Snow on a flat roof or a conservatory: what actually carries the load
Ordinary British snow is not the problem. Where it piles up, and where the meltwater goes when it thaws, is the problem.
Ordinary snowfall is designed for, and most flat roofs and conservatories shrug it off. What causes failures is drift, where snow piles against a wall or a change of roof level, and meltwater that cannot leave a frozen outlet. A conservatory is the exception: many are exempt from building control, so nobody ever checked.
The short version
- Snow load is a designed-for quantity, but there is no national figure. It depends on where the house is, how high it sits and the shape of the roof.
- A conservatory at ground level with a floor area not exceeding 30m² is an exempt building. No inspector, and no point at which anybody checked it against a snow load.
- Drifting is the danger, not depth. Snow banked against the house wall above a conservatory is where the load concentrates and where things break.
- On a flat roof the outlet matters more than the roof. A frozen or leaf-blocked outlet turns light snow into standing water, and water is much heavier.
- Do not go up to clear it. Snow hides every edge, rooflight and fragile panel, and a conservatory roof will not carry a person at all.
The design side
Snow load is a number somebody worked out
Every roof built under the Building Regulations has had a snow load applied to it. Structure is Part A, and Approved Document A covers “the loadings on a building, and the construction of the structural elements including the foundations, walls, floors, roofs and chimneys”.
The number comes from Eurocode 1: BS EN 1991-1-3:2003+A1:2015, snow loads, read with its UK National Annex. It starts with the snow load on the ground, read off a map of the country and corrected for how high above sea level the site sits, then applies shape coefficients that turn ground snow into roof snow, separately for undrifted snow, drifted snow and exceptional drifts.
So there is no national answer. A house near sea level starts from a gentler figure than one at three hundred metres on the Pennines, and a steep pitch sheds snow where a flat roof and a step between roof levels collect it.
The useful question is not how much your roof can hold, but whether anything has changed since it was worked out. A parapet added, a wall raised alongside, a neighbour’s extension going up tight against your lower roof: each creates somewhere for snow to pile that the original calculation never saw, and nobody recalculates a roof when the thing next to it changes.
The conservatory
Your conservatory may never have been checked at all
Most British conservatories have never been near building control, because they never had to be. Schedule 2 of the Building Regulations 2010 lists exempt buildings and work.
Class 7 covers “the extension of a building by the addition at ground level of a conservatory, porch, covered yard or covered way”, or a carport open on at least two sides, “where the floor area of that extension does not exceed 30m2”. Regulation 9 then does the work: these Regulations “do not apply to the erection of any building or extension of a kind described in Schedule 2”.
Read that plainly. No building notice, no full plans, no inspector on site, and so no moment at which anybody applied Part A, or Eurocode 1, or a snow load, to the structure bolted onto the back of your house. Not because it is unsafe, but because nobody had to.
The exemption is conditional
Exempt does not mean anything goes. Three conditions sit inside Class 7 itself: the extension has to be at ground level, the floor area must not exceed 30m², and where a conservatory or porch is wholly or partly glazed the glazing still has to satisfy the safety glazing requirements in Part K of Schedule 1. Regulation 9 keeps two more things alive: Part P electrical safety, where the extension takes its power from a source shared with or inside the dwelling, and the water requirements in G1 and G3 where it takes a supply from the house. Exemption also says nothing about planning permission, which is a separate regime. What is exempt is the rest of Schedule 1, and the structural check sits in there.
What your conservatory carries therefore depends on the system the installer chose and on how well they fixed it back to the house. Some are properly engineered with published load data. Some are aluminium bar and polycarbonate put up to a brochure, and from the garden you cannot tell them apart.
Two things follow. If you are buying a house with a conservatory, ask for the paperwork, and if there is none that is normal, not alarming, but nobody can hand you a design. And if it is larger than 30m², or is not at ground level, Class 7 does not cover it and it should have gone through building control.
Where it goes wrong
Drifting is the danger, not depth
Failures happen where the snow is deepest, and it is never deepest where you measured it. Wind scours the open part of a roof and dumps what it picks up in the lee of anything standing proud.
The traps are always the same four: against a parapet, at a step where a lower roof meets a higher wall, in a valley, and against the house wall where a conservatory or lean-to abuts. The last one matters most, because that is where the structure is thinnest and where the fixings back into the house are doing all of the work.
So look at the drift, not the average. Half a metre banked up the wall above a conservatory roof is a different problem from the same half metre lying evenly across it. Depth alone tells you little anyway, because snow varies enormously in weight.
| Condition | Effect on the load | What it means for you |
|---|---|---|
| Fresh dry snow | The lightest case, mostly air | The depth looks alarming and rarely is |
| Snow left to settle | Packs down. Less depth, more weight per centimetre | A shorter drift is not a lighter one |
| Rain onto lying snow | Absorbed and held rather than running off | Often the rain after the snow is the problem |
| Refrozen snow and ice | Dense, and welded to what it sits on | It will not slide off a pitch, and it plugs outlets |
| Drift at an abutment | Far deeper than the general covering, over the weakest part | Where failures happen. Look here first |
| Meltwater on a blocked roof | The full weight of water, no air in it | 100mm of standing water is 100kg per square metre |
The signs that matter. A door or window in the conservatory that will not close. A new crack at a plasterboard joint. A dip in a roof line that was straight. Creaking. Water at the abutment with the house. Any of those and you get everybody out of that room, shut the door and ring somebody instead of going up to look.
Flat roofs
The outlet matters more than the roof
Most flat roofs that get into trouble in a thaw are not overloaded by snow. They are overloaded by water that has nowhere to go.
Water is far heavier than the snow it came out of, because snow is mostly air. A hundred millimetres of standing water is a hundred kilograms on every square metre of deck. The sequence never varies: the snow melts, the water runs to the outlet, the outlet is frozen or plugged with leaves, and the roof fills to the height of the upstand and stays there. The deck deflects under it, which makes a dish, which collects more.
BS 6229:2025, the code of practice for flat roofs with continuously supported flexible waterproof coverings, works in falls rather than depths and sets a minimum completed fall of 1:80. If yours ponds after ordinary summer rain it has nowhere to send meltwater in February, and September is a better month to deal with that than a thaw is. The covering matters too, and GRP, EPDM and felt differ most in how they cope with standing water.
All of the useful checks are ground-level ones. Clear the outlets, gutters and hoppers in autumn before anything freezes. In a thaw, watch the foot of the downpipe rather than the top of the roof, because if water is plainly coming off the roof and nothing is leaving the pipe it is still up there. And find out whether the roof has an overflow through the upstand for when the main outlet blocks. Older roofs often have none.
One thing surprises people. Snow lying on your roof longer than on your neighbour’s is good news about your insulation rather than bad news about your roof. A roof losing heat melts snow from underneath, and that heat loss is behind most of what British homeowners call an ice dam, which is very often condensation instead. Whether the insulation sits above or below the deck is the difference between a warm and a cold roof.
Safety
Do not go up there, and do not let anybody else go up either
The instinct is to fetch a ladder and clear it off, and that is close to the worst thing a homeowner can do.
Before you fetch the ladder. The Health and Safety Executive’s published position is: “Do not consider going on any roof in poor weather conditions such as rain, ice, frost or strong winds (particularly gusting) or if slippery conditions exist on the roof.” The same page puts a number on the wind: “Winds in excess of 23mph (Force 5) will affect a persons balance.” 23mph is a fresh breeze, not a gale, and it is far lower than anybody guesses.
A snow covered roof is the most slippery surface that roof will ever present, and it hides everything you would otherwise use to stay alive on it: where the roof stops and the gutter begins, the rooflight, the fragile panel in a garage roof. Roof work already accounts for almost one in five deaths in construction, and those are people who do it for a living.
A conservatory roof will not carry a person. Not the glass, not the polycarbonate, not the glazing bars, not just as far as the middle. Neither will a garage roof or any rooflight. Treat them all as fragile until somebody has proved otherwise, which on a conservatory nobody ever has.
Snow rakes, the long bladed poles you will read about, are an American product answering an American problem. If anything is cleared at all, and most winters nothing needs to be, it is done from the ground with a soft brush on a pole, standing out from under the eaves rather than underneath the load you are about to release. Nothing with a metal edge goes near a membrane or a polycarbonate sheet.
And do not let anybody else up there either. Somebody knocking during a cold snap offering to clear your roof today is telling you how they find work, and the doorstep version of this follows a familiar script. That applies to us as well.
Next
What to do, in the order it is worth doing
For most houses in most winters the honest answer is that you do nothing. What is worth doing is the autumn work, because on a flat roof it is the drainage that fails rather than the timber.
- Clear the outlets, gutters and hoppers in autumn, from a safe position, before the first hard frost.
- Note where drifts will collect: parapets, valleys, steps in the roof, and the wall above any conservatory or lean-to.
- If a flat roof ponds after ordinary rain, get the falls looked at now. Ponding in July is a warning about February.
- If a door stops closing, a roof line dips or something creaks, clear the room and ring somebody. Do not go up to check.
Snow rarely arrives on its own here. It comes attached to weather that has already been named, and a named storm and its warning colour tell you two different things. If something does come off the roof, the first 24 hours are mostly about what you do indoors, and the winter checks you can make from the ground catch most of it beforehand.
If you want a flat roof, or the abutment above a conservatory, looked at before the weather turns, that is part of what our roofing work covers. Thomas comes out and prices it himself, and he will tell you if it needs nothing. You can ask for a written itemised quote.
Questions
Common questions
How much snow can a flat roof hold?
No single figure says how much snow a flat roof can hold. UK roofs are designed to Eurocode 1, which starts from a ground snow load that varies with location and height above sea level, then adjusts it for the shape of the roof and for snow drifting against obstructions. A roof that drains freely and has nothing to drift against is a very different case from one with a parapet and a blocked outlet.
Is snow on a conservatory roof safe?
Usually, but nobody has ever checked. A conservatory added at ground level with a floor area not exceeding 30 square metres is an exempt building under Schedule 2 of the Building Regulations 2010, so it never went through building control. Watch the drift banked against the house wall more than the snow lying in the middle.
Should I clear snow off my roof?
Almost always no. The Health and Safety Executive says not to consider going on any roof in poor weather, and a snow covered roof hides every edge, rooflight and fragile panel. If a drift genuinely needs taking down, it is done from the ground with a soft brush on a pole, standing clear of the eaves.
Can I stand on a conservatory roof to clear the snow?
No, not the glass, not the polycarbonate and not the glazing bars. A conservatory roof is not a working platform in any weather and it will not carry a person. The same goes for a garage roof and for any rooflight.
What is the first sign a roof is overloaded?
Usually a door or window that will not close, a new crack at a plasterboard joint, a dip in a roof line that was straight, or creaking. Clear the room, shut the door and get advice. Do not go up to look and do not stand underneath it.
Read next
Related advice
The guides below pick up where this one leaves off, and the roofing page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
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