Falls
Flat Roof Falls and Ponding
A flat roof isn't flat: it should fall towards the outlet. BS 6229:2025, the code of practice for flat roofs, makes 1 in 80 the minimum finished fall on most flat roofs, and a design fall of 1 in 60 is a common starting point. Water that sits for days means the fall is lost.
What 1 in 80 means on your roof, why water sits, and how it's put right.
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What 1 in 80 means
- 1 in 80minimum finished fall on most flat roofs (BS 6229:2025)
- 1 in 60a common design fall: built steeper so it's still 1 in 80 once the deck settles
- 50 mmdrop over a 4 m roof at 1 in 80
- 67 mmdrop over a 4 m roof at 1 in 60
On a 4 m deep extension, 1 in 80 is a 50 mm drop from the wall to the gutter. It's small enough that a sagging joist or a soft board undoes it.
Why water sits
- The roof was built level or close to it.
- Joists have sagged, or the deck has gone soft between them.
- The outlet sits higher than the lowest part of the roof.
- Layers of old felt built up unevenly over the years.
Standing water isn't a leak by itself, but it makes every seam and crack it covers work harder, and it holds debris.
How a fall is put right
- Firrings: tapered timber strips on the joists, under a new deck. The usual fix when the deck is replaced.
- Tapered insulation: insulation boards cut to a fall, which gives a warm roof and a fall in one.
- Another outlet at the low spot, where the shape allows.
Ask what fall the new roof will have, and how it's made.
Flat roof jobs we cover
Flat roof replacementExtensions, garages and dormers. €3,000 to €6,500 on a kitchen extension.More on flat roof replacement →
Flat roof repairLeaks, blisters, split seams, outlets. €250 to €900.More on flat roof repair →
EPDM rubber roofsOne sheet, no flame. €70 to €140 a m² laid.More on epdm rubber roofs →
GRP fibreglass roofsHard and walkable. €75 to €140 a m² laid.More on grp fibreglass roofs →
Torch-on felt roofsThe lowest cost. €55 to €90 a m² laid.More on torch-on felt roofs →
Warm flat roofsInsulation above the deck. €130 to €190 a m².More on warm flat roofs →Flat roofs in every county
- Carlow1.4% flat main roofs
- Cavan0.8% flat main roofs
- Clare1.2% flat main roofs
- Cork2.5% flat main roofs
- Donegal1.0% flat main roofs
- Dublin3.0% flat main roofs
- Galway1.2% flat main roofs
- Kerry1.8% flat main roofs
- Kildare1.0% flat main roofs
- Kilkenny1.4% flat main roofs
- Laois1.2% flat main roofs
- Leitrim1.2% flat main roofs
- Limerick1.6% flat main roofs
- Longford0.9% flat main roofs
- Louth2.3% flat main roofs
- Mayo2.3% flat main roofs
- Meath1.3% flat main roofs
- Monaghan0.9% flat main roofs
- Offaly1.3% flat main roofs
- Roscommon1.2% flat main roofs
- Sligo1.4% flat main roofs
- Tipperary1.6% flat main roofs
- Waterford2.2% flat main roofs
- Westmeath1.5% flat main roofs
- Wexford2.7% flat main roofs
- Wicklow2.7% flat main roofs
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Questions people ask
What fall does a flat roof need?
At least 1 in 80 once it's finished, on most flat roofs under BS 6229:2025.
Is ponding on a flat roof normal?
A little water for a short time after rain, in a shallow dip, is common. Water that's still there days later points to a lost fall.
Can ponding be fixed without a new roof?
Sometimes, with an extra outlet at the low spot. Usually the fall is fixed when the roof is replaced, with firrings or tapered insulation.
Does ponding void a guarantee?
Some membrane guarantees exclude standing water. Read the guarantee before you sign.
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