Can You Replace a Velux with a Flat Rooflight? Step-by-Step Guide

If you have an existing Velux window that is leaking, ageing or no longer suits the look of your home, replacing it with a flat rooflight is a well-considered option, particularly on flat or very low-pitched roofs where a purpose-built flat rooflight is a better long-term fit than a pitched roof window product. In most cases, yes a Velux can be replaced with a flat rooflight. The process requires careful preparation to get the sizing right, the upstand correctly built and the new unit properly sealed. This guide walks you through each stage.

The First: Understand the Difference Between a Velux and a Flat Rooflight

A standard Velux roof window is designed to sit within a pitched roof at the same angle as the surrounding roof covering. It is engineered to drain at a pitch, which means it relies on the slope of the roof to move water away from the unit. On a flat roof, a pitched roof window cannot drain correctly and should not be used.

A flat rooflight is engineered specifically for flat or very low-pitched roofs. It is set on an upstand above the roof surface, with water draining freely away from all sides. This is the correct product for a flat roof replacement and it will outperform a Velux in this application in terms of weatherproofing, thermal performance and long-term appearance.

If you are replacing a Velux flat roof product such as the CFP or CVP range rather than a pitched roof window, the process is very similar. The key steps remain the same regardless of which Velux product is being replaced.

The Step-by-Step: How to Replace a Velux with a Flat Rooflight

Step 1: Assess the Existing Installation

Before ordering anything, inspect the existing Velux and the surrounding roof area. Look for:

The condition of the existing upstand — can it be reused or does it need rebuilding?
Signs of water damage or rot in the surrounding roof structure
The condition of the existing flashing and waterproofing membrane around the unit
The type of roof covering — single-ply membrane, felt, EPDM or another material — as this affects how the new unit will be sealed

If the upstand is sound and the surrounding structure is dry and intact, the replacement is straightforward. If there is water damage, this must be addressed before the new unit is installed.

Step 2: Measure the Internal Opening

The size to order is the internal opening dimension — the clear aperture inside the upstand that the new rooflight will sit over. Do not measure the external frame of the existing Velux unit or the overall upstand footprint.

Measure the internal width and internal length of the opening in millimetres. If the existing upstand is being reused, measure from the inside face of the upstand on all four sides. If a new upstand is being built, measure the internal opening that will be created once the upstand is in place.

Standard flat rooflights from Rooflights and Skylights UK are available from 500x500mm to 1000x3000mm (internal dimensions). If the existing opening falls outside this range, a custom size is available with delivery in 7 to 10 working days.

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Step 3: Remove the Existing Velux Unit

Once measurements are confirmed and the new flat rooflight is ordered, the existing unit can be removed. This should be carried out by a qualified roofer to avoid damaging the surrounding roof membrane or structure. The process typically involves:

Removing the existing flashing and any cover strips around the unit
Carefully lifting the existing Velux unit from the upstand
Inspecting the upstand and surrounding membrane once the unit is clear
Repairing or rebuilding the upstand if necessary before the new unit arrives

Important: Do not remove the existing unit until the replacement flat rooflight is confirmed and dispatched. Leaving an open roof void is a weather risk, particularly in the UK climate.

Step 4: Prepare the Upstand

A flat rooflight is set on an upstand — a raised kerb around the roof opening that lifts the unit above the flat roof surface and prevents water ingress. The upstand must be:

Level: Checked across all 4 corners with a spirit level. An unlevel upstand causes the rooflight frame to sit incorrectly and can compromise the weatherproof seal.

The correct height: Typically a minimum of 150mm above the finished roof surface, though this varies by roof type and covering. The roofer will advise on the correct height for the specific roof.

Waterproofed: The upstand itself must be wrapped in the roof membrane before the rooflight is installed, with the membrane turned up and secured to the inside face of the upstand.

Structurally sound: The upstand must support the weight of the new rooflight and any incidental loading such as snow.

Step 5: Choose Your Flat Rooflight

Once the internal opening dimensions are confirmed, select the right flat rooflight for the application.

Frameless rooflights have a minimal visible frame profile for a clean, contemporary appearance. Available from £165 ex VAT. Well suited to modern homes where the rooflight is intended to be as unobtrusive as possible.

Framed rooflights have a defined thermally broken aluminium frame in Anthracite Grey (RAL 7016) or Jet Black (RAL 9005). Available from £392 ex VAT. Straightforward to install and suitable for a wide range of architectural styles.

Both are available with double or triple glazing, with clear Low-E, solar control or satin privacy glass options. For a replacement on an existing flat roof, the framed rooflight is typically the simpler installation, as the frame sits directly on the upstand with a clean, defined edge for sealing.

Step 6: Install the Flat Rooflight

All flat rooflights from Rooflights and Skylights UK are dispatched as complete units, ready to install. Standard sizes are dispatched within 3 to 4 working days with free delivery to all UK mainland addresses.

On installation day:

Position the rooflight onto the prepared upstand and confirm it sits level with even contact on all four sides
Apply a continuous bead of appropriate sealant around the top of the upstand before lowering the rooflight into position
Secure the frame to the upstand using the fixings specified in the installation guide supplied with the unit
Apply the final waterproofing detail around the base of the frame, integrating with the roof membrane
Check the internal finish once the unit is in place and confirm no gaps or cold bridges are visible around the frame perimeter

Full installation guidance is provided with every order. Installation should be carried out by a qualified roofer or builder with experience of flat roof glazing.

Step 7: Inspect and Sign Off

Once the installation is complete, carry out a final inspection before the roofer leaves the site.

Check the rooflight sits level and the frame is fully sealed on all four sides
Confirm the internal finish is clean with no gaps around the frame perimeter
Check the glazing unit for any damage or contamination from the installation
Confirm the surrounding membrane is intact and all upstand waterproofing is complete

Every flat rooflight from Rooflights and Skylights UK carries a 10-year guarantee. Contact our team on 0331 6303 671 or via WhatsApp on 07840 049231 if you have any questions after installation.

UK-made flat rooflights — framed and frameless, standard sizes from 500x500mm to 1000x3000mm, custom sizes available. Standard sizes dispatched in 3 to 4 working days with free UK mainland delivery and a 10-year guarantee. Browse the full range at rooflightsandskylightsuk.co.uk/collections/rooflights-skylights

What to Tell Your Roofer

When briefing a roofer for a Velux replacement, provide the following information from the outset to ensure the project runs smoothly.

The internal opening dimension you have measured and the size of the rooflight you have ordered
The type of roof covering (EPDM, single-ply membrane, felt etc.) so they can prepare the correct waterproofing detail
Whether the existing upstand is being reused or rebuilt
The delivery date for the new rooflight, so they can time the removal of the existing unit accordingly
The installation guide supplied with the unit, which they should review before work begins

Yes, in most cases. If the existing Velux is on a flat or very low-pitched roof, replacing it with a purpose-built flat rooflight is the correct long-term solution. A standard Velux pitched roof window is not engineered for flat roof drainage and should be replaced with a unit designed specifically for the application. The key steps are measuring the internal opening correctly, preparing the upstand and ordering the correct size flat rooflight.

Measure the internal opening of the existing upstand — the clear aperture on the inside of the raised kerb around the opening. This is the dimension to order, not the external frame size of the existing Velux unit. Standard flat rooflights from Rooflights and Skylights UK are available from 500x500mm to 1000x3000mm. If the opening falls outside this range, custom sizes are available and delivered within 7 to 10 working days.

Not necessarily. If the existing upstand is sound, level, correctly waterproofed and the right height (typically 150mm minimum above the roof surface), it can be reused. If there is any water damage, deterioration, or if the upstand is the wrong height for the new unit, it will need to be rebuilt. A qualified roofer can assess this before the replacement is carried out.

The installation itself typically takes half a day to a full day depending on the condition of the existing upstand and the complexity of the waterproofing detail. Standard size flat rooflights from Rooflights and Skylights UK are dispatched within 3 to 4 working days, so the full process from ordering to installation can be completed within a week for most straightforward projects.

Most like-for-like rooflight replacements are covered under permitted development rights and do not require a planning application, provided the installation remains within the existing roofline and meets the relevant height restrictions. Owners of listed buildings or properties within a conservation area should seek written confirmation from their local planning authority before any works commence.

The rooflight unit itself starts from £165 ex VAT for a 500x500mm frameless unit and £392 ex VAT for a framed unit from Rooflights and Skylights UK. Installation costs vary by location and the condition of the existing upstand, but a straightforward replacement typically takes half a day to a full day of a qualified roofer's time. Our team is available on 0331 6303 671 to advise on selecting the right unit for your opening.

A framed flat rooflight is typically the simpler installation for a Velux replacement because the defined aluminium frame sits clearly on the upstand with a clean edge for sealing. The frameless design offers a more minimal finish but requires precise upstand preparation. Both are supplied with full installation guidance. Our team is available on 0331 6303 671 to advise on which product suits your specific installation.

Every flat rooflight from Rooflights and Skylights UK carries a 10-year guarantee as standard. Each unit is individually inspected before dispatch to confirm it matches the specification and meets our quality standards. We have been recognised with the Professional Builder Top Product Award in both 2024 and 2025, voted for by tradespeople and builders across the UK.