The short answer
For a flat roof with a flexible waterproof covering, BS 6229:2025 keeps a minimum finished fall of 1:80 for general areas and internal gutters. Because building tolerances eat into the slope, IKO advises designing for 1:40 so the finished roof still clears the minimum. The standard is for membranes such as felt and EPDM, not for lead.
The finished fall and the design fall
A fall is the gentle slope that moves rain across a roof to its outlet. It is written as a ratio: 1:80 means the roof drops 1 unit for every 80 units of run, so 12.5 mm over a metre. A fall of 1:40 drops 25 mm over a metre.
The current standard is BS 6229:2025, which deals with flat roofs whose waterproof covering is flexible and fully supported. Its minimum finished fall is 1:80 across the main roof area and internal gutters. The 2018 edition has been withdrawn, so BS 6229:2018 should not be cited as current.
The finished fall is the slope the completed roof actually has. It is lower than the slope drawn on a plan, because decks deflect, timbers settle and boards are never perfectly laid. IKO, a membrane maker, therefore advises designing at 1:40, leaving room for these tolerances so that the built roof still reaches 1:80.
Treat the two numbers as a pair. Design for 1:40, and check that the finished fall meets 1:80. A roof designed only to the minimum has no allowance, and one low spot can pond.
Ponds that remain after rain are a warning that a fall has been lost somewhere.
What the standard covers, and what it does not
BS 6229:2025 is aimed at roofs pitched at 10 degrees or less and covered with flexible waterproof membranes. That includes felt, EPDM and other single-ply systems laid on a continuous deck. It is the reference for falls, drainage and detailing of those coverings.
It does not cover lead. Lead sheet roofs follow a different standard, BS 6915, which Approved Document C cites for lead, and lead has its own rules for falls, bay lengths and fixings. A roofer working with lead refers to that guidance and not to BS 6229.
GRP is a rigid system rather than a flexible membrane, and manufacturers give their own minimum falls for it. Read the manufacturer's data sheet for the covering you plan to use, since minimums vary between makers and systems.
Gutters have their own guidance. Brett Martin suggests a gutter fall of 1 mm for every 600 mm, a far gentler slope than a roof, and BS EN 12056-3 governs roof drainage design generally. A roof gutter that is too flat will hold water, and one with too much fall can overshoot an outlet.
None of this is a legal rule on its own. Building Regulations require roofs to resist rain, and these standards are how that is achieved in practice.
Creating a fall with tapered insulation
On a new roof the fall can be built into the structure by laying the joists to a slope, called firrings, or by laying the deck at an angle. On an existing roof that is too flat, the usual way to create one is tapered insulation: boards cut to a gradually changing thickness, laid over the deck so that the surface slopes towards the outlets.
The tapered scheme is drawn up from a plan showing the outlet positions and the lie of the deck, and the boards are numbered for laying. It also provides the insulation required by Approved Document L, so one layer does two jobs. Our page on tapered insulation on a flat roof explains how it is specified.
Where the deck itself has sagged or is uneven, the fix may need new firrings or a new deck, because tapered boards laid over a deep dip only hide it. A thorough roofer measures the existing levels before choosing the method.
When only the waterproof layer is being replaced, Approved Document L treats that as renovation and applies Table 4.3. The insulation target under that table is 0.16 W/(m²·K).
If the fall is wrong, a new covering laid on top of it simply repeats the old problem.
Checking the fall on your own roof
Plenty can be learned from ground level. After rain, look for standing water, dark rings left where ponds have dried, and growth of moss or algae in the same place. Water that lingers into the second day after rain points to a loss of fall or a blocked outlet.
A roofer measures the fall directly, using a long straight edge and a level, or a digital level, along the line of flow to each outlet. We also look at the deck from below where we can, check outlet heights and note any sag between joists.
If the finished fall falls short of 1:80, the options depend on the cause. A blocked outlet is a simple clear-out. A deck that has sagged may need packing or new firrings. A roof built too flat needs tapered boards or an extra deck layer, best done while the covering is being renewed anyway.
We show you the photographs and the readings, and say plainly what each option involves. A roof that ponds slightly but is otherwise sound may only need monitoring. You receive one written price for whichever work is needed.
The why water ponds on a flat roof page covers causes in more detail, and the flat roofing hub sets out the coverings.
