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Minimum Freeboard Values

In gutters where overflow can enter the structure, it is necessary to have freeboard to allow for wave action, obstructions, and other unforeseen circumstances. Capacity Calculations allow the these minimum freeboard values.

Minimum Freeboard Values

Gutter TypeFreeboard
Internal gutters20 mm
Secret gutters 
Valleys with a pitch less than 8° 
Asymmetrical valleys 
External gutters with no overflow15 mm
Valleys with a pitch of 8° or more 
External Gutters with OverflowNo freeboard required
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/cop/roof-drainage/roof-drainage-design#minimum-freeboard-values
Revision Category: 
2 - Editing and rearrangement
Revision Detail: 

Added minimum freeboard values of 20mm to the table.