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Minimum fall for 40mm waste pipe — what does the regs actually say?
Getting into an argument with a building inspector over the fall on a 40mm basin waste. I have it at 18mm per metre (roughly 1:55) and he's saying it should be 1:40. I thought BS EN 12056 allows 1:40 to 1:70 for 40mm? Anyone got chapter and verse on this? Would rather have the paperwork to back me up than just argue the toss on site.
2 Answers
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The actual text from Part H 1.29 says for branch pipes: 40mm minimum fall 18mm/m (1:55), maximum 90mm/m (1:11). So YOUR fall of 1:55 is perfectly compliant with Approved Document H. Print page 14 of the current AD H document and hand it to your inspector. That's the compliance document for England and Wales, not BS EN 12056 directly. The inspector may be quoting the BS standard rather than the Building Regs document — they're not the same thing.
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BS EN 12056-2 Table 4 gives recommended minimum gradients — for a DN40 (40mm) pipe it's 1/DN which is 1:40. However Part H of the Building Regulations (England) refers to this and notes that for branch pipes serving a single appliance the fall can be reduced. Your inspector is technically correct on the minimum but you have grounds to argue the point for short runs.