrating flow at the eave. Add 15% to the calculated flow and consider snow-rail installation above the eavestrough to prevent direct snow-pack damage.

Reference standards (Canada)

  • NBC (National Building Code of Canada) 2020 Section 9.26 — Roof drainage, requires positive drainage to discharge.
  • NBC 2020 Appendix C — Per-city design rainfall intensity tables (10-year 5-minute).
  • CSA B406 — Roof drains and rain leader sizing methodology.
  • OBC (Ontario Building Code) Subsection 9.26 — Provincial amendment, generally aligns with NBC.
  • CRCA Roofing Manual — Practice standard for eavestrough installation.
  • Alcoa / Gentek / Royal Group manufacturer manuals — Per-product hydraulic capacities.
  • Environment Canada Climate Normals — Per-station rainfall intensity-duration-frequency curves.

Sources: NBC 2020 Section 9.26 + Appendix C; CSA B406 roof drainage; OBC Subsection 9.26; CRCA Roofing Manual; Alcoa, Gentek and Royal Group manufacturer hydraulic data; Environment Canada Climate Normals 1981-2020.

Frequently asked questions

What size eavestrough do I need for a Canadian home?
A typical Canadian two-storey home with a 1,700 sq ft projected roof area, a 6/12 pitch and the NBC 2020 Toronto/Montreal design rainfall of 100 mm/hr (10-yr 5-min from Appendix C) produces a peak flow of about 5.4 L/s (86 gpm). Split between two downspouts that's 2.7 L/s (43 gpm) per downspout — handled comfortably by 5-inch K-style aluminum (capacity ~80 gpm). Most Ontario, Quebec and BC residential builds use 5-inch K-style with 2 × 3 inch downspouts as the default. Step up to 6-inch K-style for projected areas over 2,400 sq ft, for ice-prone regions where ice-dam meltwater can fill 6-inch faster than 5-inch can drain (Manitoba, northern Ontario, interior BC), or for steep 9/12+ roof pitches that catch more wind-driven rain.
Is 5-inch or 6-inch eavestrough better for Canadian homes?
5-inch K-style is the residential default across Canada — cheaper, ships from every Home Depot and RONA, fits the standard 1¼-inch fascia depth without rework. It handles up to about 2,000 sq ft of projected roof area with two downspouts at design rainfall. 6-inch K-style is recommended for projected areas over 2,400 sq ft, for steep mountain pitches above 9/12, for ice-dam-prone regions (Northwest Ontario, Manitoba, northern Quebec, interior BC) where the eavestrough must clear ice-melt water faster than 5-inch can manage, and for properties with mature deciduous trees nearby (Toronto neighbourhoods like Lawrence Park, Rosedale; Montreal's Outremont) where leaf accumulation regularly halves effective capacity. The cost premium for 6-inch is about CAD $300–$500 on a typical 200-foot perimeter.
How does NBC 2020 size eavestroughs?
NBC 2020 Section 9.26 references CSA B406 and the local jurisdictional design rainfall (Appendix C tables for major cities) for sizing. The rational method peak flow Q = effective roof area × design rainfall intensity is the foundation. Per-eavestrough capacity tables are published by major manufacturers (Alcoa, Gentek, Royal Group) following SMACNA-equivalent hydraulic methodology. The calculator uses these capacities and matches against the per-downspout flow with a 15% reserve. Appendix C tables give 10-year 5-minute rainfall for ~120 Canadian cities — Toronto 100 mm/hr, Vancouver 60 mm/hr (the rain falls longer but at lower intensity), Halifax 110 mm/hr, Calgary 90 mm/hr, Edmonton 75 mm/hr, Quebec City 95 mm/hr.
What's the difference between K-style, half-round and box eavestroughs?
K-style is the Canadian residential default — M-shaped profile that looks like crown moulding, largest cross-section per face width, easy to seamless-roll-form on-site. Half-round is the original profile, common on heritage homes (Old Quebec City, downtown Halifax, Westmount in Montreal) and high-end residential (especially copper for Ottawa Glebe and Toronto Forest Hill); lower capacity per face width, you usually step up one size compared to K-style. Box (rectangular) eavestroughs are common on commercial flat-roof buildings and on modern minimalist residential where the trough is built into the fascia line — highest capacity but specialty install. The calculator picks the right hydraulic table for the profile you select.
How many downspouts does my eavestrough run need?
Divide total peak flow by per-downspout capacity. For a 6-inch K-style at standard slope, capacity per downspout is ~130 gpm (8.2 L/s). The companion rule of thumb for residential is one downspout per 35 linear feet of single-pitch eavestrough or per 40 feet of split-pitch. A typical 60-foot front eavestrough on a 1,700 sq ft Canadian two-storey wants two downspouts (one at each corner). Add a third midpoint downspout if the projected area exceeds 2,500 sq ft. In ice-dam-prone regions, dedicating a downspout to the lowest corner of each slope (typically the leeward eave) helps clear meltwater faster than equal-spaced downspouts.
Do I size from projected area or actual sloped area?
Projected area — the plan-view footprint, not the sloped surface area. The pitch factor in the calculation then multiplies projected area to account for the additional wind-driven rain on the inclined surface. So a 30 ft × 50 ft house with a 6/12 gable has a projected area of 1,500 sq ft and an effective drainage area of 1,500 × 1.10 = 1,650 sq ft. Don't double-count by using the slope-length area (~1,635 sq ft) and then also applying the pitch factor — that overestimates by about 8%.
What downspout size pairs with 5-inch and 6-inch K-style?
Standard Canadian pairings: 5-inch K-style with 2 × 3 inch downspout (6 sq in cross-section, drains ~600 sq ft); 6-inch K-style with 3 × 4 inch downspout (12 sq in, drains ~1,200 sq ft); 7-inch K-style with 4 × 5 inch downspout (20 sq in, drains ~2,000 sq ft). The CRCA rule of thumb is 1 sq inch of downspout cross-section per 100 sq ft of effective drainage area, conservative compared to actual hydraulic capacity but accounts for partial leaf clogging and ice formation in shoulder seasons. Underspouting is the #1 eavestrough failure in Canada — a correctly sized 6-inch K-style on undersized 2 × 3 downspouts will overflow at the high end during a heavy June thunderstorm because the downspout chokes the flow.
Are eavestrough sizes a code requirement under NBC 2020?
NBC 2020 Section 9.26 requires positive drainage from the roof to a discharge point that does not adversely affect the building, neighbouring property or public roads. CSA B406 specifies sizing methodology but is not strictly mandated for residential — most provinces accept manufacturer-certified products and the SMACNA-equivalent rational method as good practice. Provincial amendments vary: Ontario Building Code (OBC) Subsection 9.26 generally aligns with NBC; Quebec Construction Code (CCQ) has additional requirements for snow-melt drainage; BC Building Code has more stringent requirements for coastal high-precipitation regions. The Alberta New Home Warranty Program references CRCA installation standards. Check your municipal building department for any local amendments before finalizing the design.

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