Gutters, downspouts and rainwater systems

Scope of supply

A pre-engineered building with a large roof sheds a lot of water in a short storm, so gutters and downspouts must be sized, not just added. World Steel supplies the complete rainwater system as part of the accessories package: see gutters and downspouts for the principles.

Eave gutters in box or ogee shape, with end caps, outlets and expansion joints.
Valley gutters for multi-gable roofs and internal gutters where specified.
Downspouts (downpipes) with elbows, offsets, brackets and shoes.
Gutter straps or brackets fixed to the eave strut, and overflow details.
Sealant, rivets and fasteners.

Eave gutter and downspout detail on a PEB — Typical eave gutter, strap and downspout outlet.

Sizing and options

Gutters and downspouts are sized from the design rainfall intensity of the site, the roof catchment area and slope, and the outlet spacing. For each market the governing method differs, and overflow provisions are required so that a blocked outlet does not flood the building. Options include:

Larger box gutters for long roof runs, with sumps and overflow weirs.
Rainwater harvesting outlets connected to tanks by others.
Leaf guards and gutter screens for sites near trees.
Square or round downspouts, and concealed downspouts inside wall cladding where requested.

Typical rainwater components
Item	Typical range
Eave gutter girth	About 300–600 mm
Valley gutter girth	About 600–1,000 mm
Downspout size	About 100 × 75 mm to 150 × 100 mm rectangular, or 100–150 mm round
Thickness	0.5–1.0 mm coated steel; heavier for valley gutters

Materials and finishes

Gutters are usually formed from aluminium-zinc coated or pre-painted steel matching the trims; valley gutters that hold water for longer may be specified in heavier gauge, stainless steel or with a membrane lining. Downspouts can be in steel or in PVC supplied locally. Details at the eave are coordinated with trims and flashings and with the eave strut.

Compliance by market

Compliance by market
Market	Design code	Fabrication / welding	Materials	Certification notes
United States	IPC / IBC Chapter 15 roof drainage incl. overflow; SMACNA manual for details	AWS D1.3 where welded	ASTM A792 / A653 coated sheet	WSC holds IAS AC472 accreditation for metal building systems; welding qualified to AWS D1.1 (structural) and AWS D1.3 (sheet steel). AISC Certification is not held – confirm acceptance if specified.
Canada	National Plumbing Code of Canada and provincial codes	Not applicable (formed sheet)	Coated steel per ASTM accepted grades	Designed to CSA S16 / CSA S136 by WSC or the engineer of record. CSA W47.1 (CWB) company certification is a market requirement, arranged per project.
Australia / New Zealand	AS/NZS 3500.3 stormwater drainage; NZBC E1/AS1 for NZ	AS/NZS 5131 for supporting steel	AS 1397, AS/NZS 2179.1 rainwater goods	WSC is certified to AS/NZS 5131 Construction Category CC3; welding to AS/NZS 1554.1. For New Zealand, SFC certification is not held – confirm acceptance per project.
Europe	EN 12056-3 roof drainage design, EN 612 eaves gutters	EN 1090-2 / EN 1090-4 for supports	EN 10346 coated steel	WSC is certified to EN 1090-2 Execution Class EXC3 and ISO 3834-2. CE marking via EN 1090-1 factory production control to be confirmed per project.

Design references and accessories

Rainwater design references by market
Market	Typical reference
United States	International Plumbing Code (roof drainage and secondary drainage), with ASCE 7-22 Chapter 8 rain loads
Canada	National Plumbing Code of Canada with NBC 2020 rain loads
Australia / NZ	AS/NZS 3500.3 stormwater drainage; NZ Building Code E1
Europe	EN 12056-3 roof drainage; EN 612 for metal gutters and downpipes

Typical rainwater accessories
Accessory	Purpose
Rainwater head	Collects flow from a gutter outlet into the downspout and provides a visible overflow
Sump	Deepens internal or valley gutters at outlets to increase capacity
Overflow weir or spout	Discharges water outside the building if outlets block
Expansion joint	Lets long gutter runs move without tearing laps
Leaf guard	Reduces blockage near trees
Downspout shoe or boot	Directs water away from the wall or into underground pipes

See gutters and downspouts for sizing principles.

Documentation, packing and quotation

Deliverables include a roof drainage layout with outlet positions, sizing calculations to the stated code, details of joints, overflow and supports, and a component list. Gutters are nested in lengths with protective film and packed with the trims.

A quotation can be prepared quickly when the enquiry states the project location and the code to be used. Please send:

Site location or design rainfall intensity and return period.
Roof plan with slopes and any valleys.
Preferred gutter style, downspout material and colour.
Connection point to site drainage (by others).

World Steel returns a priced scope with a clear inclusions and exclusions list, the design basis assumed and a typical lead time. Enquiries go through the export supply team in Ho Chi Minh City.

Standards by region

Region	Code	Scope
United States	IPC Chapter 11	Storm drainage
Australia / NZ	AS/NZS 3500.3	Plumbing and drainage – stormwater
Europe	EN 12056-3	Gravity drainage – roof drainage
Europe	EN 612	Eaves gutters and rainwater pipes of metal sheet
Canada	NPC	National Plumbing Code of Canada

Frequently asked questions