Almost every enquiry about a flat roof rooflight begins with somebody wanting to know what is up there, and a good proportion of those people are considering going up to look. This page sets out why that is a worse idea than it appears, and how the job is done properly.
A flat roof is not a floor
The word flat does a lot of damage here. A domestic flat roof looks like a surface you could walk on, and it was not designed as one.
A floor is designed for imposed load: people, furniture, and the concentrated point loads they produce. A flat roof is designed for its own weight, the covering, wind and snow. Access is anticipated for maintenance, not for occupation, and the joists are sized accordingly.
That does not mean a flat roof cannot be walked on. It means it should be walked on knowingly, by somebody who has established what is underneath, rather than confidently by somebody who has not.
What is underneath is the whole question
The covering tells you nothing about the deck. A sound looking bitumen or GRP surface sits equally well over eighteen millimetre plywood in good order and over the same plywood that has been quietly wet for a decade.
Water reaching a timber deck does not announce itself. It is held between the covering above and the vapour control layer or ceiling below, in the dark, at moderate temperatures, which is close to ideal for decay. The deck loses strength progressively while the surface above it stays intact.
The failure mode is a foot going through, and it happens without warning because there is no warning to give.
Where the weak spots are
| Location | Why | How it presents |
|---|---|---|
| Around an existing rooflight kerb | The most penetrated detail on the roof | Soft underfoot within a metre of the kerb |
| At the outlet | Standing water where the roof is lowest | Springy, sometimes visibly dished |
| At the abutment with the house | Upstands and flashings fail here | A dark line or a stain on the ceiling below |
| Under a pond | Continuous saturation | Tide marks, moss, and a soft feel |
| At the eaves edge | Least support, most exposure | Movement when weight is applied near the edge |
Every one of those is exactly where somebody surveying a rooflight needs to stand, which is the difficulty in one sentence.
Fragile coverings, and the rooflight or skylight itself
The most dangerous surface on a flat roof is frequently the rooflight, and this is not a small point.
An old polycarbonate dome, particularly one that has been in ultraviolet for twenty five years, is brittle. It looks like it would hold weight. It will not, and a fall through a rooflight is a fall to the floor below with the glazing coming after you.
Unless a unit is specifically specified and certified as non-fragile or walk-on, it is treated as fragile without exception. That includes glass units, because a domestic rooflight is designed for wind and snow rather than for a person, and the inner laminate holds a broken pane together rather than holding a person up.
Nobody stands on a rooflight, and nobody puts a ladder against one.
Edges, and the thing that actually kills people
Falls from height are the largest single cause of fatal injury in construction, and on a domestic flat roof the edge is the hazard rather than the height.
A first floor extension roof is high enough to kill. A single storey roof is high enough to cause a serious injury, and single storey roofs are where complacency lives. The perimeter frequently has no upstand, the covering is slippery when damp, and there is nothing to grab.
Working near an unprotected edge is controlled by keeping away from it, by physical protection, or by proper fall arrest. Care and concentration are not a control measure.
Wet, damp, mossy, or frosty
Every flat roof covering is more slippery than it looks, and several are treacherous under specific conditions.
- Algae and moss on a north facing or shaded roof are slippery when wet and stay damp for days after rain.
- Mineral finished bitumen sheds its granules with age, and loose granules on a smooth substrate behave like ball bearings.
- GRP with a sand broadcast finish grips well; GRP without one is slick when wet.
- Frost on any of them, and dew on a clear autumn morning, both make an ordinary roof unsafe.
Surveys get postponed for weather more often than people expect, and that is the right call rather than an inconvenience.
Getting up there is its own hazard
A ladder is involved in a large share of the accidents on domestic work, and the errors are consistent.
A ladder at the wrong angle, on soft or uneven ground, not tied at the top, not extending far enough above the landing point, or leaned against a gutter that is not designed to take it. On a single storey rear extension the ground is frequently a garden bed or a patio with a fall on it, and neither is a good ladder footing.
Where the roof is being worked on rather than looked at, a scaffold or a tower is the answer and it is priced in rather than avoided.
What a proper survey involves
- Assess from the ground and from a window first, and get as far as possible without setting foot on anything.
- Establish safe access, with a correctly footed and tied ladder or a tower.
- Read the surface before standing on it: tide marks, moss, patch repairs, tell-tale staining.
- Test underfoot progressively from the edge inward, staying over joists where their positions can be read.
- Treat every rooflight and every fragile surface as a hole in the roof, and stay off it.
- Take the twelve kerb measurements, and photograph the details rather than trying to remember them.
- Lift a small area at the perimeter where the build-up needs to be confirmed, and make it good.
Reading joist positions from above
Standing over a joist is considerably safer than standing between them, and joists can frequently be located without opening anything.
Deck fixings show as a faint regular pattern of dimples through most coverings once you know to look. Deflection between joists is visible as a slight scalloping across the surface in raking light. And the direction is usually predictable from the shape of the extension, since joists normally span the shorter dimension.
None of that is certainty. It is enough to bias where you put your weight, which is worth having.
What we bring rather than assume
A survey that involves going onto a roof is planned rather than improvised.
Access equipment appropriate to the height rather than whatever is in the van. Crawl boards where the deck is questionable, which spread load across joists rather than concentrating it between them. A moisture meter where decay is suspected. And a second person, because working alone at height on an unknown deck is the situation where a minor incident becomes a serious one.
Bungalows, and the roof that feels safe
A great deal of our flat roof work is on bungalows, which are everywhere along this coast through Ferring, Rustington, East Preston and Goring. A bungalow flat roof is low, reachable from a short ladder, and looks like the least hazardous roof there is.
It is not, particularly. The height is lower, so a fall from the edge is less likely to be fatal and entirely likely to be serious. The deck is exactly as unknown as any other. And the low height encourages people to step on and off from a ladder rather than establishing proper access, which is when a ladder moves.
The one genuine advantage is that a bungalow roof can frequently be surveyed almost completely from a stepladder at the edge and a good camera, without anybody having to stand on it. Where that is possible it is what we do.
Two storey extensions, and where it gets serious
At the other end, a flat roof at first floor level over a two storey extension or on a rear addition changes the risk category entirely.
The fall is now potentially fatal, the access is a proper ladder or a tower rather than a step up, and the edge frequently has nothing on it. Parapet roofs, common on the Brighton and Hove terraces, are the exception in that the parapet itself is edge protection, provided it is sound and provided it is high enough to be one.
On roofs at this height the survey is planned with the access arrangement decided in advance, and the cost of that access is in the quotation rather than being avoided by somebody taking a chance.
Asbestos, on older roofs
Buildings from before 2000 may contain asbestos, and there are two places it turns up around a domestic flat roof rooflight.
Some older bitumen products and roofing felts contained asbestos fibres, and so did some cement based surrounds and flues passing through flat roofs. Neither is common on a domestic extension and neither is impossible.
The rule is not to disturb anything suspected until it has been identified. Undisturbed material in good condition is not a hazard; cutting, drilling or breaking it is. Where we suspect it we stop and have it tested rather than working on regardless, and that is a delay worth accepting.
Why we survey rather than quote from photographs
People frequently offer to send photographs, and it is a reasonable offer that we take up for the first conversation and never for a quotation.
A photograph cannot show whether the deck is sound, whether the roof ponds, what the fall is, whether the kerb is square, or what the build-up is. It also cannot show the access route or the constraints on getting a unit to the roof. Quoting from one means quoting with the four largest variables unknown, and a price built that way moves once work starts.
A fixed price before the work begins requires knowing what the work is, which requires going and looking.
What a homeowner should do instead
There is a genuinely useful amount that can be established from ground level and from an upstairs window, and none of it involves a ladder.
Photograph the roof from every window that overlooks it. Note whether there are vents at the fascia. Note the covering type and its condition. Look for standing water after rain and photograph where it sits. Note the position of the outlet, and whether the existing rooflight is glass or a plastic dome. Look at the ceiling below for stains and note when they appear.
That is a genuinely helpful survey and it is entirely safe. What it is not is a substitute for the measured one, and it is better use of an hour than getting onto the roof.
Working on the roof, once the job starts
Everything above concerns the survey. The work itself is a different proposition because the roof is opened.
An open roof has a real hole in it, which is guarded and covered whenever it is not being worked in. The roof is never left open overnight, which is a rule that exists for weather and is also a safety rule, because an unguarded opening on a roof in the dark is a serious hazard. Materials are not stacked in one place on a deck of unknown capacity. And the opening is covered rather than merely marked whenever the crew is off the roof.
The condition we find most often
Across the rear extensions in this area, the most common finding at survey is a roof that is sound in the field and soft in one specific place, and that place is almost always the same: within a metre of the existing rooflight kerb, on the uphill side.
That is the confluence of everything on this page. It is where the roof is most penetrated, where water arrives and stands, where debris collects, and where the covering has been cut and rejointed. It is also exactly where the new kerb has to bear.
Finding it before quoting is the difference between a fixed price and a conversation on the day.
What the survey covers on a flat roof
We survey with proper access, treat every rooflight as fragile, test the deck before we trust it, and postpone for weather rather than working on a slippery roof. We open up a small area at the perimeter where the build-up matters, and we make it good.
Nothing on a rooflight job is worth a fall from a single storey roof.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.