Why an existing flat roof is not ready as it stands
Most domestic flat roofs in Cambridge were built to keep the rain out and carry snow and the odd maintenance visit, not to be walked on every day. A walk-on skylight changes what the roof is for, and the roof has to change with it.
Think of the single-storey rear extensions behind the gault brick terraces in Romsey and off Mill Road, or the flat-roofed garage and kitchen additions on 1930s semis in Cherry Hinton and Chesterton. Their joists were sized for a non-accessible roof. The deck is often chipboard or plywood, laid to a gentle fall towards a gutter or outlet, and finished in felt, single-ply membrane or glass reinforced plastic. None of that was designed for a table, four chairs and a group of people standing around on a warm evening.
Fitting walk-on glass into that roof is really two jobs. One is forming an opening that the glass can bear on evenly. The other is turning the roof around it into a surface people can use safely, which is the part owners tend to underestimate. The walk-on rooflight installation hub covers the whole service; this page looks at the retrofit case only.
The structure: joists, trimmers and the engineer
Cutting an opening means removing lengths of joist, and the load they carried has to go somewhere else. On an accessible roof that load is larger, so the numbers need working out properly rather than guessed.
In a typical retrofit the joists are cut back and new trimmers are fixed across the ends to pick them up, with trimming joists either side of the opening taking the extra load down to the walls. Whether the existing joists can take the higher loads of a terrace at all is a separate question, and on an older extension the answer is sometimes no without strengthening. Where the opening runs close to a wall or a steel beam, the bearing details change again.
These are decisions for a structural engineer, not for an installer or a product brochure. The engineer sizes the trimmers, checks the existing joists and bearings against the terrace loads, and specifies how the glass frame is supported. The page on when walk-on glass needs a structural engineer explains what they look at, and the guide to the structural opening for walk-on glass covers the level, square bearing the glass itself asks for.
Falls, levels and the terrace build-up
A flat roof is never quite flat: it falls so water drains away. Walk-on glass wants to sit level and flush with the surface around it, so the finished terrace is usually built up on top of the old roof rather than walked on directly.
The usual approach keeps the waterproof layer where it does its job, below the walking surface, and lays a level deck above it. Paving on adjustable pedestals or a raised deck lets the finished surface come up level while the membrane underneath keeps its falls and drains to its outlets. The glass is then set so its top sits flush with that finished surface, while its own kerb and weathering still rise above the membrane beneath. From the terrace you see a flat plane of glass and paving; underneath, water is still running away from the unit.
Each layer adds height. The existing roof, any new insulation to bring it up to current thermal standards, the membrane, the pedestals and the paving together can raise the finished level noticeably, and that affects everything that meets it:
- the threshold of the door or window you step out through, which may have to be lowered, raised or replaced;
- the height of any parapet or guarding, measured from the new finished surface;
- the flashings and upstands against the house wall, which need enough height above the membrane to stay watertight;
- the outlets and gutters, which must stay reachable for clearing.
Water around flush glass needs particular care, because there is no raised upstand throwing it clear. The guide to drainage around flush walk-on rooflights goes into the detail, and walk-on rooflights for roof terraces looks at keeping the terrace usable once the glass is in.
Safety, rules and permission
Turning a roof people rarely visit into one they use changes the regulations that apply. It is the terrace, as much as the glass, that brings them in.
Building Regulations cover the structure, the thermal performance of the roof and the new glazing, and guarding wherever people can reach an edge. An accessible roof normally needs a guard at the edges, and the glass itself must carry the loads it is rated for. Where the work is notifiable we handle the Building Control notification as part of the job. The guide to Building Regulations for walk-on rooflights sets out the parts that apply, and whether a walk-on rooflight needs a guard rail covers guarding specifically.
Planning is a separate test. Adding glass to an existing flat roof can be modest, but creating a new roof terrace, balcony or raised platform is generally not something permitted development allows, because of overlooking. In Cambridge conservation areas such as Mill Road, De Freville Avenue or Newnham, the council will also weigh how the guarding looks from the street and from neighbours' gardens. Check with Cambridge City Council or South Cambridgeshire District Council before committing; the page on whether a walk-on rooflight needs planning permission explains where the lines usually fall.
What the light will do in the room below
Horizontal glass under open sky takes in more daylight than a window of the same size, and in summer it throws a hard, bright pool of sun onto the floor below. On a retrofit, where the opening often has to fall between existing joists or walls, that pool will not always land where you would have chosen.
That is why we plan the position before the roof is opened. At the survey we look at where the sun sits over the roof at different times of year, which part of the room below is used most, and whether a sandblasted or fritted finish would spread the light more softly. A finish that improves grip on top also diffuses the light underneath, and it adds privacy when people are standing over a kitchen or bathroom. See glare and pools of sunlight below walk-on glass for how the patch moves through the day. Every quote comes with a written light plan, so you can see the reasoning before anything is cut. You can read more about the method on how we plan the light.
Questions people also ask
Can I just swap my existing flat rooflight for a walk-on one?
Rarely as a straight swap. A standard flat rooflight sits on a raised upstand, while walk-on glass needs a level, flush setting with continuous edge support and a surrounding surface people can stand on. Replacing one with the other normally means reforming the opening and building up the roof around it.
Will the existing waterproofing have to be replaced?
Often it is worth renewing, because once paving or decking goes on top it becomes hard to reach. The condition, age and type of the existing covering, and how it will meet the new glass kerb, decide whether it stays. A survey of the roof answers that.
How heavy is the glass the roof has to carry?
Walk-on units are much heavier than standard flat rooflights because of their thick laminated top pane, and the terrace adds its own weight on top. Figures vary by product and size, so the engineer works from the maker's data. See how heavy a walk-on rooflight is for the general picture.
Does fitting walk-on glass take longer on an existing roof?
Usually, because the structural work and terrace build-up come before the glass goes in, and each depends on the roof you have. The installation itself is only one stage of the programme; the guide to installation time sets out the stages. If you would like the roof looked at, book a light survey.
