Is a roof lantern strong enough for snow?

Snow is weight, and weight is what a lantern is built to carry. Understanding how it lands shows why the design is reassuring.

Fresh, dry snow is light. Wet snow and snow that has thawed and refrozen are much heavier, because they hold water. A lantern frame sees that weight as a downward load spread over its sloping panes. Because the panes are pitched, a lantern sheds snow more readily than a flat pane does. Steeper slopes hold less. Much of what settles on a lantern slides off as the glass warms from inside.

The glass is not the weakest part of that system, and it is not meant to be. Each sealed unit sits on aluminium bars that act as beams, spanning from the ridge down to the eaves. The bars are sized for the span and the load, which is why a larger lantern has deeper or more closely spaced bars. Our page on roof lantern glazing bars explains how the layout follows the size.

What matters for a home in Crawley is the local design load, not a worst case from an Alpine valley. Crawley sits low, on the Weald clay, well below the heights where snow loads climb. Sussex does get heavy falls now and again, so the design still allows for them.

The design load comes from published standards, not from guesswork by the installer.

Snow loading on roofs in the UK follows the Eurocode for snow actions and its UK national annex, which give a ground snow load for the site based on its location and altitude. That figure is adjusted for the shape of the roof and for how exposed it is. Lantern manufacturers then test or calculate the frame and the glass against loads at or above that level, and a good supplier will give you a written statement of the loads the lantern is designed for.

We ask the supplier for that statement and compare it with the site. Height above sea level, how open the garden is, and whether the lantern sits beside a taller part of the house all feed into the decision. The result goes into the quote, in plain words, alongside the glazing build. You can see how that written approach works on the quiet spec page.

Snow travels through a chain of parts on its way to the walls. Each link has to be as strong as the one above it.

The glass is the layer people worry about, and it is the one we are least concerned about. A laminated inner pane also stays together if the outer is ever damaged. The layers below it are where attention belongs. A lantern is heavy, and snow adds to that weight. Whether the existing roof can carry it is a separate check, and whether a roof lantern needs a steel beam around the opening covers that decision. The kerb under the frame is described in our page on roof lantern kerbs and upstands.

A lantern stands proud of a flat roof, and that changes where snow settles.

Wind carries snow across a flat roof and drops it where it meets an obstacle. The lantern's upstand is exactly that. Snow piles against the sides, sometimes deeper than the general cover on the rest of the roof. This is why design loads include an allowance for drifting against raised features, and why the pile next to a lantern can look alarming after a windy fall.

The same heap can melt from below. The warm room under a lantern gives off heat that thaws the snow first against the glass, and meltwater runs down the pane and across the flat roof. If the roof has a fall to drain it, such as 1:80, the water leaves as usual. If outlets are blocked with leaves, water can pool, so it makes sense to clear gullies in autumn before the first snow. Our guide to cleaning a roof lantern safely covers access.

For most lanterns, the best response to snow is to wait. If you do need to clear it, how you do it matters more than how much you remove.

• Stay off the roof. A frosted flat roof is slippery, and a lantern is not a surface to lean on.
• Use a soft brush on a long pole from a safe position, pushing snow away from the glass rather than scraping it.
• Do not pour hot water onto cold glass. The sudden change can crack a pane.
• Avoid metal shovels and hard tools, which scratch coatings and can damage seals.
• Leave heavy, compacted ice to thaw. Forcing it off risks the frame.

If a lantern has electric vents, leave them closed while snow is on the glass, because vents can jam against ice. Our page on electric and opening rooflight installation explains how those controls work.

Will snow make the lantern dark inside?

For a while, yes. A covering of snow blocks daylight until it slides or melts. A lantern's pitch and the heat from the room below usually clear it within a day or so, although a deep cold spell can leave it longer.

Are flat rooflights as strong under snow as lanterns?

Flat units are also designed to a specified load, but because they do not shed snow they carry it for longer. Our page on the flat roof rooflight installation service covers how they are specified.

Does a bigger lantern cope worse with snow?

Not if the frame is sized for it. A larger span needs deeper bars, but it can be designed to the same standard as a smaller one. See the roof lantern size guide for how size affects the build.

Should I ask for the load rating before I order?

Yes. A written statement of the loads the lantern is designed for should be part of any quote. You can get a quote and we will include it.

The wider job, from survey to handover, is on our roof lantern installation page.