The flat roof around a lantern needs a minimum finished fall of 1:80, and because timber deflects and firrings are never perfect, it should be designed and built to 1:60 or 1:40 to guarantee that 1:80 is actually achieved. The fall must run away from the lantern upstand towards the outlet, never towards it. A dead-flat deck ponds, and standing water against an upstand is the beginning of every upstand failure we see.
“Flat roof” is a misnomer that has caused an enormous amount of damage. No flat roof should be flat. It should be a shallow ramp with a designed route for water to leave by.
What the numbers mean
A fall of 1:80 means the roof drops 1 unit vertically for every 80 horizontally: 12.5mm of drop per metre run. Over a 4m roof that is 50mm from the high point to the outlet. It is a shallow gradient and it is easy to lose.
It gets lost because joists deflect under load, firring pieces get cut inconsistently, insulation boards compress unevenly, and the deck sags slightly between supports. Each of those takes a few millimetres. Design to exactly 1:80 and you will finish somewhere below it in the middle of the span, which is precisely where the puddle forms.
| Fall | Drop per metre | Use |
|---|---|---|
| 1:80 | 12.5mm | Minimum finished fall. The target after all deflection. |
| 1:60 | 16.7mm | Common design fall on short spans to allow for deflection. |
| 1:40 | 25mm | Design fall on longer spans, or where the deck is over 4m. |
| Dead flat | 0mm | Ponding, accelerated covering degradation, upstand saturation. |
Why direction matters more than gradient
A lantern upstand is an obstruction in the middle of a drainage plane. Water arriving at it has to go round it, and how it goes round is a design decision that somebody has to make deliberately.
Get it right and the fall runs from the upstand outwards to the perimeter and the outlet, so water never dwells against the vertical face. Get it wrong, most often by siting the lantern on the low side of the roof, and the upstand becomes a dam. Every shower leaves a pool against it. The covering at the base of the upstand stays wet, the laps there work harder than anywhere else on the roof, and the timber inside eventually finds out.
On wider lanterns, the roof behind the upstand also needs thinking about. A 3m wide lantern sitting across the fall creates a sheltered strip on its upslope side where water collects and debris accumulates. The fix is to form cross falls that shed water round both ends of the upstand rather than letting it queue against it. That has to be built in when the roof is constructed, not adjusted afterwards.
A puddle is not cosmetic. Standing water accelerates degradation of every covering type, holds silt and organic matter that keeps the surface damp between showers, and in Suffolk adds a ring of pollen and, near the coast, salt residue. On EPDM and single-ply it shortens service life. On felt it ends it early.
Retrofitting into a roof that has no fall
This is a common situation on extensions built in the 1970s and 1980s, where the deck was laid level and the covering was expected to cope. It cannot, and it does not.
If we survey a roof with no usable fall and you want a lantern on it, there are two honest options and one dishonest one. The honest options are to re-form the fall with firrings or tapered insulation as part of the works, or to site the lantern where the existing high point happens to be and accept the constraint on position. The dishonest option is to build the upstand tall enough to sit above the ponding and call it solved, which protects the lantern while leaving the rest of the roof to fail.
We say which of these applies at survey, before you have paid anything. If the roof needs re-forming, that is a cost you should know about while it is still a decision rather than a discovery.
Local factors in Suffolk and north Essex
Rainfall in East Anglia is low by UK standards, and that has produced a long local habit of under-specifying falls on the assumption the roof will not see much water. The assumption is wrong in the way that matters. Annual totals are modest, but individual intense summer downpours are not, and a roof that ponds gets its damage from the intense events and from the slow degradation of never fully drying out.
On the coastal belt at Felixstowe, Harwich, Frinton and Aldeburgh there is a second factor. Driven rain arrives at an angle, so the effective load on the upwind side of an upstand is higher than the rainfall figure suggests, and water is physically pushed up the vertical face. This is why we build upstands to 200mm there rather than 150mm, and why the fall around them wants to be generous rather than minimal. The estuarine towns, Woodbridge, Wivenhoe, Manningtree and Shotley, get the wind exposure across open water without the full salt loading, and the drainage argument applies there just as strongly.
Inland, on the older timber-framed stock across mid Suffolk, the issue is deflection. Joist spans on those roofs are often irregular and undersized by modern standards, so the built fall degrades more than the calculation allows for. Designing to 1:40 rather than 1:60 is the sensible response.
Related questions
- What is a roof lantern upstand? covers the detail the fall protects.
- How big should a roof lantern be for the room? includes siting away from the low corner.
- Why do roof lanterns leak at the corners? explains where saturated details give way first.
See also how a roof lantern upstand is built and when the upstand is the problem, under roof lantern installation.
Have the fall checked before you order
We check the fall of the existing roof at survey with a level, not by eye, and we tell you what we find before pricing. Every installation carries a 10-year workmanship guarantee. Request a fixed quote.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.