Minimum Roof Pitch for a Metal Roof in Malaysia: How Flat Is Too Flat?

What is the minimum roof pitch for a metal roof?

The minimum roof pitch for a metal roof depends on the sheet profile, not on metal roofing in general. In Lysaght's Australian roofing and walling installation manual, the minimum recommended slope runs from 1 degree (1 in 50) for deep concealed-fix profiles to 5 degrees (1 in 12) for its corrugated CUSTOM ORB, and the manual advises designing for at least 1 in 30 (2 degrees) wherever possible.

Pitch is usually written in degrees or as a ratio of rise to run. A slope of 1 in 30 rises 1 unit for every 30 units across, which is about 2 degrees; 1 in 12 is about 5 degrees. The manual's own wording on the general rule is direct:

"As a guide, wherever possible, you should design for a minimum slope of 1 in 30 (2°)."

A slope below a profile's listed minimum is not automatically ruled out, but it is a special case: Lysaght says lower slopes "may be available subject to enquiry" and "may require additional provisions."

How does the sheet profile change the minimum pitch?

The sheet profile changes the minimum pitch because the width and depth of a profile's pans decide how much water it can carry, and each profile is given its own minimum slope. That is why the profile is chosen together with the pitch, not after it.

Lysaght's specification table for its roofing profiles shows the spread (minimum recommended slope, by profile):

Swipe sideways to see every column

These figures are for Lysaght's own profiles in its Australian manual; Malaysian product data can differ, so check the documents for the exact sheet. The same profile can carry a different minimum in a different thickness: Lysaght lists KLIP-LOK 700 HI-STRENGTH at 2 degrees in 0.42 mm and 1 degree in 0.48 mm.

Read the minimum for the exact profile and thickness on the drawings, from the manufacturer's own documentation. Our guide to choosing metal deck roofing explains how the profile itself is selected.

What goes wrong when a metal roof is too flat?

When a metal roof is too flat, rain can stand on it or back up into the laps. Lysaght's manual warns that without adequate positive fall "there is danger of ponding, which can lead to a reduced service life, or reduced rainwater capacity," and that reduced service life matters most in more severe environments.

Very low slopes also leave no margin for building tolerances. At around 2 degrees or less, the manual says, "all roof supports must be in the one plane because slight variations can result in zero or negative fall." It lists the causes: construction tolerances, settlement, timber warping or shrinking, and extra loads such as air-conditioners or suspended services.

A low-slope metal roof therefore depends on the purlins being set level and true, which our guide to purlin and batten spacing covers.

How do pitch and rainfall decide how long the roof can run?

Pitch and rainfall together decide how long a metal roof can run from ridge to gutter. In heavy rain the valleys of the sheet must carry all the water falling on the run above them; if they overfill, water can flow into the roof through the side-laps and flashings. A flatter pitch drains more slowly, so the maximum run gets shorter.

Lysaght lists the factors as the width and depth of the valleys, the pitch ("rain flows faster on a steeper pitch"), the rainfall intensity for the area, the length from ridge to gutter, and penetrations that divert water into neighbouring valleys. Its drainage table shows how much the profile matters. At a peak rainfall intensity of 200 mm per hour:

• Corrugated CUSTOM ORB at 5 degrees can drain a run of about 15 m from ridge to gutter.
• KLIP-LOK 700 HI-STRENGTH at 1 degree can drain about 172 m, and 214 m at 2 degrees.

These are Lysaght's table values for its own profiles at an example intensity, not a Malaysian design rainfall.

Malaysia's rain makes this calculation worth doing. MetMalaysia notes that over the east coast states "November, December and January are the months with maximum rainfall," and that the southwest coast of the Peninsula is "much affected by early morning 'Sumatras' from May to August."

The design rainfall intensity for a site comes from local data, and the manual recommends "a suitably qualified engineer" for checking roof and flow design. The gutters at the bottom of the run are sized from the same rainfall figure; see how roof gutters and downpipes are sized.

What extra details does a low-pitch metal roof need?

A low-pitch metal roof needs better sealing at the ends and laps of the sheets, because water drains more slowly and is more likely to be driven or drawn into a joint. Lysaght's manual sets two thresholds: below 15 degrees (1 in 4) end-laps should be sealed, and below 10 degrees (1 in 5) infill strips should be fitted.

1. Use full-length sheets where possible. Lysaght notes that sheets can be supplied long enough to cover many roofs without end-lapping, and that its KLIP-LOK 700 profiles are not recommended to be end-lapped at all; an expansion joint is used instead.
2. Seal end-laps below 15 degrees. The manual specifies two runs of sealant, one at the low end of the lap "to prevent moisture being drawn in by capillary action" and one at the high end to stop condensation running into the lap, with an end-lap of 200 to 300 mm.
3. Fit infill strips below 10 degrees. "Where roof pitches are below 1 in 5 (10°), you should incorporate infill strips to maximise resistance to water entry," the manual says, using strips that do not absorb water.
4. Keep the supports in one plane. The manual requires this at around 2 degrees or less, where slight variations can leave zero or negative fall.

How is the pitch chosen for a new metal roof in Malaysia?

The pitch for a new metal roof in Malaysia is chosen from the building's design, the sheet profile's minimum slope and the length of the run the roof must drain.

The decision is easier when the drawings state three things together: the pitch, the profile and thickness, and the longest run from ridge to gutter. With those, the profile's minimum slope and drainage length can be checked against the manufacturer's tables before the roof is ordered. On an existing roof that ponds or leaks at the laps, our guide to repairing, reroofing or replacing a metal roof explains the options.

How can Ducero help with a low-slope metal roof?

Ducero's work covers metal deck roofing, cladding and reroofing, alongside light-gauge steel roof trusses, as its published project archive shows. Send the building's location, the roof plan dimensions and the pitch shown on the drawings through the contact page, or call +603 3123 1348. Planning the whole roof? See how to choose a steel roof truss system.