Roof Lantern or Flat Rooflight?

What each one actually is

A roof lantern is a glazed box with a pitched or hipped top, usually in aluminium or timber with glass on all slopes, standing on an upstand (a low raised edge) built around the opening in the roof. It rises into the room as a vaulted ceiling feature. A flat rooflight, sometimes called a flat glass rooflight or a lay light, is a sealed double or triple glazed panel sitting on a raised kerb, with a very gentle slope or none at all, so the glass is close to the plane of the roof. Both sit over a framed opening in a flat roof, and both rely on the same kerb and weatherproofing detail at the base.

Our wider guide to flat roof rooflights covers kerbs, glazing choices and fixing in more depth, so this page keeps to the comparison.

Light, headroom and the feel of the room

A lantern gathers light from several faces, so the room gets changing light through the day and a bright central zone rather than a single patch. The extra height gives a sense of space and lets warm air rise away from head level. The price is that more glass faces the sun, and a south-facing lantern over a kitchen diner can get hot without solar control glazing or blinds.

A flat rooflight delivers a clean sheet of daylight that suits a corridor, bathroom, utility or a deep extension where the middle gets dark. It is less showy but easier to control. On a wide span, a run of two or three rooflights can spread light evenly. Its glass is closer to horizontal, so it collects more dirt and bird mess and needs wiping more often than a steep pane.

Heat, performance and regulation

Glass loses heat faster than an insulated roof, so the more of it you add, the more the glazing specification matters. For an existing dwelling in England, Part L sets a limit of 2.2 W/m²K for rooflights, so look for the figure on the data sheet. A lantern has more joints and a bigger frame, so its overall performance depends heavily on a thermally broken frame and good glass. Solar control coatings, which cut the sun's heat gain, are worth considering for any glazing facing south or west.

Both options are works that building control may need to approve, because they alter the structure and the roof's insulation. A lantern is heavier, so the opening needs a stout supporting frame, while a flat rooflight needs the kerb built and insulated carefully so it is not a cold bridge. Our page on roof window building regulations explains what building control checks. Planning is separate: a lantern projects well above the roof, so ask the local planning department what it will accept before you order.

Cost, maintenance and which to pick

As a rule, a lantern costs considerably more than a flat rooflight of a similar footprint, because it is a larger fabricated structure with more glass, more framing and more labour to fit. A flat unit is the lower-cost route and the neater choice where you want light without changing the outline of the roof. Both need the surrounding roof covering, whether EPDM, GRP or felt, to be tied in properly to the kerb, and a failure there is the commonest source of leaks.

A lantern suits an open-plan kitchen or a room where you want a focal point. A flat rooflight suits narrow spaces, bathrooms and anywhere restrained looks matter, such as a conservation area. If the room is a loft or a pitched slope instead, a standard opening window may suit better, which is covered in roof window types.

Where each one goes wrong

The failures are rarely in the glass. On a lantern, the weak points are the ridge and hip joints, the sealing at the foot of the frame and the condensation channels that carry moisture out of the aluminium sections. If those are blocked with dirt or poorly sealed, water or damp collects at the bar joints. On a flat rooflight the usual trouble is the kerb: an upstand that is too low lets the roof covering peel away from it, and insulation that stops short of the glass edge leaves a cold line where condensation forms. Either way, a regular wipe of the glass and a look at the seals once a year keeps small problems small.

We survey the opening, check the structure and photograph the roof so you can see what will be involved, then give a fixed written quote. Start from the roof windows in Basingstoke page if you want to see the full range of work.