Roof Drain Calculator Project Checklist

Roof Drainage Checklist

Before you size, buy, or install anything, run through this checklist. It covers every input the calculation needs, every design decision around it, and every verification step after — the difference between a system that passes inspection and drains for decades, and one that ponds, leaks, or gets red-tagged. Work it top to bottom; don’t skip the verification section at the end. Print it or keep it open on your phone during the roof walk — a checklist you don’t have with you is a checklist you won’t use.

The sizing checkpoint in the middle uses the Roof Drain Calculator — the checklist tells you what to feed it and what to check after. The full design background is in the complete guide.

1. Measurement Checklist

• ☐ Roof footprint measured (length × width, horizontal plan area) — in ft² or m²
• ☐ Upper roofs, penthouses, and canopies discharging onto this roof measured and added
• ☐ Measurements verified on the actual roof, not just the drawings
• ☐ Roof divided into drainage zones of roughly equal area — sketch on file
• ☐ True low points identified (post-rain walk preferred over drawings)
• ☐ Parapet heights and existing penetrations noted

2. Design Data Checklist

• ☐ Code edition enforced by the local AHJ identified (IPC/UPC/national code + year)
• ☐ Design storm intensity obtained from the code’s published data — in/hr and mm/hr recorded
• ☐ Runoff coefficient selected (0.95–1.0 for roofs; 1.0 for conservative design)
• ☐ Drain capacity data sourced (manufacturer ratings at a stated design head)
• ☐ Local amendments checked: discharge rules, material restrictions, detention requirements
• ☐ Permit requirements confirmed with the building department

3. Sizing Checklist

• ☐ Peak flow computed: Q = C × I × A (cross-checked in both unit systems)
• ☐ Drain count = Q ÷ capacity per drain, rounded up
• ☐ Minimum two drains (or drain locations) regardless of the arithmetic
• ☐ Each zone’s flow verified within its drain’s capacity
• ☐ Flow paths from zone edges to drains under ~50 ft (15 m)
• ☐ Leaders sized for the combined flow of all drains they serve
• ☐ No leader smaller than the largest drain outlet feeding it

The sizing math is derived step by step in the formula guide, and the measurement-to-takeoff workflow in the estimating guide.

4. Overflow (Secondary) Checklist

• ☐ Independent secondary system designed — not sharing pipe with the primary
• ☐ Secondary sized for the full design flow Q on its own
• ☐ Scuppers (or secondary drains) set 2–4 in (50–100 mm) above primary level
• ☐ Overflow discharge path verified: visible, safe, legal (not onto walkways or neighbors)
• ☐ Secondary system shown as a separate line item in the estimate/bid

5. Materials Checklist

• ☐ Drain body material matched to membrane lifespan and environment
• ☐ Clamping rings specified for every drain (same material family as body)
• ☐ Membrane-compatible flashing for every penetration
• ☐ Strainers: cast-iron or aluminum in trafficked areas
• ☐ Leader material code-approved for the occupancy (check plastic pipe restrictions)
• ☐ Cleanout at the base of every leader
• ☐ Dissimilar metals separated (no galvanic couples in the water path)

Material trade-offs are compared in the material guide.

6. Installation Verification

• ☐ Drain bodies recessed slightly below the surrounding membrane surface
• ☐ Clamping rings torqued; membrane properly dressed into each drain
• ☐ Leaders strapped at every story and at the base
• ☐ Scupper openings clear; discharge path unobstructed
• ☐ Roof flood-tested (or hose-tested) at each drain before sign-off
• ☐ Ponding check: no standing water beyond ~48 hours after the test
• ☐ Inspector sign-off obtained; calculations and product data sheets filed

7. Maintenance Schedule

• ☐ Strainer and gutter cleaning: twice yearly minimum (spring + late fall)
• ☐ Leader flush: annually from the top
• ☐ Post-storm roof walk: after every major event in the first year, then seasonally
• ☐ Photo log: dated drain-condition photos each visit
• ☐ Overhanging vegetation trimmed before leaf season
• ☐ Maintenance responsibility assigned in writing (owner, manager, or service contract)
• ☐ Spare strainers kept on site — a cracked strainer replaced the same day beats a week of unprotected drain

A maintenance routine with the reasoning behind each item is in the pro tips guide.

Related Guides

• Complete Guide — roof drainage design from start to finish.
• Estimating Guide — from roof measurement to material takeoff.
• Code Compliance — primary vs. secondary drainage and inspection checkpoints.
• Common Mistakes — the errors this checklist prevents.
• Roof Drain FAQ — quick answers to the most asked questions.

Frequently Asked Questions

When should I use this checklist?

Three times: during design (sections 1–5), during installation (section 6), and ongoing (section 7). It also works as an audit tool for existing roofs — run sections 1–4 against what’s already built.

What’s the most-skipped checklist item?

Verifying low points on the actual roof. Designs get built from drawings, drawings show intended slopes, and the built roof disagrees — a post-rain walk catches it before the drains go in the wrong places.

Do I need all of this for a simple gutter replacement?

No — like-for-like maintenance replacements need only the relevant items (sizing verification and the maintenance schedule). The full checklist is for new systems, added drains, and redesigns.

Who signs off on the completed checklist?

The installing contractor for the work, and the AHJ inspector for code compliance. Keep the completed checklist with the building’s records — the next owner and the next roofer will both want it.

Get Your Number

A checklist is only as good as the numbers in it. The Roof Drain Calculator supplies the sizing core — peak flow, drain count, both unit systems — and this checklist wraps everything else around it: honest measurements, the real design storm, independent overflow, verified installation, and a maintenance habit that keeps the system at rated capacity.

Disclaimer: This checklist is general planning guidance. Actual drainage requirements depend on local design storms, building geometry, and code requirements that vary by jurisdiction. Verify sizing with your local building authority and consult a licensed roofing or drainage professional for design and installation decisions.