Can a roof lantern go on a flat roof with no fall?

What “flat” means in a roof specification

No flat roof is flat. The term describes a pitch below 10 degrees, and British practice has for decades assumed a designed minimum fall of 1 in 40, with 1 in 80 accepted as the finished minimum after allowing for deflection and construction tolerance. That distinction is the whole point: you design at 1 in 40 so that when the deck sags a little under load you still have 1 in 80 left.

A roof built dead level therefore has no margin at all. The moment the joists take up any deflection, parts of it fall the wrong way. Which is why level roofs almost always turn out to have a slight fall towards the middle of the span rather than towards the outlet.

Why a lantern makes a level roof worse

A lantern occupies the centre of the deck and stands proud of it. Water running across the roof now has to go round it, so the flow divides at the upslope edge of the kerb and rejoins downslope. On a roof with a proper fall that happens quickly and the two streams merge again within a metre.

On a level roof there is no flow to divide. Water simply stands, and it stands against the upslope face of the kerb, which becomes a small dam. That is the worst place on the roof for standing water, because it is exactly where the covering has been cut, turned up and terminated against a new element. Depth also matters: 20mm of standing water over a large area is a meaningful load, and it sits there for weeks in winter.

Finding out whether your roof falls at all

Most people do not know, and the answer is not obvious from the ground. Three checks settle it. First, look for a tide mark: dried silt, algae and a dull ring on the covering show exactly where water habitually sits. Second, go up after rain has stopped and see what is still there an hour later, because a roof with fall is dry and a roof without one is not. Third, take levels properly, with a laser or a long straightedge and a spirit level, from the outlet back to the highest point.

The third is what we do at survey, and it produces a number rather than an impression. The number is what the specification gets written against.

Building the fall back in

Three routes, and the right one depends on what the roof is made of and whether the ceiling below is coming down anyway.

• Firrings on top of the joists. Tapered timbers running with the joists, thick at the high end and thin at the low, with new decking over. Cheap and reliable, needs the old covering and deck off, and raises the roof line slightly.
• Tapered insulation. Rigid boards cut to a designed scheme by the manufacturer, creating the fall within the insulation layer itself. It suits warm roofs, gives excellent thermal performance, and is designed to a drawing that routes water around obstructions including your lantern kerb.
• A new deck entirely. Where the joists themselves run the wrong way or have deflected badly, the structure is set out again. This is the expensive option and occasionally the only honest one.

Tapered insulation is usually the neatest answer where a lantern is involved, because the scheme can be drawn to shed water away from the kerb on all four sides rather than round it.

Where the lantern sits relative to the outlet

Position is a drainage decision before it is an aesthetic one. A lantern placed directly upslope of the only outlet blocks the natural route and forces water into two side channels that may not have been designed for it. A lantern placed at the low corner sits in the wettest part of the roof.

The better arrangement is a lantern in the upper half of the fall, with clear runs down both sides to the outlet, and enough width between kerb and parapet or edge for water to move. Half a metre of clear covering all round is a reasonable working minimum. On a small roof where that cannot be achieved, a second outlet is often cheaper than moving anything.

The kerb height that buys you margin

Whatever the fall, the kerb is your insurance. The frame of the lantern must sit high enough above the finished covering that ponded water, driven rain and a winter’s worth of debris cannot reach the point where the covering terminates.

150mm above the finished surface is the figure worth holding to, and it is measured from the finished covering, not from the deck. Get that wrong by building the kerb before the insulation and covering thicknesses are known and you can lose 100mm of it without noticing. On a roof with a marginal fall, going to 200mm costs a length of timber and buys a great deal of tolerance.

Ponding, and what it does over time

Standing water does not go through a sound covering. What it does is age it. Ultraviolet exposure combined with permanent moisture degrades felt and single-ply faster than either does alone, algae and silt build a mat that holds more water still, and in a hard frost the ice lifts laps and works at the edges of the membrane. Ten years of ponding is not the same as ten years of a dry roof.

It also removes your margin for error. A dry roof with a small imperfection in a lap will never show it. The same imperfection under 15mm of permanent water shows within a season.

When we would decline the job as asked

If a survey finds a dead level deck, saturated insulation, or a covering already at the end of its life, we will say that the lantern is not the first job. We install and replace rooflights. We build kerbs, form the opening and weather the perimeter properly, and we will happily work alongside a roofer who is re-laying the covering and forming the falls, which is the natural order of things.

What we will not do is set a lantern into a roof that we know holds water, because the ten-year guarantee on our workmanship means something and the perimeter detail is what it is guaranteeing. A survey settles this in an hour. Call 01865 704245 and we will tell you which roof you have before anybody quotes for a lantern.