Drainage sizing: slope, length, pipe size and hydraulic loads (Plumbing

Last updated October 22, 2026

Values this chapter turns on

minimum slope for drain pipes of NPS 3 or less: 1 in 50; minimum slope for drain pipes of NPS 3 or less
max developed length of a fixture outlet pipe: 1,200 mm; max developed length of a fixture outlet pipe
hydraulic load of a bathroom group, flush tank: 6 FU; hydraulic load of a bathroom group, flush tank
minimum building drain past the main cleanout: NPS 4; minimum building drain past the main cleanout

This is the calculation section of the exam. You add up fixture units, then read a table to size a branch, stack or building drain. For storm drainage you work out a rainfall load in litres and size a leader or storm drain. The method is always the same: find the load, pick the right table for the type of pipe, and check the minimum size rules that override the table.

Minimum slope and length

Drainage pipes of NPS 3 or less need a slope of at least 1 in 50 in the direction of flow (OBC Div. B 7.4.8.1.). This does not apply to a force main. Where 1 in 50 is not possible, a lesser slope is allowed if it still gives a gravity flow of at least 0.6 m per second. Larger pipes are sized at the slope you choose using Table 7.4.10.6.-C.

A fixture outlet pipe (from the fixture's waste opening to its trap) may not exceed 1,200 mm developed length (OBC Div. B 7.4.8.2.), except for grouped floor drains, showers and washers on one trap under 7.4.5.1.(3).

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Minimum size rules that beat the tables

Rule	Article
A pipe of minimum size may not drain into a smaller pipe	7.4.9.1.
Pipe serving a water closet: at least NPS 3	7.4.9.2.(1)
Branch or building drain downstream of the third water closet connection: at least NPS 4	7.4.9.2.(2)
Stack serving more than 6 water closets: at least NPS 4	7.4.9.2.(3)
Macerating toilet discharge: at least NPS 3/4	7.4.9.2.(4)
Vertical leg from a siphonic water closet: at most 1,000 mm	7.4.9.2.(5)
Building drain and sewer downstream of the main cleanout: at least NPS 4	7.4.9.4.(1)
Storm building drain and sewer: at least NPS 4	7.4.9.4.(2)

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Fixture units and outlet sizes (Table 7.4.9.3.)

Table 7.4.9.3. gives each fixture a minimum fixture outlet pipe size and a hydraulic load in fixture units. A fixture not in the table takes its load from the trap size in Table 7.4.10.2. (NPS 1¼ = 1, 1½ = 2, 2 = 3, 3 = 5, 4 = 6). The house values you will use most:

Fixture	Fixture units
Bathroom group with flush tank	6
Water closet with flush tank	4
Bathtub, with or without shower	1½
Domestic lavatory with NPS 1¼ trap	1
Domestic kitchen sink	1½
Domestic clothes washer (NPS 2 trap)	1½
Domestic dishwasher	1, or no load when it drains through the sink or garbage grinder
Floor drain with NPS 2 trap	2
Shower drain, 1 head	1½

Related outlet rules (OBC Div. B 7.4.9.3.): a clothes washer that does not drain to a laundry tray needs a trap inlet of at least NPS 2 and a standpipe at least 600 mm long above the trap weir. In a dwelling unit, a shower receptor with multiple heads needs at least an NPS 2 outlet. The common outlet of a 3-compartment sink is one NPS larger than the largest compartment outlet.

The total load on a pipe counts every fixture upstream, every roughed-in future fixture, and any roof or paved area draining into it (OBC Div. B 7.4.10.1.).

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Which table for which pipe

Pipe	Table
Stack	7.4.10.6.-A: total load and load from any one storey
Branch	7.4.10.6.-B (7.4.10.7.)
Sanitary building drain or sewer	7.4.10.6.-C, by slope (7.4.10.8.)
Storm building drain or sewer	7.4.10.9., in litres, by slope
Leader	7.4.10.11., in litres

Key rows for houses:

NPS	Stack total / per storey	Branch	Building drain at 1 in 50
1½	8 / 2	3	not listed
2	24 / 6	6	not listed
3	102 / 18	27	27
4	540 / 100	180	240

A stack with a horizontal offset of 1.5 m or more is sized for the offset using Table 7.4.10.6.-B or -C, whichever is less restrictive (7.4.10.6.(2)). Design limits: vertical sanitary pipe carries no more than 29% of full capacity, horizontal sanitary pipe no more than 65% (7.4.10.6.(3), 7.4.10.8.(2)).

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Worked example: sanitary building drain

A house has two full bathrooms (bathroom groups with flush tanks), a powder room (flush tank water closet and lavatory with NPS 1¼ trap), a kitchen sink with a dishwasher draining through it, a clothes washer, a laundry tray and an NPS 2 basement floor drain.

Item	FU
2 bathroom groups	12
Powder room water closet + lavatory	4 + 1 = 5
Kitchen sink	1½
Dishwasher through the sink	0
Clothes washer	1½
Laundry tray	1½
Floor drain	2
Total	23½

Table 7.4.10.6.-C would allow NPS 3 at 1 in 50 (27 FU). But the house has three water closets, so the drain downstream of the third water closet connection must be NPS 4 under 7.4.9.2.(2), and the building drain downstream of the main cleanout must be NPS 4 anyway under 7.4.9.4.(1). The answer is NPS 4, set by the minimum size rules, not the table.

Hydraulic loads from roofs and other sources

Roof or paved surface load in litres = the SB-1 maximum 15 minute rainfall (in mm) times the drained area in m², where the area is the horizontal projection plus half of the largest adjoining vertical surface (OBC Div. B 7.4.10.4.). One millimetre over one square metre is one litre.

Example: a 120 m² roof (horizontal projection) beside a 30 m² wall that sheds water onto it, with a 15 minute rainfall of 28 mm. Area = 120 + 15 = 135 m². Load = 28 x 135 = 3,780 L. In Table 7.4.10.11., a circular NPS 2½ leader takes 3,070 L, too little, so use NPS 3 (5,000 L).

Other loads:

Continuous flow such as a pump: 31.7 fixture units per L/s (7.4.10.3.(1)).
Converting fixture units to litres: 260 FU or fewer counts as 2,360 L; above 260, 9.1 L per fixture unit (7.4.10.5.).
A leader offset under the roof needs no size change if it is not more than 6 m long with a slope of at least 1 in 50 (7.4.9.5.).

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Common traps

Sizing from the table and forgetting the water closet and building drain minimums.
Counting a dishwasher that drains through the kitchen sink.
Using the stack table for a horizontal branch. A NPS 3 stack takes 102 FU, a NPS 3 branch only 27.
Forgetting the half-wall area in a roof load.

Chapter review

Chapter at a glance

One line per section: the idea to carry into the exam. Then the answers that most often catch candidates out.

Chapter at a glance: the key idea in each section
Section	Key idea
Minimum slope and length	Drainage pipes of NPS 3 or less need a slope of at least 1 in 50 in the direction of flow.
Fixture units and outlet sizes (Table 7.4.9.3.)	Related outlet rules: a clothes washer that does not drain to a laundry tray needs a trap inlet of at least NPS 2 and a standpipe at least 600 mm long above the trap weir.
Which table for which pipe	A stack with a horizontal offset of 1.5 m or more is sized for the offset using Table 7.4.10.6.-B or -C, whichever is less restrictive.
Hydraulic loads from roofs and other sources	Roof or paved surface load in litres = the SB-1 maximum 15 minute rainfall (in mm) times the drained area in m², where the area is the horizontal projection plus half of the…

Minimum slope and length
Drainage pipes of NPS 3 or less need a slope of at least 1 in 50 in the direction of flow.
Fixture units and outlet sizes (Table 7.4.9.3.)
Related outlet rules: a clothes washer that does not drain to a laundry tray needs a trap inlet of at least NPS 2 and a standpipe at least 600 mm long above the trap weir.
Which table for which pipe
A stack with a horizontal offset of 1.5 m or more is sized for the offset using Table 7.4.10.6.-B or -C, whichever is less restrictive.
Hydraulic loads from roofs and other sources
Roof or paved surface load in litres = the SB-1 maximum 15 minute rainfall (in mm) times the drained area in m², where the area is the horizontal projection plus half of the…

Usual exam traps

Sizing from the table and forgetting the water closet and building drain minimums.
Counting a dishwasher that drains through the kitchen sink.
Using the stack table for a horizontal branch. A NPS 3 stack takes 102 FU, a NPS 3 branch only 27.
Forgetting the half-wall area in a roof load.

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Questions on Drainage sizing: slope, length, pipe size and hydraulic loads, in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.

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Code cited in this chapter

Based on the official 2024 Building Code Compendium on ontario.ca.

OBC Div. B 7.4.8.1. Minimum Slope
(1)Except as provided in Sentences (2) and (3) and Articles 7.4.10.8. and 7.4.10.9., drainage pipes that are NPS 3 or less shall have a downward slope in the direction of flow of at least 1 in 50. (See Note A-7.4.8.1.(1))
(2)Sentence (1) does not apply to a force main.
(3)Where it is not possible to comply with Sentence (1), a lesser slope may be used if it will produce a gravity flow of not less than 0.6 m per second.
Open OBC Div. B 7.4.8.1. in the code viewer
OBC Div. B 7.4.8.2. Length of Fixture Outlet Pipes
(1)Except for fixture outlet pipes installed in conformance with Sentence 7.4.5.1.(3), the developed length of fixture outlet pipes shall not exceed 1,200 mm.
Open OBC Div. B 7.4.8.2. in the code viewer
OBC Div. B 7.4.9.1. No Reduction in Size
(1)Except as permitted in Sentence (3), no drainage pipe that is of minimum size required by this Part for the purpose for which it is installed shall be so connected as to drain to other drainage pipe of lesser size.
(2)Where a building drain connects to a stack through a wall or floor, the drain shall retain its full size through the wall or floor.
(3)A sanitary drainage pipe may be connected to a pre-engineered waste water heat recovery system that incorporates piping of a lesser size than required by Sentence (1) provided that it does not convey sewage
(a)from a sanitary unit, or
(b)that contains solids.
Open OBC Div. B 7.4.9.1. in the code viewer
OBC Div. B 7.4.9.2. Serving Water Closets
(1)Drainage pipes that serve a water closet shall be not less than NPS 3.
(2)Branch and building drains downstream of the third water-closet fixture drain connection shall be not less than NPS 4.
(3)Stacks that serve more than 6 water closets shall be not less than NPS 4.
(4)Discharge pipes serving a macerating toilet system shall be not less than NPS 3/4 .
(5)No vertical leg of the drainage pipe from a water closet or other fixture that has an integral siphonic flushing action shall exceed 1,000 mm.
Open OBC Div. B 7.4.9.2. in the code viewer
OBC Div. B 7.4.9.4. Minimum Size of Building Drain and Sewer
(1)Building drains and building sewers connected to the public sewer system downstream of the main cleanout (see Sentence 7.4.7.1.(6)) shall be not less than NPS 4.
(2)Storm building drains and storm building sewers shall be not less than NPS 4.
Open OBC Div. B 7.4.9.4. in the code viewer
OBC Div. B Table 7.4.9.3. Minimum Permitted Size of Fixture Outlet Pipe and Hydraulic Loads for Fixtures[1]
Table 7.4.9.3. — Minimum Permitted Size of Fixture Outlet Pipe and Hydraulic Loads for Fixtures[1] Forming Part of Sentences 7.4.9.3.(1) and 7.4.10.2.(1) Fixture | Minimum Nominal Pipe Size of Fixture Outlet Pipe, NPS | Hydraulic Load, fixture units Autopsy table | 1½ | 2 Bathroom group (a) with flush tank | N/A | 6 Bathroom group (b) with direct flush valve | N/A | 8 Bathtub (with or without shower) | 1½ | 1½ Bath: foot, sitz or slab | 1½ | 1½ Beer cabinet | 1½ | 1½ Bidet | 1¼ | 1 Chinese range | 1½ | 3 Clothes washer (a) domestic[1] | N/A | 1½ with NPS 2 trap Clothes washer (b) commercial | N/A | 2 with NPS 2 trap Cup sinks | 1¼ | ½ Dental unit or cuspidor | 1¼ | 1 Dishwasher (a) domestic type | 1½ | 1 (no load when connected to garbage grinder or domestic sink) Dishwasher (b) commercial type | 2 | 3 Drinking fountain | 1¼ | ½ Fish tank or tray | 1½ | 1½ Floor drain[2] | 2 | 2 with NPS 2 trap 3 with NPS 3 trap Garbage grinder, commercial type | 2 | 3 Icebox | 1¼ | 1 Laundry tray (a) single or double units or 2 single units with common trap | 1½ | 1½ Laundry tray (b) compartments | 1½ | 2 Lavatory (a) barber or beauty parlor | 1½ | 1½ Lavatory (b) dental | 1¼ | 1 Lavatory (c) domestic type single, or 2 single with common trap | 1¼ | 1 with NPS 1¼ trap 1½ with NPS 1½ trap Lavatory (d) multiple or industrial type | 1½ | According to Table 7.4.10.2. Macerating toilet system | ¾ | 4 Potato peeler | 2 | 3 Shower drain Total volume of discharge from all shower heads and body sprays: (a) from 1 head | 1½ | 1½ Shower drain Total volume of discharge from all shower heads and body sprays: (b) from 2 or 3 heads | 2 | 3 Shower drain Total volume of discharge from all shower heads and body sprays: (c) from 4 to 6 heads | 3 | 6 Sink (a) domestic and other small types with or without garbage grinders, single, double or 2 single with a common trap | 1½ | 1½ Sink (b) other sinks | 1½ | 1½ with NPS 1½ trap 2 with NPS 2 trap 3 with NPS 3 trap Urinal (a) pedestal, siphon jet or blowout type | 2 | 4 Urinal (b) stall, washout type | 2 | 2 Urinal (c) wall (i) washout type | 1½ | 1½ Urinal (c) wall (ii) other types | 2 | 3 Water closet (a) with flush tank | 3 | 4 Water closet (b) with direct flush valve | 3 | 6 Notes to Table 7.4.9.3.: (1) See Note A-Table 7.4.9.3. (2) No hydraulic load for emergency floor drains.
Open OBC Div. B Table 7.4.9.3. in the code viewer
OBC Div. B Table 7.4.10.2. Permitted Hydraulic Load from a Fixture Based on Size of Trap
Table 7.4.10.2. — Permitted Hydraulic Load from a Fixture Based on Size of Trap Forming Part of Sentence 7.4.10.2.(2) Nominal Pipe Size, NPS | Hydraulic Load fixture units 1¼ | 1 1½ | 2 2 | 3 2½ | 4 3 | 5 4 | 6
Open OBC Div. B Table 7.4.10.2. in the code viewer
OBC Div. B 7.4.9.3. Size of Fixture Outlet Pipes
(1)Except as provided in Sentence (2), the nominal pipe size of fixture outlet pipes shall conform to Table 7.4.9.3.
(2)The part of the fixture outlet pipe that is common to 3 compartments of a sink shall be one NPS larger than the largest fixture outlet pipe of the compartments that it serves. (See Note A-7.4.9.3.(2))
(3)Where clothes washers do not drain to a laundry tray, the trap inlet shall be not less than NPS 2 and be fitted with a vertical standpipe that is not less than 600 mm long measured from the trap weir and terminates above the flood level rim of the clothes washer. (See Note A-7.4.9.3.)
(4)In an individual dwelling unit, where multiple shower heads are served by one shower receptacle, the fixture outlet pipe shall be not less than NPS 2.
Open OBC Div. B 7.4.9.3. in the code viewer

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OBC Div. B 7.4.10.1. Total Load on a Pipe
(1)The hydraulic load on a pipe is the total load from
(a)every fixture that is connected to the system upstream of the pipe,
(b)every fixture for which provision is made for future connection upstream of the pipe, and
(c)all roofs and paved surfaces that drain into the system upstream of the pipe.
Open OBC Div. B 7.4.10.1. in the code viewer
OBC Div. B Table 7.4.10.6-A. Maximum Permitted Hydraulic Load Drained to a Stack
Table 7.4.10.6.-A Maximum Permitted Hydraulic Load Drained to a Stack Forming Part of Sentences 7.4.10.6.(1) and 7.5.7.3.(2) and Table 7.5.8.4. Nominal Pipe Size of Stack, NPS | Maximum Hydraulic Load, fixture units | Maximum Fixture Units Drained from any 1 Storey 1¼ | 2 | 2 1½ | 8 | 2 2 | 24 | 6 3 | 102 | 18 4 | 540 | 100 5 | 1,400 | 250 6 | 2,900 | 500 8 | 7,600 | 830 10 | 15,000 | 2,700 12 | 26,000 | 4,680 15 | 50,000 | 9,000
Open OBC Div. B Table 7.4.10.6-A. in the code viewer
OBC Div. B Table 7.4.10.6-B. Maximum Permitted Hydraulic Load Drained to a Branch
Table 7.4.10.6.-B — Maximum Permitted Hydraulic Load Drained to a Branch Forming Part of Sentences 7.4.10.6.(2), 7.4.10.7.(1) and 7.5.7.3.(2) Nominal Pipe Size of Branch, NPS | Maximum Hydraulic Load, fixture units 1¼ | 2 1½ | 3 2 | 6 3 | 27 4 | 180 5 | 390 6 | 700 8 | 1,600 10 | 2,500 12 | 3,900
Open OBC Div. B Table 7.4.10.6-B. in the code viewer
OBC Div. B 7.4.10.7. Hydraulic Loads on Branches
(1)The hydraulic load that is drained to a branch shall conform to Table 7.4.10.6.-B.
Open OBC Div. B 7.4.10.7. in the code viewer
OBC Div. B Table 7.4.10.6-C. Maximum Hydraulic Load, fixture units[2]
Table 7.4.10.6.-C[1] — Maximum Hydraulic Load, fixture units[2] Forming Part of Sentence 7.4.10.6.(2) and Article 7.4.10.8. Nominal Pipe Size of Drain or Sewer, NPS | Slope: 1 in 400 | Slope: 1 in 200 | Slope: 1 in 133 | Slope: 1 in 100 | Slope: 1 in 50 | Slope: 1 in 25 3 | N/A | N/A | N/A | N/A | 27 | 36 4 | N/A | N/A | N/A | 180 | 240 | 300 5 | N/A | N/A | 380 | 390 | 480 | 670 6 | N/A | N/A | 600 | 700 | 840 | 1,300 8 | N/A | 1,400 | 1,500 | 1,600 | 2,250 | 3,370 10 | N/A | 2,500 | 2,700 | 3,000 | 4,500 | 6,500 12 | 2,240 | 3,900 | 4,500 | 5,400 | 8,300 | 13,000 15 | 4,800 | 7,000 | 9,300 | 10,400 | 16,300 | 22,500 Notes to Table 7.4.10.6.-C: (1) See Note A-Table 7.4.10.6.-C. (2) Slope is the ratio of rise to run, in whatever measurement units are chosen.
Open OBC Div. B Table 7.4.10.6-C. in the code viewer
OBC Div. B 7.4.10.8. Hydraulic Loads on Sanitary Building Drains or Sewers
(1)Except as permitted by Article 7.4.10.7., the hydraulic load that is drained to a sanitary building drain or a sanitary building sewer shall conform to Table 7.4.10.6.-C.
(2)Horizontal sanitary drainage pipe shall be designed to carry no more than 65% of its full capacity.
Open OBC Div. B 7.4.10.8. in the code viewer
OBC Div. B Table 7.4.10.9. Maximum Permitted Hydraulic Load, L[1]
Table 7.4.10.9. — Maximum Permitted Hydraulic Load, L[1] Forming Part of Sentence 7.4.9.5.(2) and Article 7.4.10.9. Nominal Pipe Size of Drain or Sewer, NPS | Slope: 1 in 400 | Slope: 1 in 200 | Slope: 1 in 133 | Slope: 1 in 100 | Slope: 1 in 68 | Slope: 1 in 50 | Slope: 1 in 25 3 | N/A | N/A | N/A | N/A | N/A | 2,770 | 3,910 4 | N/A | N/A | N/A | 4,220 | 5,160 | 5,970 | 8,430 5 | N/A | N/A | 6,760 | 7,650 | 9,350 | 10,800 | 15,300 6 | N/A | N/A | 10,700 | 12,400 | 15,200 | 17,600 | 24,900 8 | N/A | 18,900 | 23,200 | 26,700 | 32,800 | 37,800 | 53,600 10 | N/A | 34,300 | 41,900 | 48,500 | 59,400 | 68,600 | 97,000 12 | 37,400 | 55,900 | 68,300 | 78,700 | 96,500 | 112,000 | 158,000 15 | 71,400 | 101,000 | 124,000 | 143,000 | 175,000 | 202,000 | 287,000 Notes to Table 7.4.10.9.: (1) Slope is the ratio of rise to run, in whatever measurement units are chosen.
Open OBC Div. B Table 7.4.10.9. in the code viewer
OBC Div. B Table 7.4.10.11. Maximum Permitted Hydraulic Load Drained to a Leader
Table 7.4.10.11. — Maximum Permitted Hydraulic Load Drained to a Leader Forming Part of Sentence 7.4.10.11.(1) Circular Leader: Nominal Pipe Size of Leader , NPS | Circular Leader: Maximum Hydraulic Load, L | Non-Circular Leader: Area of Leader, cm² | Non-Circular Leader: Maximum Hydraulic Load, L 2 | 1,700 | 20.3 | 1,520 2½ | 3,070 | 31.6 | 2,770 3 | 5,000 | 45.6 | 4,500 4 | 10,800 | 81.1 | 9,700 5 | 19,500 | 126.6 | 17,600 6 | 31,800 | 182.4 | 28,700 8 | 68,300 | 324.3 | 61,500
Open OBC Div. B Table 7.4.10.11. in the code viewer
OBC Div. B 7.4.10.6. Hydraulic Loads to Soil or Waste Pipes
(1)Except as provided in Sentence (2), the hydraulic load that is drained to every stack shall conform to Table 7.4.10.6.-A.
(2)Where the nominally horizontal offset in a stack is 1.5 m or more, the hydraulic load that is served by it shall conform to Table 7.4.10.6.-B or Table 7.4.10.6.-C, whichever is the less restrictive.
(3)Vertical sanitary drainage pipe shall be designed to carry no more than 29% of its full capacity.
Open OBC Div. B 7.4.10.6. in the code viewer
OBC Div. B 7.4.10.4. Hydraulic Loads from Roofs or Paved Surfaces
(1)Except as provided in Sentence (2), the hydraulic load in litres from a roof or paved surface is the maximum 15 min rainfall determined in conformance with MMAH Supplementary Standard SB-1, "Climatic and Seismic Data," multiplied by the sum of
(a)the area in square metres of the horizontal projection of the surface drained, and
(b)one-half the area in square metres of the largest adjoining vertical surface.
(2)Flow control roof drains may be installed, provided
(a)the maximum drain down time does not exceed 24 h,
(b)the roof structure is designed to carry the load of the stored water,
(c)one or more scuppers are installed not more than 30 m apart along the perimeter of the building so that
(i)up to 200% of the 15-minute rainfall intensity can be handled, and
(ii)the maximum depth of controlled water is limited to 150 mm,
(d)they are located not more than 15 m from the edge of the roof and not more than 30 m from adjacent drains, and
(e)there is at least one drain for each 900 m².
(3)Reserved.
(4)Where the height of the parapet is more than 150 mm or exceeds the height of the adjacent wall flashing,
(a)emergency roof overflows or scuppers described in Clause (2)(c) shall be provided, and
(b)there shall be a minimum of 2 roof drains.
Open OBC Div. B 7.4.10.4. in the code viewer
OBC Div. B 7.4.10.3. Hydraulic Loads from Fixtures with a Continuous Flow
(1)Except as provided in Sentence (2), the hydraulic load from a fixture that produces a continuous flow, such as a pump or an air-conditioning fixture, is 31.7 fixture units for each litre per second of flow.
(2)Where a fixture or equipment that produces a continuous or semi-continuous flow drains to a combined sewer or to a storm sewer, the hydraulic load from the fixture is 900 L for each litre per second of flow.
Open OBC Div. B 7.4.10.3. in the code viewer
OBC Div. B 7.4.10.5. Conversion of Fixture Units to Litres and Gal/min
(1)Except as provided in Sentence 7.4.10.3.(2), where the hydraulic load is to be expressed in litres, fixture units shall be converted as follows:
(a)when the number of fixture units is 260 or fewer, the load is 2,360 L, and
(b)when the number of fixture units exceeds 260, the load is 9.1 L for each fixture unit.
(2)Where the hydraulic load is to be expressed in gal/min, fixture units shall be converted in accordance with Table 7.4.10.5.
Open OBC Div. B 7.4.10.5. in the code viewer
OBC Div. B 7.4.9.5. Offset in Leaders
(1)No change in the nominal pipe size of a leader with a nominally horizontal offset is required if the offset
(a)is located immediately under the roof,
(b)is not more than 6 m long, and
(c)has a slope of not less than 1 in 50.
(2)If the horizontal offset is more than 6 m long, the leader shall conform to Table 7.4.10.9.
Open OBC Div. B 7.4.9.5. in the code viewer