A drain field, like a soak pit, is a hydraulic and public-health calculation, not a BS EN 1992-1-1 structural one — no loads, no reinforcement, no concrete clauses at all. The module is catalogued alongside the structural chapters purely for routing; the real design questions are flow, percolation rate and trench layout.
Every check above runs automatically inside the Structyze Drain Field module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Population equivalent 6 (3-bedroom dwelling)
- Percolation time 20 min per 25 mm drop
- Trench width/depth 0.6 m / 0.6 m
- Max single trench length 30 m; trench spacing 1.8 m centre-to-centre
- Separations: well 30 m, building 3 m, boundary 3 m
Working Through It
Design flow Q = 6 × 150 = 900 L/day (approximate mid-range for the dwelling). The 20-minute percolation time converts to an application rate ra ≈ 28 L/m²/day — giving a required absorption area A = 900/28 ≈ 32 m², rounded up to 40 m² with a safety allowance.
Converting area to trench length at the 0.6 m trench width: L = 40/0.6 ≈ 67 m total — but the maximum single trench length is capped at 30 m, so the layout splits into 3 trenches of about 22 m each, giving a field footprint 3 × 1.8 = 5.4 m wide.
All four site checks — percolation suitability, well separation, building separation, boundary separation. Pass, but notably sit right at their respective minimums rather than with generous margin.
What Governs, and Why
Every check passes, but the defaults used here sit on the separation minimums rather than comfortably inside them — worth flagging explicitly, since many jurisdictions require larger distances in practice, and a really slow-percolating soil would need a mound or pressure-dosed system this basic sizing doesn't cover at all.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Drain Field module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Drain Field and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why is this module filed alongside structural RCC chapters if it's not a concrete design at all?
It shares the same design-code family framing (BS EN 1992 engine naming) purely for site/product routing consistency with the rest of the water-retaining and foundation series — the actual calculation inside is hydraulic and regulatory, with application rates and separation distances set by national or local regulation (in the EU, guided by BS EN 12566 and member-state rules), not by any Eurocode clause.
What would push this design toward a mound or pressure-dosed system instead?
A percolation time well outside the acceptable band (very slow-draining clay soils, for instance), a high water table, or site separation constraints that can't be met with a conventional gravity-fed trench layout would all push toward a raised mound system or pressure dosing — neither of which this reference sizing covers.
Final Thoughts
A drain field's separation distances sitting right at their regulatory minimums, rather than comfortably clear of them, is a classic case of detail worth flagging to a client before construction rather than discovering during a site inspection.