Asymmetrical Valleys

Where opposing roofs of different pitches discharge into a valley‚ an asymmetrical valley is required. As these have reduced cross-section area compared to a symmetrical valley at the same (lower) pitch, it is often necessary to increase the valley dimensions. Increasing the depth has the biggest effect on capacity. Greater depth can be gained by using 10 mm ply valley boards, standing purlins on edge, or fitting valley boards flush with the rafter. The consequences that a deeper valley will have on the capacity of the gutter it discharges into must also be considered.

A valley baffle is required in all cases where a valley has a change of angle or when the difference in roof pitch exceeds 10°. Valley baffles are also helpful in wooded locations to minimise lodging of debris under the roof overhang.

With asymmetrical valleys the lower side is the most critical. The width of the upper side can be less than the lower side as long as the vertical height of the upper side is not less than the height of the lower side.

Valley Baffle

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