Roof windows are one of the most effective ways to flood a loft room or attic conversion with natural light, but choosing the right size can feel baffling when every catalogue entry is just a code like “FK06” or “MK04”. This guide explains exactly how Velux size codes work, lists every standard UK size, and helps you pick the right option for your rafter spacing and room layout.
How Velux Size Codes Work
Velux — the dominant brand in the UK roof window market — uses a two-part alphanumeric code to describe each window’s width and height:
- Letters indicate the width group (e.g. MK = 78 cm wide, FK = 66 cm wide, UK = 134 cm wide)
- Numbers indicate the height (e.g. 04 = 98 cm, 06 = 118 cm, 08 = 140 cm)
The code is always Width Group + Height Number. So MK06 = 78 cm wide × 118 cm tall, and FK04 = 66 cm wide × 98 cm tall.
Other manufacturers (Fakro, Keylite, RoofLITE) follow the same convention and their codes are largely interchangeable in terms of rough opening sizes — useful when comparing prices.
Standard UK Roof Window Sizes
The table below lists every common UK size, the Velux code, nominal dimensions (width × height in cm), and the rough opening required in the rafter structure.
| Velux Code | Width (cm) | Height (cm) | Typical Use | Supply Price (£) |
|---|---|---|---|---|
| CK02 | 55 | 78 | Tight rafter bays, bathrooms | 220–320 |
| CK04 | 55 | 98 | Narrow rafter bays | 260–370 |
| FK04 | 66 | 98 | Single-rafter bay, shower rooms | 280–390 |
| FK06 | 66 | 118 | Popular bedroom size | 300–430 |
| MK04 | 78 | 98 | Single-rafter bay, standard | 300–450 |
| MK06 | 78 | 118 | Most popular UK size | 320–520 |
| PK10 | 94 | 160 | Large bedrooms, open-plan | 420–650 |
| SK06 | 114 | 118 | Double-rafter bay, living rooms | 480–720 |
| SK10 | 114 | 160 | Maximum light, large lofts | 560–850 |
| UK08 | 134 | 140 | Panoramic views, wide bays | 620–900 |
Prices are supply-only, ex-VAT, Q1 2026. Installation adds £300–£600 per window depending on complexity.
What Does “Rough Opening” Mean?
The rough opening is the hole cut between rafters to accept the window frame. Velux windows are sized to fit standard rafter centres:
- 600 mm centres (most common in UK roofs built before 1990) suit FK and MK series without trimming
- 800 mm or 900 mm centres (common in engineered truss roofs post-1990) suit SK and UK series
You must never cut a principal or load-bearing rafter without first consulting a structural engineer. Headers (trimmers) must be fitted above and below the opening and sized to BS 5268 or Eurocode 5.
Centre-Pivot vs Top-Hung: Which Is Right for You?
| Feature | Centre-Pivot (e.g. GGL series) | Top-Hung (e.g. GGU/GPU series) |
|---|---|---|
| Opening style | Rotates 180° for easy cleaning | Hinges at top, opens outward |
| Minimum head height | 1.5 m comfortable | 1.8 m recommended |
| Unobstructed view | Sash rotates into room | Cleaner view when closed |
| Best for | Loft bedrooms, accessible roofs | Roof terraces, balconies |
| Typical price uplift | Base model | +£80–£150 over centre-pivot |
Centre-pivot is the default for most UK loft conversions because it allows the sash to flip for external cleaning from inside the room — important when a roofer is not regularly on site.
Planning & Building Regulations
Planning permission is generally not required for roof windows that:
- Do not protrude more than 150 mm above the roof plane
- Are not installed on a principal elevation (front-facing in many Conservation Areas)
- Do not exceed the highest part of the existing ridge
Under Part L of the Building Regulations (England, 2022), replacement roof windows in existing dwellings must achieve a minimum U-value of 1.4 W/m²K. New-build roof windows in a loft conversion must meet Part L1A targets, typically requiring triple-glazed or high-performance double-glazed units with a U-value of 1.2 W/m²K or better.
For loft conversions, Part B (Fire Safety) is equally critical: if the converted loft is a habitable room above two storeys, the roof window must provide an escape opening of at least 0.33 m² with a minimum 450 mm dimension on each side.
How to Choose the Right Size
- Measure your rafter centres before ordering — the window must fit within an existing bay or between newly fitted trimmer rafters.
- Consider head height: a taller window (e.g. height code 10 = 160 cm) feels more generous if your ridge height allows.
- Check orientation: south- and west-facing roof windows may need solar-control glazing (Velux Energy or Fakro equivalent) to prevent summer overheating.
- Budget for the full package: flashings (lead or pre-formed aluminium) add £80–£180 per window; blackout blinds typically cost £80–£220 depending on size.
[!tip] Installer’s Rule of Thumb For a standard loft bedroom, specify the largest window that fits your rafter bay without structural alteration. Daylight levels in a roof window are far harder to increase retrospectively than to reduce with a blind.
Cost Summary
| Item | Typical Cost (per window) |
|---|---|
| Standard double-glazed roof window (FK06/MK06) | £300–£520 |
| High-performance triple-glazed equivalent | £480–£750 |
| Lead flashing kit | £120–£200 |
| Blackout or solar blind | £80–£220 |
| Installation (carpenter + roofer) | £300–£600 |
| Total installed (mid-size window) | £800–£1,500 |
Velux’s own installation service covers most of the UK and quotes per window; independent roofers are often 15–25% cheaper. Always ask to see the installer’s public liability insurance and check they are familiar with the roof covering type (clay tile, concrete tile, slate) as each requires different flashing adaptors.
Common Mistakes to Avoid
- Ordering the wrong flashing type: each window needs a flashing kit matched to your tile profile. A standard flashing will not seal against deep-profile interlocking tiles.
- Ignoring solar gain: a large south-facing roof window without solar-control glass can raise room temperatures by 8–12°C on sunny days, making the space unusable in summer.
- Skimping on the lintel: the double or triple trimmer above the opening must be properly engineered. An undersized lintel will cause rafter deflection and the window to rack and leak within a few years.
- Forgetting condensation control: roof windows need trickle ventilation or mechanical extract in bathrooms and kitchens to comply with Part F of the Building Regulations.
