Flat Roof Falls & Drainage: BS 6229 Sizing Guide UK

Quick Answer: BS 6229:2018 requires flat roofs to have a minimum design fall of 1:80 (12mm per metre) to avoid standing water; 1:40 (25mm/m) is recommended for new build to allow construction tolerance. Falls are formed by firring strips, tapered insulation, or sloped deck. Each outlet drains a maximum 50–60m² for domestic; sized per BS EN 12056-3 using design rainfall (typically 75mm/h for UK 1-in-50 year). Overflow weirs or scuppers required as secondary defence.

Summary

Flat roof failure is overwhelmingly a falls and drainage problem. Standing water above 25mm depth is sufficient to penetrate most membrane seams, slow membrane ageing, encourage moss and lichen, and exceed the design loading of the deck. UK rainfall is variable but design intensities of 75mm/h (1-in-50 year) and 150mm/h (1-in-100 year + climate change) are typical. A roof that ponds in moderate rain cannot drain a design storm.

BS 6229:2018 is the UK code for flat roofs with continuously supported coverings (EPDM, GRP, single-ply, asphalt, felt). It sets the minimum design fall at 1:80 (12mm per metre) and the recommended fall at 1:40 (25mm per metre) for new construction. The reason for the gap: construction tolerance. A design fall of 1:80 with typical timber deflection, ply thickness tolerance and adhesive thickness variation can easily reduce to zero in places — producing ponds despite design intention. A 1:40 design retains positive fall even with normal tolerances.

The falls must be designed into the structure — into the deck, the firring strips, or the tapered insulation. Trying to "add fall" by raising the perimeter after the roof is built does not work. For new design, the falls are part of the architectural drawings. For retrofit, the choice is between dismantling and re-sloping the deck, or laying tapered insulation over the existing structure. See cold flat roof problems for cold roof design issues and warm flat roof detail for warm roof construction.

Key Facts

Quick Reference Table

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Fall Design Mathematical fall Construction tolerance to zero fall? Recommended for
1:80 (12mm/m) Minimum per BS 6229 Yes — likely ponding Avoid for new
1:60 (17mm/m) Acceptable Possible ponding Refurbishment max
1:40 (25mm/m) Recommended Unlikely ponding New build standard
1:30 (33mm/m) Generous No ponding Heavy rainfall area
Zero (level) None Guaranteed ponding Never

Outlet sizing (BS EN 12056-3, 75mm/h design rainfall):

Outlet Diameter Maximum Drained Area
50mm ~15 m²
75mm ~40 m²
100mm ~80 m²
150mm ~180 m²

Detailed Guidance

Why falls matter

Flat roofs are not flat. They are gently sloped surfaces with positive drainage toward outlets. The term "flat roof" describes the visual appearance, not the surface profile.

Roof at design fall 1:40:
                                              outlet
  ─────────────────────────────────────────────│
  ↘ ─ ── ── ── ── ── ── ── ── ── ── ── ── ── ──↓
  ↘                                            │
  ↘  Water flows positively toward outlet      │
  ─────────────────────────────────────────────┘

Roof at level (zero fall):
                                              outlet
  ─────────────────────────────────────────────│
   ▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪▪│
   ▪▪▪▪▪▪▪▪▪ standing water ponds ▪▪▪▪▪▪▪▪▪▪│
  ─────────────────────────────────────────────┘

Ponded water:

Falls design approaches

Sloped deck: Joists are designed at the required slope. New build only — retrofit cannot economically re-slope joists. Common in commercial new build with metal deck.

Firring strips: Tapered timber battens fixed on top of joists. Standard 1:40 firrings supplied in 50 × 50mm tapered to nothing over the length. Fixed at right angles to the slope direction. The firrings give the slope; the joists remain level.

Firring strips on level joists:
           ←─ firring 50 × 0 mm tapered ─→
   ┌────────────────────────────────────┐
   │  firring (1:40 taper)              │
   ├────────────────────────────────────┤
   │  level joist                       │
   └────────────────────────────────────┘

Tapered insulation: Pre-cut PIR or polyurethane boards with built-in fall. Most common for warm-roof construction. Manufacturer designs the layout (boards combine to create slope to outlets); installer follows layout drawing.

Tapered insulation layout (warm roof):
              ↓ outlet centre
   ╱╲╱╲╱╲╱╲╱╲╱╲╱╲╱╲╱╲╱╲╱╲╱╲ tapered PIR
   ────────────────────────────────────  level deck

Tapered insulation packs can include sumps (deeper around outlets) and crickets (raised valleys between outlets to direct flow).

Drainage layout

For new build, the design starts with outlet positions and works back to fall directions:

For a typical 6m × 4m extension:

Outlet sizing

BS EN 12056-3 calculates outlet capacity based on:

Typical outlet capacity at 75mm/h:

A single outlet on a 100m² roof is undersized — split into two 75mm outlets or use 110mm.

Overflow weirs

Where the primary outlet is blocked (leaves, ice, debris), water level rises. Without overflow, water can:

Provide a secondary overflow above the primary outlet:

For parapet roofs, overflow is mandatory under Building Regulations Part C and BS 6229.

Box gutters

Box gutters are concealed channels at the base of a parapet wall. Used where:

Requirements:

Internal drainage

Outlets in the middle of the roof drain via:

Internal drainage requires careful detailing — any leak inside the building damages internal finishes. Use solvent-weld plastic pipe with tested joints.

Construction tolerances

The most overlooked aspect of falls is construction tolerance. Even with a design fall of 1:80, real-world construction adds:

Stack these tolerances and a designed 1:80 can drop to 1:160 or even slope back upward locally. This is why 1:40 design is the practical standard for new build — even with 25mm of cumulative tolerance loss, positive fall remains.

Retrofit fall correction

Retrofit fall correction options:

  1. Strip and re-board with firrings — most thorough; full re-cover; most expensive
  2. Tapered insulation over existing — adds thickness; raises door thresholds; needs new abutment detailing
  3. Localised re-board with firrings under low points — targeted; only fixes specific ponds
  4. Re-balance outlets — sometimes the issue is outlet position, not fall; adding new outlet at low point

Option 1 is the only definitive fix. Options 2–4 are remedial and may need to be repeated.

Frequently Asked Questions

Why does my customer's flat roof pond in heavy rain?

Either the design fall is too low (designed at 1:80, real world below that after tolerance), the outlet is blocked or undersized, or the deck has deflected over time. Inspect for: bird's nests in outlets, leaves in gutters, deck deflection, undersized outlets. Often more than one cause.

Can I lay membrane over a level deck and "trust the water to run off"?

No. A level deck holds water indefinitely. No membrane is designed for permanent submersion. Falls must be positive — designed into the deck or added with firrings/tapered insulation before the membrane.

What's the minimum acceptable fall on a refurbishment?

For complete refurbishment (deck stripped), 1:40 is the standard. For minor membrane replacement without deck work, the existing falls are what you have — if these are below 1:80 the customer should know that future ponding remains a risk. Document what you found and what you advised.

How do I size the outlet for a typical garage roof?

For a 5 × 6m garage (30m²) in a typical UK location at 75mm/h design rainfall: single 75mm outlet capacity is ~40m² — adequate. For redundancy, install two outlets. Add overflow if parapet roof or any risk of water rising above upstand.

Are tapered insulation systems expensive?

Modest premium over flat boards — typically 15–30% extra cost for the same square metre but with the slope built in. The labour saving (no firrings needed, no separate slope formation) often offsets the material cost. For warm-roof construction, tapered insulation is the standard approach.

What's the most common cause of flat roof leaks?

Membrane seams and termination details — almost never the membrane field. Failure points: lap seams insufficiently bonded, perimeter termination bar loose, outlet boot seal failed, upstand chase mortar fallen out. Falls and drainage matter for membrane life expectancy and for avoiding ponding-driven failures.

Do I need approval for an internal rainwater pipe?

Building Control review for new build or substantial works (extensions) — internal pipework is part of the design. Surface water discharge route may need approval (water company adoption of soakaway, sewer connection consent). For maintenance / replacement, normally not notifiable.

Regulations & Standards