Short answer: choose a roof lantern when the room is deep, the ceiling is low and you want the middle of the room to feel taller and brighter. Choose a flat rooflight when you want a calm roofline, fewer joints to look after, or the roof has little height to spare. Both are rooflights under the Building Regulations, and both can be fitted to last.
What each one does to a room
A roof lantern rises above the flat roof in a pitched frame of glass. From inside, the ceiling opens upward into that frame, which adds height over the part of the room beneath it. Because its glass slopes on two or four sides, it takes light from a wide stretch of sky, including low morning and evening sun, and spreads it across a long kitchen or dining space.
A flat rooflight is a single glazed unit lying almost level on a kerb. From inside it reads as a clean, bright rectangle in an unbroken ceiling. It gives strong, even daylight straight down, and the view up is a simple frame of sky. In a narrow room, a landing or a bathroom, that is often all the light the space needs, and a lantern would crowd it.
How they compare over the years
This is where the two differ most, and the part we go through carefully at survey. A flat rooflight has one frame, one glass unit and a short run of seals. A lantern has a base frame, glazing bars, a ridge, top caps, end caps and a sealed unit in every bay. More joints means more places for a gasket to age and more small parts to look at in later years.
| Question | Roof lantern | Flat rooflight |
|---|---|---|
| Best room shape | Deep, open-plan | Narrow, compact or long and thin |
| Roofline | Raised and visible | Low and quiet |
| Parts that wear | Gaskets, caps, several glass units | One glass unit, frame seals |
| One pane fails | Often that pane alone, from above | The glass unit or the whole light |
A lantern is not the weaker choice because it has more parts. On a well-made aluminium lantern, a single misted pane can usually be lifted out by releasing the caps either side and a new unit set in, with the roof left untouched. A flat rooflight with bonded glass may have to come off its kerb whole when its unit fails. So the better question is how each part is replaced, and the life plan in your quote answers it for the exact product we propose.
Planning and the regulations
For the energy rules, both are rooflights. The 2021 edition of Approved Document L holds rooflights fitted to existing homes to a U-value no higher than 2.2 W/m²K (its Table 4.2), and a lantern is judged against that figure just as a flat unit is. Cutting a new opening in the roof is structural work, so either one needs Building Regulations approval when it goes into an existing roof, and we handle the Building Control notification.
Planning can separate them. Permitted development for a house allows rooflights that stand no more than 0.15 metres above the roof plane and no higher than the top of the roof. A low flat rooflight is far more likely to sit within that allowance. A lantern often stands higher, so it may need planning permission depending on its height and the house. We check your address and the design at the survey. For the full list of conditions, see rooflights, planning and Building Regulations.
Heat, glare and fresh air
A lantern has more glass for its footprint, so it lets in more light and, on a sunny day, more heat. Facing south over a kitchen, that can mean a warm room by mid-afternoon in summer. Coated solar control glass cuts that heat gain without dimming the room much, and vents near the ridge let hot air out at the highest point.
A flat rooflight has less glass and less solar gain for the same opening. If it opens, it vents at ceiling level. Where a room already gets sun through glazed doors, a flat rooflight can give the light without adding as much warmth.
Height above the roof and the view from upstairs
A lantern changes the look of the back of the house from the garden and from the windows above it. Some owners like that; it reads as a feature. On a terrace or semi where a bedroom window looks straight down onto the extension, some prefer the quieter line of a flat rooflight. If the extension roof sits close under a first-floor sill, the height of a lantern may also be limited by what fits.
What drives the price of each
We do not quote figures without seeing the roof, but the reasons behind them are simple. A lantern has more frame, more glass units and more assembly on the roof than a flat rooflight over the same opening, so for a like-for-like footprint it is the larger job. For either one, the bigger costs often sit outside the glass: whether joists must be trimmed, whether the upstand is new, and how the covering is joined.
Which do we suggest?
We do not lean either way before seeing the room. As a starting point, a deep kitchen or orangery that wants a focal point usually suits a lantern, and a narrow galley, a bathroom or a room under a low parapet usually suits a flat rooflight. Some rooms take two flat rooflights better than one large lantern.
The free survey ends in a fixed written quote for the option you choose, with its life plan, so you can see what you are buying and how it will be looked after. Read more on our pages for roof lanterns and flat roof rooflights.