Snow and standing water on a flat rooflight

What snow actually weighs

People overestimate fresh snow and underestimate what happens to it after a few days.

Fresh dry snow is around 100kg per cubic metre, so 200mm of it is roughly 20kg per square metre. Settled snow after a thaw and refreeze cycle can reach 300kg per cubic metre or more, and wet snow driven by rain is heavier again. A rooflight carrying 300mm of consolidated snow is carrying something in the order of 90kg per square metre, which on a 1000 x 1500 unit is about 135kg of load sitting on a single pane.

Design snow load across Essex

The figures come from BS EN 1991-1-3 and its UK National Annex, and they are altitude and location dependent.

Essex is low lying and near the south east coast, so the characteristic ground snow load for most of the county sits at the bottom of the national range, generally around 0.4 to 0.5 kN per square metre at sea level, which is roughly 40 to 50kg per square metre. The shape coefficient for a flat or near flat roof is 0.8, and then drift allowances get added where a roof abuts a taller wall. In practical terms, a rooflight glass make-up is designed to a load well above anything an ordinary Essex winter delivers, and the units on the market carry substantial margin. This is not a marginal calculation.

Drifting against a parapet or a taller wall

The exception, and it is a real one, is drift.

Where a flat roof runs up to a taller wall, which describes almost every rear extension on a two storey house, wind pushes snow into the angle and it accumulates there far deeper than on the open roof. The design allowance for a drift against an obstruction can be two or three times the uniform load. A rooflight positioned within a metre or so of the house wall, or tight against a parapet on a side return, is in the zone where that matters. Positioning a unit away from that angle is free at design stage and impossible afterwards.

How the glass make-up carries the load

The pane spans between its supported edges, and the governing case for a flat rooflight in the UK is usually snow rather than wind, because wind on a horizontal surface acts mostly as uplift and suction while snow acts straight down onto glass in bending.

What controls it is thickness, aspect ratio and the laminate build. A 6mm toughened outer over a 6.8mm laminated inner is comfortable at modest sizes. As the pane grows past about 1.5 square metres, deflection under the design snow load becomes the limit and the make-up steps up to 8mm or 10mm outer with an 8.8mm or 10.8mm laminate. This is why unit weight climbs faster than area, and why an unusually large rooflight is never a straight scale-up of a small one.

What the frame and kerb are doing underneath

The load does not stop at the glass. It goes into the frame, then into the kerb, then into the deck and the joists.

A rooflight opening removes joists and puts their load onto trimmers, which means the trimmers around a large opening are carrying more per metre than the joists they replaced, and now they are carrying snow on glass as well as on deck. On a purpose-designed extension that is in the calculations. On an opening cut into an existing roof, particularly an old garage or outbuilding roof with light joists, it is the thing worth checking before the unit is chosen rather than after.

Why snow rarely slides off a flat rooflight

On a pitched roof window at 35 degrees, snow slides. On a flat rooflight glazed at three to five degrees, it does not, and that is by design rather than by accident.

Five degrees is nowhere near the angle at which snow overcomes friction on glass, so it sits until it melts. This is generally preferable. Snow sliding off a rooflight has to land somewhere, and on a lantern or a steeper unit above a doorway or a path that is a genuine consideration. On a flat rooflight the snow simply stays put and melts from underneath, because the glass is losing a small amount of heat from the room below.

Snow as insulation, and what that does to the glass

A layer of settled snow is a decent insulator, and it changes the thermal picture in a way that is briefly counter-intuitive.

With snow on top, the outer pane runs warmer than it would under clear cold sky, because it is no longer radiating to space. The inner pane runs warmer too, so condensation on the room side actually reduces during a snowfall. What people notice instead is the light: a covered rooflight goes from a bright rectangle to a dim grey one, and a room that relies on overhead light alone becomes noticeably duller until it clears.

Meltwater, refreezing and the perimeter

This is the mechanism worth understanding, because it is the one that finds a poor detail.

Heat escaping through the glass melts the snow directly above it. That meltwater runs to the edge of the pane, reaches the frame and the surrounding roof surface, which are colder because they are better insulated, and refreezes there. Repeat across a week and a ridge of ice builds around the perimeter of the unit, holding subsequent meltwater on the glass. It is the same physics as an ice dam at a gutter, at a smaller scale.

A well insulated unit with a warm edge spacer and a thermally broken frame melts less snow in the first place, so it forms less of a ridge. A well drained roof clears the water once it does melt.

Standing water on the glass surface

Water sitting on a rooflight is a different complaint from water sitting on a roof, and it has a different cause.

Modern flush glazed units are built with a fall of three to five degrees within the frame, so the glass drains even when the roof deck is close to level. Where water genuinely sits on the pane, the usual causes are a unit fitted level rather than to its intended fall, a unit fitted with the fall running towards a raised edge, or debris along the low edge acting as a small dam. None of those is about the glass.

Where standing water on the roof is the real issue

Water pooling on the roof surface around the kerb is the more consequential version of this, and it is a roof design matter rather than a rooflight one.

A flat roof is laid to a fall, conventionally 1:80 as designed to allow for deflection and construction tolerance. Where that fall is absent or has been lost, water stands, and standing water against an upstand is the condition that tests every detail in the junction. That subject, along with drainage layout and outlet positions, is covered on the main flat roof rooflight page. The short version is that the fall gets sorted first and the glass is set into a roof that drains.

Tide marks, and why the glass looks dirty in bands

A very common observation after a wet winter, and it is cosmetic rather than structural.

Water carries airborne dust and pollen across the pane. Where it drains cleanly it leaves nothing. Where it slows and evaporates in place, on a low edge or against a frame upstand, the dissolved solids stay behind as a rim. Over several cycles that rim becomes a visible tide line. It cleans off with warm water and a soft cloth, and it comes back if the drainage geometry that caused it stays the same.

Self cleaning coatings in winter conditions

Self cleaning glass works in two stages. A photocatalytic titanium dioxide coating breaks down organic dirt under ultraviolet, then the surface behaves hydrophilically so rain sheets across the pane rather than beading, carrying the loosened material off.

Both stages need conditions winter does not reliably supply. Short days and heavy cloud reduce the ultraviolet available for the first stage, and a pane under snow gets none at all. The coating is worth having, particularly on a rooflight nobody wants to be climbing up to, but it is a reduction in cleaning rather than an elimination of it, and its weakest season is exactly the one people notice the glass in.

Clearing snow safely, and when to leave it

The usual answer is to leave it. Essex snowfall almost never approaches the design load, it clears within days, and the roof is a worse place to be in icy conditions than at any other time of year.

Nobody should stand on a rooflight to clear it. Standard flat rooflights are not designed to be walked on, and the load case for a person is quite different from the load case for distributed snow. Where glass genuinely needs to take foot traffic, that is a different product with a designed structural build-up, covered under walk-on rooflight installation. Where snow does need reducing on a large unit, it is done with a soft roof rake from the edge of the roof or from a ladder, never with a shovel and never from on top of the glass.

Distinguishing condensation from a genuine water problem

A wet mark appearing near a rooflight during a cold spell is not automatically water coming through the roof, and the distinction saves a great deal of anxiety.

Surface condensation on the room side of the glass appears as an even film or as droplets across the whole pane, worst in the mornings, and it dries as the room warms. Condensation between the panes means the sealed unit has lost its seal and no ventilation will touch it. Water arriving from outside behaves differently: it is intermittent, it follows rain rather than temperature, it appears at a specific point rather than across an area, and it often shows on the reveal rather than on the glass. Timing is the clearest evidence. Something that appears on cold clear mornings with no rain is condensation, and a unit misting internally has reached the end of its life and wants replacing.

Specifying for winter on an exposed Essex roof

Exposure varies more across this county than the flat map suggests. A bungalow on Canvey or a house on the Blackwater estuary edge takes wind and driven rain from an open fetch with nothing in the way, while a terrace in the middle of Chelmsford is sheltered on three sides.

On an exposed site the specification hardens sensibly: a taller upstand than the minimum, a thermally broken frame with a warm edge spacer so less snow melts at the perimeter, a laminated outer pane which damps rain noise as well as adding strength, and generous drainage capacity so meltwater and rain leave quickly. None of that is exotic and all of it is decided at order rather than added later. Typical figures and what moves them sit under costs.