Roof Window Installation: Flashing Kit Selection by Pitch, Structural Header Requirements and Part L Compliance

Summary

Roof window installation is one of the most consistently sold roofing/carpentry jobs in the UK. Adding a Velux or similar roof window to a bedroom, bathroom or landing can transform a dark space, and most homeowners consider it a straightforward upgrade.

The complexity is in the detail: incorrect flashing selection for the roof profile and pitch is the most common failure mode, causing water ingress within the first 12 months. The flashing must match the specific tile or slate type (flat interlocking concrete, clay plain tile, natural slate, profiled tile), and each manufacturer produces a different kit for each profile.

The structural opening is frequently underestimated — many older loft conversions have rafters at 400–600mm centres and inserting a 550mm-wide window requires removing one or more rafters and installing a structural trimmer arrangement that must carry the loads of the removed rafters.

Key Facts

• Minimum pitch — Most standard Velux/Fakro windows: 15° minimum pitch. Recessed/in-roof installations: 10°. Check the specific window manufacturer's installation guide.
• Flashing kit codes (Velux example):
  • EDL — Standard tile flashing kit, for profiled interlocking concrete tiles (gauge 90–120mm)
  • EDM — Standard tile flashing kit for plain tiles and natural slate (gauge 60–100mm)
  • BFX/BPX — Low pitch (10–20°) insulating flashing kit
  • EDB — Insulating flashing kit for colder climates or where thermal performance is critical
  • EDW — For slates and plain tiles, extended sill
  • Numbers following code letters indicate window size
• Structural trimmer arrangement — Remove rafters to create opening. Install: (1) header (trimmer rafter across top of opening) — same depth as existing rafters, doubled; (2) trimmer rafters (each side of opening — doubled rafters); (3) sill joist if required.
• Span check — Header joist spans between trimmer rafters. For standard 550mm window, header is short; for large windows (1340mm wide), header span requires engineer check.
• Thermal compliance — BS EN 14351-1 U-value declarations required. Part L1B (replacement) requires ≤1.8 W/m²K for roof window units. Building Control notification required.
• FENSA/CERTASS — Roof window replacement is notifiable. Installer should be registered; FENSA/CERTASS certificate issued on completion.
• Moisture barrier — Roofing underlay (breathable membrane) must lap up and over the window sill flashing; side flashings must lap under the underlay at jambs.
• Safety glass — BS EN 12600: roof windows must have safety glass (toughened outer, laminated inner is standard for quality units). Always check glass specification before ordering.
• Ventilation value — Opening roof windows provide Part F-compliant background ventilation if they have an open trickle vent.

Quick Reference Table

Detailed Guidance

Sizing and Positioning

Structural constraints:

• Rafter spacing is typically 400–600mm on pitch
• For a 550mm-wide window (Velux C02 or GGU standard), one rafter is removed and two trimmers installed
• The window CW dimension (centre-to-centre of trimmers) must match the manufacturer's rough opening specification exactly — not room for adjustment
• Vertically: position with top of window at least 200mm below ridge board; at least one full tile course above eaves to maintain structural integrity of lower courses

Positioning for head height:

• Window sill at seated height: approx 600–700mm from floor level = good for seated work/views
• Window sill at standing height: approx 900–1000mm from floor = privacy and glare reduction
• For loft conversion rooms: measure from finished floor level upward

Structural Opening

1. Mark the rafter(s) to remove — Allow for trimmer rafter width (50mm each side) plus window CW dimension
2. Temporary propping — Install temporary prop (Acrow) under the ridge area of the rafter(s) to be removed before cutting
3. Cut rafters — Cut cleanly at the header joist position (top) and sill position (bottom)
4. Install trimmers — Two full-depth rafters each side, nailed through ridge board/purlin at top; toenailed to wall plate at bottom
5. Install header joist — Same depth as rafters; double up if span exceeds 900mm; secure with joist hangers or skew nails
6. Install sill joist — At bottom of opening; same depth; doubles up on each side if needed
7. Check opening — Measure diagonals; must be within 5mm of square. CW and CH must match manufacturer's opening size spec.

Flashing Installation

The flashing sequence is critical — incorrect overlapping leads to water ingress:

1. Prepare underlay — Cut underlay to opening. Fold outer edges down inside opening (don't cut flush — leave 30mm). Fold lower edge down over sill position.
2. Fix roof window frame — Drive fixing brackets through tile battens into rafters as per manufacturer guide. Check level; pack as needed.
3. Install bottom flashing (sill) — Insert under bottom tile course; lay under lower battens. Overlap with side flashings.
4. Install side flashings — Step flashings (multiple pieces) or continuous side flashing per kit. Each piece tucks under the tile on the upslope side and laps over the tile on the downslope side. Check manufacturer's guide for exact sequence — it varies by kit.
5. Install top flashing — Tuck under the tile course above; lead/aluminium cover plate over upper apron.
6. Re-bed cut tiles — Cut and bed any partial tiles around the flashing perimeter with mortar (if necessary for coverage).
7. Seal all joints — Apply sealant to all metal-to-metal joints; check for uplift gaps under high-profile tiles.

Part L Compliance and Notification

Replacement window:

• U-value ≤1.8 W/m²K required
• Submit building regulations notification: FENSA/CERTASS preferred
• Retain U-value declaration sheet from manufacturer

New window in new extension or loft conversion:

• Part L1B/SAP energy calculation may require the window to contribute to the overall energy balance
• New loft conversion: Part B escape window may be required (see escape window sizing below)

Escape windows (Part B):

• Loft conversion bedrooms require an escape window if no protected stairway below
• Minimum opening area 0.33m², minimum 450mm high and 450mm wide
• Velux GGL/GGU sizes MK04 (78×98) provides 0.29m² — insufficient. Minimum Velux C04 (55×98) provides adequate dimensions with the 78×118 being the common compliant choice.

Frequently Asked Questions

Do all Velux windows use the same flashing kit?

No — each window series (GGL/GGU/GPL, etc.) uses specific flashing kits in specific size codes. The kit code on the Velux website must match both the window reference code and the tile type. Check the product page for your specific window before ordering.

Can I install a roof window without removing a rafter?

Only if the opening fits exactly between two existing rafters (window width ≤ rafter spacing minus 100mm for trimmers). A 550mm-wide window at 400mm rafter centres is not possible without removing a rafter. A 550mm window at 600mm rafter centres — possible in theory (500mm clear space needed for CW, minus 50mm each trimmer = 400mm clear) — check manufacturer's minimum opening dimensions.

My roof window leaks only in heavy, wind-driven rain — what is wrong?

Wind-driven rain exposes any gaps between the flashing and tile or between flashing components. Check: step flashing upper edges (should tuck under tile not just butt up to it), the junction between side and top flashings (must be sealed), and the gap under high-profile tiles at the sill. If flashing is correct but leaks persist, confirm the window itself is closed properly (some Velux models have been known to have handle latch issues).

Regulations & Standards

• Approved Document L1A/L1B — Energy: U-value ≤1.8 W/m²K for roof windows

• Approved Document B (Volume 1) — Fire: escape window requirements for loft conversion bedrooms

• Approved Document F — Ventilation: opening windows contribute to background ventilation

• BS 5534:2014+A2:2018 — Slating and tiling for pitched roofs: fixing requirements around roof windows

• BS EN 14351-1:2006+A2:2016 — Product standard for windows and doorsets

• Velux Installation Guidance — manufacturer-specific flashing selection

• Fakro Installation Guides

• NFRC — Roofing specification guides

• rooflights skylights — Flat roof rooflights and skylights

• loft conversions — Full loft conversion process

• roof trusses — Structural considerations when modifying roof structure

• lighting design — Lighting to complement natural light from roof windows