Does a walk-on rooflight or skylight need an engineer?
For the opening and the structure carrying it, almost always. For the glass itself, the manufacturer does the design, working from the span, the support conditions and the loading you give them. What an engineer is doing is the part beneath: making sure the trimming and the deck carry floor loading rather than roof loading, which is a considerable difference.
Two designs, not one
The panel and the structure.
A manufacturer specifies the glass build up for a stated span and load. An engineer specifies what the panel sits on and how that load reaches the walls.
Both are needed and they are done by different people.
Why the structure is the harder half
Because floor loading is much larger than roof loading.
A roof is designed for wind, snow and access for maintenance. A terrace is designed for people, furniture and everything a floor carries.
What that means for an existing roof
It probably will not do. A flat roof deck built as a roof is not automatically adequate as a terrace, and turning one into the other is a structural exercise rather than a covering one.
That case is on can a walk on rooflight be retrofitted.
What the trimming has to do
Carry the panel and what stands on it.
The members around the opening take the weight of a very heavy glass unit plus the imposed load on it, and hand that into the surrounding structure.
Where a steel appears
On anything but a small opening. Beyond a modest span, doubled timbers are not enough and a designed steel or engineered timber member is what carries the opening.
Deflection of the structure
As important as its strength. Glass does not tolerate the supports moving unevenly beneath it, so the structure is designed to a deflection limit as well as a strength one.
Why that limit is tighter than usual
Because glass is brittle. A timber floor can flex a little without complaint. A glass panel bearing on a frame that flexes gets loaded at its corners, and that is where glass breaks.
Level bearing
The detail that follows from it. The frame has to be set level and true, on a structure that will stay level, because the panel is rigid and unforgiving of a support that is not.
What the manufacturer needs from you
Four things. The clear span in both directions, whether it is supported on two edges or four, the imposed loading required, and whether the panel is insulating a heated space.
Support on two edges or four
It changes the specification substantially.
A panel held on all four sides is far stiffer than the same panel held on two, so the frame design and the glass design are settled together.
The consequences are on what is the maximum span for walk on glass.
The loading to specify
Domestic or something more.
A private terrace and an area with public access carry different design loads, and specifying the lower one where the higher applies is the mistake to avoid.
The figures are on how much weight can a walk on rooflight take.
Where a specialist assesses it
On a basement or a lower ground conversion.
Those projects generally have an engineer involved already, and the walk on panel is one element in a scheme rather than a job on its own.
Whether a supplier will design the structure
Generally not, and they should not.
A glass manufacturer designs glass. What the panel sits on is a matter for somebody who has seen the building and knows what is beneath it.
Where building control comes in
They check the design rather than write it.
Calculations are submitted, the trimming is inspected before it is covered, and the glazing specification is checked against what is required overhead.
What applies is on does a walk on rooflight need building regulations.
What a full plans application gives you
The details checked before anybody starts.
On work with a designed beam and a heavy glass panel, having the design agreed in advance is more comfortable than a building notice.
What it costs
A fee, and it buys certainty.
An engineer’s fee on a walk on installation is modest against the glass and the structural work, and it removes the largest risk in the job.
Where it sits is on how much does a walk on rooflight cost.
What happens without one
A panel on a structure nobody checked.
The glass will usually be fine and the deck beneath it may not be, and a floor that deflects under load will eventually take the panel with it.
Where a standard unit still needs checking
Because the rating is for a size.
An off the shelf panel is rated for a stated span, and putting one over a larger opening removes exactly the margin that made it safe.
Why is on is walk on glass safe.
Waterproofing is not their job
Worth separating.
An engineer designs the structure. The junction between the terrace waterproofing and the frame is a roofing detail and it is designed by whoever is doing that.
Why it matters is on do walk on rooflights leak.
What to keep afterwards
The calculations and the specification. They are what tells a future owner, a surveyor or a contractor that the panel is genuinely load rated and what it was designed for.
Who we involve
We involve an engineer on any walk on installation over an existing structure, because the panel is rarely the risk and what it is standing on almost always is.
The whole picture is on the walk on rooflight installation page.
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.