Minimum Pitch

Q: Is there a standard for minimum pitch?

A: AS1562.1 details fixing requirements for metal roofs but doe not specify pitches. It states:

“4.1 GENERAL The cladding system shall be installed in accordance with the design specifications or the manufacturer’s installation specification, as appropriate.

All sheets shall be supported over the full width of the sheet.”

Regarding wind pressures on roofs:
“5.5 RESISTANCE TO WIND PRESSURES FOR NON CYCLONE REGIONS

5.5.1 Serviceability test. When the cladding system is subjected to the test pressure for serviceability limit state, in accordance with AS4040.2, the maximum deflection of the cladding relative to the supporting members shall not exceed (S120+p/30). Further, no de-indexing, unclipping, permanent local deformation, or fracture or failure of any part of the sheeting or of the fastenings shall occur………..” It also somewhere states “no leaking”.

A:For Bluescope Steel products: http://www.bluescopesteel.com.au/go/building-products/roofing

A: The minimum pitch recommended by manufacturers is “after deflection”. Long term deflection (such as timber creep) is an issue.

In addition, roof structures reduced to the minimum by the engineer will result in a low pitched roof rising and lowering with the pressures in high winds (If this is with driving rain you have a problem – rigid flashings, seals etc. give way, and water is driven upwards).

Penetrations are where most problems arise, and minimum roof pitches exacerbate the weatherproofing issue and demand conscientious workmanship.

A:Care should be taken when working on the really low pitches (1 or 2 deg)as the minimum roof pitches recommended by manufacturers, to ensure that deflections are taken into account. A structure with a Klip-lok roof at 1 deg can have so much structural deflection in high winds that it results in ponding and flooding at the side laps. The only solution in this case is to re-pitch the entire roof.