Flat Roof Windows, Rooflights & Lanterns: What's the Difference & Which One do I Need?

If you’re looking to bring natural light and/or ventilation into a flat roof extension, garage or other area with a non-pitched roof, the first thing you’ll notice is that the industry can’t decide what to call the products.

One website says "skylight," another says "window," and your builder might just call it a "rooflight". Often you’ll see all 3 on the same page.

But are they really different? And how do you go about selecting the right type of flat roof glazing product? Answers below.

In this article:

Difference between flat roof window, rooflight & skylight explained

5 main types of flat roof glazing

Glazing materials: glass vs. polycarbonate

Examples of common problems & most suitable flat glazing option

Rough guide to flat roof window costs

Is There a Tangible Difference Between a Flat Roof Window, Rooflight and Skylight?

In the context of flat roofing, the difference between these three is mostly linguistic:

• Rooflight: The technical industry term. If you’re talking to a builder or architect, this is the word they’ll probably use for opening or fixed flat roof glazing.
• Skylight: A common consumer term. In the UK, it generally refers to any glazed unit on a roof.
• Roof window: Technically a roof window is a unit built to window standards (EN 14351-1). While often associated with pitched roofs, many flat roof "opening" units fall into this category.

In reality: On a flat roof, these all usually refer to a flat, glazed unit that sits on a raised frame (upstand or kerb).

The Five Main Types of Flat Roof Glazing

Now the general names are cleared up, you need to know the physical style of different units. Each one solves a slightly different structural or aesthetic problem:

Flat Glass Rooflights/Windows/Skylights

These are contemporary units usually consisting of a single, flat pane of glass (though double/multi units are also available) and they can open or be fixed. If opening, they usually come with the option for manual or electric operating systems. They sit low to the roofline for a sleek, minimalist look and are generally considered the "gold standard" for modern kitchen extensions.

Roof Domes (Polycarbonate or Polycarbonate + Glass)

A flat roof dome is a curved unit usually made from high-impact plastic (like polycarbonate). The curved shape is functional; it allows rainwater to run off easily and provides excellent light diffusion. These are often the most cost-effective and durable options for utility rooms or hallways.

Roof Lanterns (or Pyramid Skylights)

Unlike a flat unit, a roof lantern is a 3D "pitched" structure that sits proud of the roof and looks like a small glass pyramid. These are designed to be more of an architectural feature, adding height to a room and catching light from all angles. They also allow excellent water-run off given their pitched construction.

Access Hatches

If you need to get onto your roof for maintenance or as a fire escape, you need an Access Hatch. These look like standard flat rooflights but are engineered with heavy-duty hinges and gas struts that allow the unit to open to 90°, allowing a person to climb through safely.

Walk-on Rooflights

In some projects, the roof of an extension might also be a usable balcony or terrace. In these cases, you need Walk-on Glass. This is significantly thicker, reinforced glass designed to support the weight of people walking across it without compromising the light in the room below.

Flat Roof Glazing Materials: Glass vs. Polycarbonate

Your choice of material affects the price, the sound insulation and the look of the project:

• Glass: Offers the best clarity and thermal performance and is the most common glazing material in flat roof windows/rooflights. It’s heavier and more expensive than plastic but it’s also much quieter during heavy rain. This makes it ideal for living areas like kitchens and bedrooms.
• Polycarbonate: A lightweight, virtually unbreakable plastic. It’s more affordable than glass and very easy to install. While it can be noisier in the rain, it’s still a brilliant, practical choice for garages, outbuildings or budget-conscious renovations. As mentioned above, domed rooflights are usually made from polycarbonate.

Which Flat Roof Glazing Product do you Need?

Here are some suggestions based on common real-world projects and goals:

I want to bring natural light into a single-storey extension

Best option: Fixed flat glass rooflight

• If light is the only consideration, this is the best option (otherwise see below re: ventilation)
• Choose glass for a clean look and better thermal/sound performance

I need ventilation in a bathroom, kitchen or hallway

Best option: Manual or electric-opening flat rooflight

• Electric versions are ideal for hard-to-reach ceilings
• Choose models with rain sensors or thermostatic controls if needed

I need to access a roof terrace

Best option: Roof access hatch

• Designed for occasional foot traffic
• Includes reinforced frames, assisted opening and secure locking

I want a visual centrepiece for an open-plan room

Best option: Roof lantern

• Ideal for large kitchen diners or vaulted ceiling extensions
• Requires structural consideration and parapet clearance

I’m replacing an old dome or rooflight

Best option: Like-for-like sizing in polycarbonate or glass

• Match dimensions to existing kerb if reusing
• Where relevant, consider upgrading to glass for better insulation and appearance

How Much do Flat Rooflights & Other Glazing Cost?

Flat rooflight prices can vary widely based on size and whether the unit is fixed or opening. Larger sizes and premium materials and features can cost significantly more. But the price ranges below are a rough guideline for base glazing units in popular sizes (incl. VAT but excluding delivery or installation):

• Fixed Polycarbonate Dome: £180 – £500
• Fixed Flat Glass Rooflight: £450 – £1,500
• Opening (Electric) Rooflight: £800 – £2,400
• Roof Lantern: £1,000 – £3,500
• Access Hatch: £700 – £2,000

A Note on Installation: Upstands and Pitch

Every flat roof window needs two things to function correctly:

1. An upstand (kerb): The window can’t sit directly on the roof material. It must sit on a raised frame (usually 150mm high) to keep it clear of standing water and snow.
2. Drainage pitch: Even "flat" glass units should be installed at a slight angle (usually 3–5 degrees) so that rainwater runs off. If it’s perfectly flat, water will pool and leave dirt marks on the glass.

A lot of units are sold with an upstand as an extra so look out for it when you're shopping around (if you're replacing you might be able to keep the exisiting one - check with your installer if you're not sure).

Where to next?

If you're still not sure which flat glazing unit is going to be best for your specific project, give our friendly customer team a call or drop them an e-mail and they'll be happy to discuss your options. Or keep exploring more ideas and products below.