Can You Add a Skylight to an Existing Roof?

January 2026

Adding a skylight to an existing roof is not only possible, but in many cases it is one of the most effective ways to transform a flat roof extension. For homeowners who have extended at the rear of their property, skylights can restore lost daylight, improve ventilation, and reconnect old and new spaces into one bright, liveable environment. 

In this guide we focus specifically on flat roof extensions, explaining what’s possible, how different roof coverings affect the process, and which products are best suited for low-pitch and flat roofs. Explore our modern solutions for natural light in flat roofs here.

Why Add a Skylight to a Flat Roof Extension?

When a rear extension is added, the original external wall of the house becomes internal. This often means: 

• The new extension is bright and open

• The existing room behind becomes dark and gloomy 

Natural daylight typically only travels up to 4 metres before it significantly deteriorates. As a result, the original part of the home can feel darker like a cave. 

A flat roof skylight or lantern solves this problem by: 

• Introducing daylight from above

• Illuminating both the new extension and the original house

• Creating a stronger visual connection between old and new spaces 

In short, skylights are one of the most effective daylighting solutions for flat roof extensions. 

Can a Skylight Be Added to an Existing Flat Roof?

Yes, in most cases, a skylight can be retrofitted into an existing flat roof. However, feasibility depends on several critical factors: 

• Roof structure and joist layout

• Roof pitch (even “flat” roofs often have a fall)

• Roof covering type (felt, GRP, or EPDM)

• Size and position of the proposed opening 

Because cutting an opening affects the structure, a professional assessment is essential. A structural engineer or experienced roofing contractor will determine whether trimming joists or adding support is required.

Understanding Flat Roof Coverings

Adding a Skylight to a Felt Roof

Traditional felt roofs can accept skylights, but detailing is crucial. 

Key considerations: 

• A timber upstand is formed above roof level

• Felt is dressed and bonded up the upstand

• Proper flashing and weathering are essential

• Felt roofs are often found on older extensions, so structural checks are especially important.

Adding a Skylight to a GRP (Fibreglass) Roof

GRP roofs are well suited to skylights because: 

• They form a seamless waterproof finish

• Skylights can be fully laminated into the roof surface

• Long-term durability is excellent 

GRP skylight installations must be carried out by experienced installers to ensure correct curing and waterproofing. 

Adding a Skylight to an EPDM Roof 

EPDM rubber roofs are one of the most common modern flat roof systems. 

Important points: 

• Skylights require proprietary EPDM-compatible trims

• The membrane is sealed around the upstand

• Manufacturer-approved details should always be followed 

When installed correctly, EPDM skylights offer excellent longevity and watertight performance. 

Roof Pitch: Why It Matters

Most flat roof skylights are designed to work on roof pitches between 0° and 15°. This allows rainwater to drain correctly and prevents pooling on the glass. 

However, some roofs are: 

• Shallow pitched but tiled or slated

• Below 15° but not suitable for standard flat roof skylights 

In these cases, low-pitch roof windows may be a better solution.

Low-Pitch Roof Window Options

• Keylite QFS – designed specifically for low pitch roofs 

• VELUX GBL – suitable for pitches below traditional roof window limits 

These products allow homeowners to introduce roof windows where standard flat roof skylights are not suitable.

Structural Considerations: Why Professional Advice Is Essential

Installing a skylight means cutting into the roof structure. This often involves: 

• Trimming existing joists

• Installing doubled or tripled support timbers

• Ensuring load paths remain compliant with Building Regulations 

A professional assessment ensures: 

• Structural integrity is maintained

• The opening is correctly sized

• The skylight performs safely and efficiently 

Never cut a roof opening without expert advice. Read more about things to consider before installing a skylight here.

Skylight Options for Flat Roofs

If your roof is suitable, there are several excellent options: 

Flat Roof Skylights 

• Fixed or opening designs

• Excellent for kitchens and living spaces

• Can be installed singly or in multiples 

Flat Roof Lanterns 

• Ideal for larger extensions

• Create a strong architectural feature

• Flood the space with daylight 

Multiple Smaller Skylights 

• Better daylight distribution than one large unit

• Reduce shadowed areas

• Create a balanced, modern aesthetic

Positioning Skylights to Maximise Daylight

For pitched roofs, roof windows are best positioned close to the existing external wall, allowing daylight to be pushed back into the original house. 

For flat roofs: 

• Skylights or lanterns should be positioned to serve both spaces

• Multiple units often outperform one large opening

• Thoughtful placement prevents glare and overheating 

Good daylight design connects spaces visually and functionally.

Why Buy Skylights from Sterlingbuild?

Sterlingbuild offers: 

• A wide range of flat roof skylights and lanterns

• Low-pitch roof window solutions

• Expert technical support and product guidance 

Referencing trusted suppliers ensures you choose products that are: 

• Suitable for your roof type

• Compliant with UK standards

• Designed for long-term performance

Final Thoughts

So, can you add a skylight to an existing roof? 

In most cases, yes – especially with flat roof extensions. 

The key is: 

• Understanding your roof covering

• Checking roof pitch

• Assessing structural requirements

• Choosing the right skylight or roof window 

With professional advice and quality products from Sterlingbuild, adding a skylight can completely transform your extension, turning dark spaces into bright, welcoming rooms filled with natural light. 

If you’re considering a flat roof skylight, the right design doesn’t just add light – it adds value, comfort, and enjoyment to your home.