Thinking About a Window for Your Flat Roof?

When customers ask me about adding a window to a flat roof, they’re usually really asking one of three things:

  • “Will it actually make the room brighter and feel bigger?”
  • “Is it going to leak?”
  • “What type of rooflight is best for my home?”

In my 18 years on the roof around Brighton, Hove and Worthing, I’ve fitted and repaired all sorts of flat roof windows and skylights – from simple fixed units over kitchens to walk-on glass on roof terraces. This page isn’t about me selling you a specific product. It’s my straight, on-the-roof advice on how to choose the right window for a flat roof, where to put it, and what to watch out for so you don’t end up with leaks or condensation problems a few winters down the line.

The Main Types of Windows for a Flat Roof (And When I’d Use Each)

When you say “window for flat roof”, most people actually mean one of these:

  • Flat rooflights (frame sits above roof, looks low profile from outside)
  • Lanterns (pitched, usually rectangular, lots of glass and light)
  • Domes (traditional plastic units, often on older extensions)
  • Walk-on glass (for balconies and roof terraces)
  • Access hatches (for occasional roof access, ladders etc.)

Here is my honest take on each, based on what I see working well in Sussex homes.

Flat Rooflights – The Modern, Clean-Looking Option

If you ask me, a good quality flat rooflight is usually the best all-round choice for most homeowners. They sit fairly low, look neat from outside, and give you a lot of natural light without stealing too much attention from the rest of the roof.

I tend to recommend flat rooflights when:

  • You’ve got a kitchen or dining room extension that feels a bit gloomy.
  • You want more light, but not a huge glass lantern dominating the space.
  • You’re trying to keep the roofline simple and easy to waterproof.

The key detail (which many people never see) is the upstand – the raised frame that the rooflight sits on. Get this wrong and you’ll have ponding water and leaks later. Get it right, and you’ve got a tidy, reliable window that blends into a modern flat roof nicely.

Lantern Roofs – Great Light, Bigger Statement

Lanterns are what many people picture when they think “window in a flat roof” – a glazed structure sticking up in the middle of the room, usually over a kitchen island or dining table.

I usually suggest a lantern when:

  • The room is quite deep and really starved of natural light.
  • You want a bit of a “wow” feature when you walk into the room.
  • You don’t mind seeing more of the structure from outside.

From a roofing point of view, lanterns are more fiddly. More angles, more joints, more potential for someone to cut corners. If your builder is doing the carpentry and I’m doing the roof covering, I always make sure we agree exact dimensions and upstand details before anyone starts cutting holes in the roof. That planning step often saves a lot of headaches later.

Domed Roof Windows – Practical, But Not to Everyone’s Taste

Plastic domes (often on older extensions or stairwells) are very practical: they shed water easily and give decent light. They’re not always the prettiest solution, but they do a job.

I normally only recommend a dome when:

  • We’re replacing an existing dome on a budget.
  • The roof is quite exposed and you want something very robust and simple.
  • You don’t really see the roof from the garden or upstairs windows, so looks aren’t a big concern.

If you’re refurbishing an older flat roof with an old, yellowed dome, it can be a good time to think about upgrading the roof covering at the same time. I can often combine a modern SBS felt system with an upgraded rooflight so the whole thing works together instead of being a patchwork of old and new.

Walk-On Glass – For Roof Terraces and Balconies

Walk-on units are a different game. Here you’re combining structure, safety and waterproofing in one package. These windows are heavy and need to be properly supported, but when they’re done right they can transform the room underneath.

I consider walk-on glass where:

  • You’ve got a terrace above a living room or kitchen.
  • You want to keep usable outdoor space, but still get light below.
  • You’re prepared for the higher cost and more involved installation.

On these projects I always pay particular attention to how the window frame, waterproofing layer and any decking or tiles all meet. This is where most of the leaks I get called to fix tend to start, especially on balconies and roof terraces around Brighton and Hove.

If you’re planning a balcony or terrace at the same time as a roof window, it’s worth looking at how I approach balcony and roof terrace waterproofing, because the thinking is very similar.

Choosing the Right Size and Position for Your Flat Roof Window

Once you’ve picked the general type of window, the next big questions are: how big, and where?

Here’s how I normally talk this through on a site visit.

How Big Should a Flat Roof Window Be?

Bigger isn’t always better. In my experience you get the best results when the size and shape of the window match the way you actually use the room.

Things I look at include:

  • Room size and layout – A long, narrow kitchen often suits two smaller rooflights spaced along the worktop, rather than one huge one in the middle.
  • Furniture position – There’s no point putting a window exactly where you’ll later want ceiling lights, a fan, or a range hood.
  • Heat and glare – Over glazed isn’t fun in July when the room turns into a greenhouse.

As a rough guide, many people aim for roof glazing equal to about 10–20% of the floor area, but I always adjust that based on how bright the room already is and what direction the garden faces.

Where to Position a Flat Roof Window

From a roofing point of view, some positions are simply better than others. Here’s what I try to avoid when I can:

  • Too close to parapet walls or upstands – These junctions are already busy. Moving the window in a bit usually gives a more reliable job.
  • Too close to existing vents, pipes or other roof penetrations – Again, less clutter usually means fewer leak risks.
  • Directly over high-moisture spots (showers, cookers) without planning for ventilation and extraction.

Inside the room, I try to line the window up with natural focal points – like above a dining table, kitchen island or stairwell. That way the light actually lands where you feel it most, not just in a random patch of floor.

Fixed vs Opening Flat Roof Windows

Another decision people often don’t think about until late is whether the window should open. Here’s how I tend to break it down with customers.

When a Fixed Rooflight Makes Sense

A fixed unit is usually simpler, more secure and a bit cheaper. I’ll normally recommend fixed when:

  • You already have decent background ventilation in the room.
  • The main goal is natural light, not airflow.
  • The window would be high or awkward to operate anyway.

Fixed windows generally have fewer moving parts to go wrong. From a waterproofing point of view, that’s always a good thing.

When an Opening Roof Window is Worth It

On the other hand, in some rooms an opening unit is a game-changer. I often suggest an opening rooflight when:

  • The room is a kitchen, bathroom or utility with lots of steam.
  • You don’t have many other opening windows in the space.
  • You want to help hot air escape in summer.

Most modern opening rooflights can be fitted with electric motors and rain sensors, but you need to think about wiring routes and control switches early – ideally before the plastering and decorating stage. I’ve seen too many projects where the roof is finished nicely but nobody planned where the cables were going, so the electrician has a nightmare later.

The Hidden Part: How the Window and Flat Roof Work Together

From the inside, you mostly see glass and plaster. From the outside, you mainly notice the frame. But the really important bit – and the part my job focuses on – is how the window, upstand and roof covering are all tied together.

Upstands: The Bit That Keeps the Water Out

Every good flat roof window sits on an upstand. This is basically a raised frame that lifts the window above the main roof surface. Done properly it:

  • Keeps water from pooling against the frame.
  • Gives the roof membrane something solid to wrap up and over.
  • Helps the window meet the manufacturer’s warranty conditions.

I always make sure upstands are tall enough, square and properly fixed, and that the roof’s falls (slopes) are directing water away from them, not towards them. It sounds simple, but I spend a lot of my time fixing jobs where that basic planning was ignored.

Insulation and Condensation Around Flat Roof Windows

In my 18 years, one of the most common complaints I hear isn’t actually leaks – it’s condensation: water droplets forming on the inside of the glass or around the reveals.

To reduce that risk, I pay attention to:

  • Insulating the upstand and reveals – Not leaving cold timber or plasterboard bridges.
  • Using decent double or triple glazing with warm edge spacers.
  • Ventilation and extraction – especially in kitchens and bathrooms.

If you already suspect you’ve got a leak or condensation issue around a roof window, you can always book a free roof inspection and I’ll tell you honestly whether it’s a roofing problem, a ventilation issue, or a bit of both.

Planning a Flat Roof and Window Together

Where possible, I prefer to plan the new roof covering and the window at the same time. That way everything is designed as one system, instead of cutting a hole into an existing roof and hoping for the best.

Why the Roof Covering Matters for Your Window Choice

The type of flat roof covering you have – felt, single-ply, GRP, asphalt – affects how the window is detailed. I specialise in modern SBS torch-on felt systems, which work very well with most modern rooflights when the upstand and flashing details are done properly.

If you’re thinking about replacing an older flat roof at the same time, it’s worth having a look at how I handle flat roof repairs and replacements in Sussex. Adding a new roof window is often much easier and more reliable when it’s done as part of an overall roof upgrade rather than as an afterthought.

Windows for Dormers, Porches and Smaller Flat Roofs

Not every flat roof window is over a big kitchen extension. I often get asked about adding light to:

  • Dormer flat roofs – to brighten up loft rooms or stairwells.
  • Small porch roofs – to stop halls feeling dark and enclosed.

On dormers, it’s really important that the flat roof, vertical cladding and existing pitched roof all work together neatly. If you’re already looking at a loft or dormer roof, my flat dormer roof service page explains the way I approach those junctions.

For porches, a smaller rooflight can make a big difference without too much extra cost, especially if we’re redoing the porch roof covering anyway. You can see how I price those roofs on my porch flat roof guide.

Common Mistakes I See with Flat Roof Windows

When I’m called out to look at problems around roof windows, the same issues come up again and again. If you’re planning a project, watching out for these can save you a lot of hassle later.

  • Upstands too low – The window frame ends up sitting almost flat on the roof, so water and debris build up around it.
  • No proper fall – The roof is dead flat, so water just sits around the window and slowly finds its way in.
  • Mixing trades without clear responsibility – Builder fits the window, roofer is expected to somehow waterproof around it after the fact.
  • Ignoring manufacturer details – Cutting corners on flashing kits or recommended upstand sizes to save a bit of money.
  • Not thinking about maintenance – Windows placed where you can’t safely clean the glass or clear debris.

My approach is simple: I take responsibility for the waterproofing details around any roof window I work on, and I only sign off on a job when I’d be happy to have the same setup on my own home.

How I Work When You Ask Me to Install a Window in Your Flat Roof

If you contact me about adding or replacing a window in your flat roof, here’s what typically happens:

1. A Straightforward Site Visit

I come out, have a proper look at your roof and the room below, and we talk through what you actually want: more light, better ventilation, access to the roof, or all three. I’ll measure up, check the current roof covering and insulation, and look for any existing issues that could affect the new window.

2. Clear, Fixed-Price Quote

After the visit, I give you a fixed-price quote covering:

  • Supply of the rooflight (or fitting a unit you’ve already chosen, if that’s what you prefer).
  • Building and insulating the upstand.
  • All roof waterproofing details around the window.
  • Any local roof repairs or upgrades needed to do the job properly.

No “day rate” guessing, no open-ended extras. If you want to get a feel for general flat roof pricing before we talk, you can use my garage roof cost calculator – it’s not specific to windows, but it does give a realistic idea of how I price smaller flat roof areas.

3. Full Transparency While I’m on the Roof

Once you’re happy with the quote and date, I book the work in and you get access to my online client portal. There you can see:

  • The agreed schedule.
  • Daily photo updates from the roof.
  • Your invoices and any warranty documents.

That way you don’t have to climb ladders or guess what’s going on above the ceiling – you can see every stage of the window and roof detail being built.

Not Sure Which Flat Roof Window Is Right for You?

If you’re in Brighton, Hove, Worthing or nearby and you’re unsure what type of window would suit your flat roof, my honest advice is: let me have a proper look. Every roof and room is slightly different, and it’s much easier for me to give you clear, sensible options once I’ve seen the layout, the existing roof build-up and how you use the space.

You can book a free roof inspection in Sussex, and I’ll walk you through the pros and cons of each option on your specific roof – no pressure, no sales patter, just the same advice I’d give a friend or family member.