Can I extend my SVP for a loft conversion WC?

Yes — common approach. Extend existing SVP up through new loft space to roof termination; or branch off existing SVP with new AAV-terminated extension if external extension impractical. Stack diameter capacity check (110mm primary takes 4 WCs + waste); check with plumber + Building Control. Branch length max 6m + 1:40 fall.

AAV vs open-vent — which?

Open-vent preferred (more reliable, no failure point); AAV acceptable alternative where open-vent impractical (loft conversion, heritage, no roof termination route). Belt-and-braces: primary external SVP open-vent + internal extensions AAV. Cost similar; choice driven by access + aesthetic.

Why does my drain smell?

Diagnose: dry trap (P or U trap below sink/bath — pour water down to refill seal); failed AAV (degraded diaphragm — replace); SVP blockage (debris in stack — drain rod or jet); main drain blockage (rare for upstairs smell — usually local). New install: HepVO waste valves (vacuum-sealed waste outlet) at every fixture provides backup against trap siphonage + smell — premium spec £25–£65 per fixture.

Drainage Stack SVP London Renovation | Builderr

How Is a Drainage Stack and Soil Vent Pipe Designed in a London Renovation?

London drainage stack design per Approved Document H: 110mm PVC main soil vent pipe (SVP) vents WC + sink + bath + shower waste; terminates 900mm above any opening window/door within 3m horizontal of stack; AAV (Durgo, Studor — air admittance valve) acceptable internal alternative where external SVP impractical (loft conversion, heritage). New SVP install £450–£1,800 depending on routing complexity. WC trap loss prevention via correctly-sized vent or AAV.

How SVP works

Drainage stack vertical pipework carries waste water + soil from upper-floor fixtures to ground-level drain. Two functions: drainage (gravity flow waste to drain) + venting (air admittance to prevent trap siphonage). When WC flushes, large slug of water descending stack creates low pressure behind — without vent, low pressure sucks water out of nearby trap = trap loses water seal = sewer gas + smell into room. Vent above highest connected fixture allows air in to equalise pressure. Vent options: open vent termination above roof (traditional — terminates 900mm above any window/opening within 3m horizontal); AAV (Durgo, Studor — internal cap on stack opens to admit air on low pressure, closes when normal, prevents sewer gas escape into building). UK Building Regs Approved Document H: at least one open vent stack per drainage system (typically primary SVP serves multiple AAV-vented branches). Stack diameter: 110mm primary for combined WC + branch waste; 100mm acceptable for waste-only but 110mm preferred for future-proofing.

Routing + termination

External SVP (traditional): black UPVC 110mm fixed to external wall up to gable or eaves; passes through roof tile with weather collar/lead slate; terminates 900mm above any opening within 3m horizontal; protected with vent cowl + bird mesh. Pros: full venting (no AAV maintenance), visible compliance. Cons: visible on building exterior (aesthetic concern for heritage); accidental damage risk; ice/frost block in extreme cold. Internal SVP (modern): runs vertically inside duct or service void; terminates inside roof void with AAV; no external roof penetration; primary external SVP elsewhere in property handles open-vent obligation. Pros: aesthetic clean (no external pipe), no roof penetration, heritage-friendly. Cons: AAV maintenance (replace 10–15 years), internal AAV failure = sewer gas in building. New stack install + termination: £450–£1,800 depending on routing length + roof penetration complexity. Loft conversion: new WC in loft requires new SVP branch + may require new full external stack or AAV internal extension. Branches: 100mm WC branch to stack, 50mm bath/shower branch, 40mm sink/basin branch; minimum 1:40 fall; maximum branch length 6m to stack.

AAV (air admittance valve)

AAV: spring-loaded one-way valve at top of stack; closed in normal operation (no smell escape); opens to admit air when negative pressure inside stack. Brands: Durgo (Sweden, market-leading), Studor MaxiVent + MiniVent, HepVO (waste valve — different application but same principle). Sizing: 110mm AAV for primary stack, 32–50mm for fixture-trap individual valve. Location: inside duct, service void, loft, accessible cupboard — must be accessible for service + replacement (5–15 year life). Compliance: BS EN 12380 + Building Regs Approved Document H1 accepted alternative to open vent — but at least one open vent per drainage system mandatory. Cost: AAV £85–£185 fitting + install; complete internal stack with AAV vs external open-vent stack — install cost similar but AAV avoids roof penetration. Maintenance: AAV diaphragm degrades over 5–15 years; replace if sewer gas detected — £85–£185 replacement. New install: external open-vent SVP preferred where roof termination feasible; AAV for loft conversion + heritage + internal duct.