Roof Slope & Singapore Rainfall Calculator

Enter the roof run and slope to calculate the drop. The rainfall check separates average daily water volume from the short-duration runoff rate used for preliminary drainage checks.

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1. Roof geometry

Edit any field; results update automatically. The Update button is provided as a backup.

Roof length

High point to drainage edge.

Length basis

Slope format

Slope percentage

2% = 20 mm/m = 1 in 50.

Roofing system advisory

Calculation text

Roof slope and Singapore rainfall calculation

ROOF GEOMETRY
Input: 5 m (horizontal run)
Vertical drop: 100.0 mm (0.1000 m)
Slope: 2.000% = 1 in 50.00 = 1.146° = 20.0 mm/m
Horizontal run: 5.000 m
Roof surface length: 5.001 m

RAINFALL CHECK
Roof area: 100.00 m²; runoff coefficient: 1.00
Daily rainfall: 6.94 mm/day → 694 L/day
Storm intensity: 93.0 mm/h → 2.58 L/s

Preview note: these are the default values. Open in a browser or use the spreadsheet to recalculate.

This text updates automatically and can be copied manually.

2. Singapore rainfall check

Daily depth estimates accumulated volume. Hourly intensity estimates how fast water reaches the drains.

Horizontal roof area

m²

Runoff coefficient1.0 is conservative for an impervious roof.

Daily rainfall

mm/day

Short-duration intensity

mm/h

Daily runoff volume694 L/day0.694 m³/day

Storm runoff rate2.58 L/s155 L/min

Do not divide the daily average by 24 to size drains; Singapore rain often falls in short, intense bursts.

Long-term all-calendar-day mean2,534.3 mm/year for 1991–2020 ≈ 6.94 mm/day.

Recent wet-day comparison2025 averaged 13.8 mm on wet days; this is not a design storm.

Short-duration comparisonThe highest observed 60-minute intensity in 2024 was 93 mm; this is historical, not a prescribed design value.

Quick drop reference

Slope	1 m run	5 m run	10 m run
1%	10 mm	50 mm	100 mm
2% (1:50)	20 mm	100 mm	200 mm
3%	30 mm	150 mm	300 mm
1°	17.5 mm	87.3 mm	174.6 mm
2°	34.9 mm	174.6 mm	349.2 mm
5°	87.5 mm	437.4 mm	874.9 mm

Required roof drop

High point to drainage edge.

Vertical drop

100.0 mm

10.0 cm · 0.1000 m · 3.94 in

Slope2.000%

Ratio1 in 50.00

Angle1.146°

Fall per metre20.0 mm/m

Horizontal run5.000 m

Surface length5.001 m

At or above 2% is a sensible preliminary membrane-roof target, subject to the complete drainage design.

Equatorial-rainfall advice

Slope alone is not enough. Outlet, scupper, gutter, downpipe and overflow capacity must match the design storm.

Membrane roof starting pointWhere feasible, use about 2% (20 mm/m or 1:50) to retain positive fall after construction tolerances.

Metal and tiled systemsFollow the exact product’s tested minimum pitch and maximum drainage length.

Overflow and maintenanceProvide an accessible primary route and a safe visible overflow route. Avoid ponding and mosquito-prone standing water.

Final sizingUse SS 525 and project-specific design rainfall, effective catchment, wind-driven rain and blockage provisions.

Reference sources

NEA/MSS 2022 climate review — 1991–2020 islandwide annual average of 2,534.3 mm.
NEA/MSS 2025 climate review — 13.8 mm average rainfall on wet days in 2025.
NEA/MSS 2024 climate review — highest observed 60-minute intensity in 2024 was 93 mm.
SS 525:2024 preview — roof-drainage design reference.
LYSAGHT KLIP-LOK® product data — an official concealed-fix example allowing pitches from 1°, subject to the selected profile and design provisions.
LYSAGHT TRIMDEK® product data — an official trapezoidal-sheet example with a 2° minimum roof slope.
LYSAGHT CUSTOM ORB® product data — the official corrugated-profile source behind the 5° example. These examples are not universal minima; the exact supplied product controls.