Summary

Dormers are one of the most complex leadwork details in domestic roofing. Unlike a simple chimney flashing or an abutment step, a dormer involves multiple converging elements — cheeks, back gutter, front apron, and the transitions between each — all of which must be watertight individually and at every junction with each other. When a dormer leaks, it is rarely the fault of a single piece of lead; more commonly it is the junction between two elements, or a detail that was correct in isolation but failed to allow for differential thermal movement between adjacent components.

Dormers are also a common source of dispute on refurbishment work because they often involve existing structure that has settled, twisted, or moved over decades. A contractor installing new lead flashings on an old dormer is not just fitting standard details onto a flat wall: they are working around uneven surfaces, out-of-square angles, and tile courses that may not align with the brick courses on the dormer cheeks. Measuring and templating before cutting is essential, and the ability to boss lead into complex three-dimensional shapes — an advanced skill covered at LEADSKILLS Level 2 and Level 3 — is often required.

Water damage from dormer failures is typically significant: the back gutter sits at the highest point of the dormer, and any overflow or leak at that point sends water into the ceiling void of the room below, often over a wide area. A small ponding issue in an under-specified back gutter can become tens of thousands of pounds of structural damage if left untreated. Getting the back gutter detail right is not optional, and using the correct minimum dimensions and codes is the difference between a 60-year installation and a callback in five years.

Key Facts

  • Dormer cheek — the vertical side wall of the dormer, usually clad in lead, tiles, slate, or timber; the lead flashing wraps from the main roof tile field up and over the cheek junction
  • Back gutter — the lead-lined gutter at the junction between the dormer roof and the main roof slope behind it; the highest-risk element because it collects water from the main roof above
  • Front apron/sill — the lead flashing at the foot of the dormer, below the window sill; directs water from the sill onto the main roof tile field
  • Back gutter minimum depth — 75 mm; this is the minimum vertical dimension of the gutter liner to ensure water does not overtop it on the main roof slope
  • Back gutter minimum width — 150 mm clear; wider for larger dormers or where the main roof above has a substantial catchment area
  • Back gutter code — Code 4 (standard); Code 5 for back gutters wider than 300 mm or where the catchment from the main roof above exceeds approximately 15 m²
  • Cheek flashing code — Code 3 soakers, Code 4 cover flashings; same as standard step-and-cover at any abutment
  • Step flashing at cheek/main roof junction — where the dormer cheek meets the main roof tile field, a step-and-cover system is used; soakers go in with tiling, cover flashing fixed after
  • Maximum back gutter bay length — 1,500 mm; longer gutters need at minimum one intermediate drip
  • Maximum cheek cover flashing bay length — 1,500 mm; same limit as any cover flashing
  • Thermal movement at back gutter — Code 4 back gutter at 1,500 mm will experience approximately 3 mm of longitudinal movement; drip detail must allow free movement
  • Outlet from back gutter — water must be directed from the back gutter outlet through the main tile field or into a valley; an outlet pipe or lead tongue piece is used
  • Tilt fillet at back gutter — a triangular timber fillet at the upslope end of the back gutter prevents water tracking up under the back gutter liner at the apex
  • Felt upstand at back gutter — the sarking felt from the main roof must be turned up and over the back gutter edges; this provides backup if the lead is ever overwhelmed or fails
  • UPVC or Velux roof windows — factory-supplied flashing kits are available; lead alternatives are preferable on heritage work and longer-lasting on new build; lead must still comply with BS 6915 bay length limits

Quick Reference Table

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Dormer Element Minimum Code Minimum Dimension Maximum Bay Length
Back gutter Code 4 75 mm deep, 150 mm wide 1,500 mm
Back gutter (large catchment) Code 5 75 mm deep, 300 mm+ wide 2,000 mm
Cheek soakers Code 3 Gauge + lap + 25 mm height N/A (one per course)
Cheek cover flashings Code 4 65 mm into joint, 75 mm lap 1,500 mm
Front apron/sill Code 4 150 mm under tiles, 75 mm over sill 1,500 mm
Stepped cover at cheek/roof junction Code 4 As per step flashing guidance 1,500 mm
Common Failure Mode Cause Prevention
Back gutter ponding Gutter too narrow; insufficient fall Size to minimum 150 mm; ensure positive fall to outlet
Back gutter overflow Inadequate outlet; debris blockage Oversize the outlet; use debris-resistant outlet cover
Cheek cover flashing blow-out Insufficient depth in joint; rigid pointing Rake joint 25–30 mm; use non-setting mastic
Back gutter cracking Bay too long; no drip Limit bays to 1,500 mm; form drips at every bay junction
Water tracking behind cheek Short soakers; tile not lapping soaker Size soakers correctly; ensure tile gauge matches
Sill apron failure Apron too short under tile Minimum 150 mm under tile field

Detailed Guidance

Dormer Anatomy and the Sequence of Lead Installation

A standard dormer has five distinct lead zones:

  1. Soakers — installed as the main roof tiling proceeds up each side of the dormer; placed at every tile course at the cheek/main-roof abutment
  2. Back gutter — installed before the dormer roof covering; sits at the junction between dormer rear and main roof slope
  3. Cheek cover flashings — installed after tiling is complete on both the main roof and the cheek cladding; tucked into the cheek wall joints
  4. Front apron — installed after the sill is in place; tucked under the window sill and over the tiles below
  5. Cheek/back gutter junction pieces — bossed or welded corner pieces where the cheek meets the back gutter; the most technically demanding part of the job

The sequence matters because some elements must go in before others can be fixed. Soakers must be installed as tiling proceeds; back gutter goes in before the dormer roof covering; cover flashings are always last. Getting the sequence wrong means lifting completed work to install omitted elements.

Back Gutter Sizing and Construction

The back gutter is the element most frequently under-specified on dormer refurbishment work. The minimum dimensions — 75 mm deep, 150 mm wide — are not negotiable.

Depth (75 mm): the gutter liner must rise 75 mm above the lowest point of the gutter before it is fixed at the sides. This prevents water overtopping the liner in heavy rainfall. On steeply pitched main roofs, water velocity at the back gutter is high; if the gutter is only 50 mm deep, heavy rainfall will run over the top of the liner and into the roof structure behind the dormer wall.

Width (150 mm): the horizontal gutter dimension must accommodate the water flow from the main roof above the dormer. For a small dormer (say 1.5 m wide) with a main roof slope of only 3–4 m above it, a 150 mm wide gutter is sufficient. For a large dormer spanning most of the roof width, with a substantial main roof slope above collecting rain, a 200–300 mm gutter may be needed. Use the Lead Sheet Manual valley width table as a guide: the back gutter behaves like a transverse valley gutter in terms of catchment area calculation.

The gutter liner is formed from a single piece of lead (or bayed pieces with drips if the dormer exceeds 1,500 mm wide). It is dressed into the gutter and up each side — the upstand at the dormer wall side and the upstand onto the main roof slope. Both upstands should be a minimum of 75 mm.

At the upslope (back) end of the gutter, a tilt fillet is nailed to the valley board or dormer wall plate. The lead is dressed over the fillet, which prevents water backing up at the top of the gutter and tracking under the back gutter liner at the apex.

At each end of the back gutter, the lead must meet the cheek flashing detail. This junction — the back gutter to cheek transition — is the most technically demanding part of a dormer lead installation. It is a three-dimensional internal angle: the gutter runs horizontally, the cheek runs vertically, and the main roof slope runs diagonally. The transition must be fully watertight under any volume of rainfall. Options are:

  • Bossed corner piece: a separate piece of lead is bossed (worked with a mallet) to form the internal three-way angle; traditional and repairable; requires skill
  • Welded corner piece: the corner is fabricated by lead-burning; acceptable on new work; requires appropriate welding equipment
  • Preformed corner piece: some lead merchants stock preformed corners for standard gutter depths; useful on commercial refurbishment for speed

Bossed corners are preferred on heritage work and are the detail tested at LEADSKILLS Level 2 and Level 3.

Back Gutter Outlets

Every back gutter must have an outlet that directs collected water away from the dormer and into the main roof drainage system. The outlet is typically positioned at one or both ends of the back gutter, immediately above the cheek/main-roof junction. Options include:

  • Lead tongue piece: a strip of Code 4 lead dressed over the tile field below the cheek, directing water from the gutter outlet between the tiles and into the main eaves gutter
  • Outlet pipe: a short lead pipe (soldered or lead-burned to the gutter liner) that directs water through the cheek detail into a valley or hopper below
  • Junction into valley gutter: where the dormer sits adjacent to a valley, the back gutter outlet can be directed into the top of the valley

Whatever outlet method is used, the outlet must be sized to handle the maximum flow from the catchment above. A 40 mm outlet pipe is generally adequate for small to medium dormers; larger dormers may need 50 mm or dual outlets.

Back gutter outlets must be checked and cleared periodically — moss, tile fragments, and leaf litter accumulate and can block the outlet, causing the gutter to pond and eventually overtop. This is a maintenance point that should be communicated to the client at handover.

Cheek Flashing Step-and-Cover Details

The cheek/main-roof junction uses standard step-and-cover flashings, with Code 3 soakers installed as tiling proceeds and Code 4 cover flashings installed after.

The soaker sizing is identical to any abutment soaker: height = tile gauge + tile lap + 25 mm; width = tile width + 25 mm. The cover flashing follows the standard rules: minimum 65 mm into the mortar joint, 75 mm lap over the soakers, maximum 1,500 mm bay length.

At the top of the cheek step-and-cover detail, the cover flashing must meet the back gutter liner. This junction is typically formed by turning the last cover flashing bay up and under the back gutter upstand on the cheek side, creating a lapped weathering joint. The back gutter upstand laps over the top of the cover flashing, shedding any water that runs down the cheek towards the back gutter.

At the foot of the cheek detail, the last soaker course must align with the eaves tile and direct water clearly into the main eaves gutter. The bottom cover flashing bay typically terminates with an end cap dressed over the fascia line.

Front Apron (Sill) Detail

The front apron is the lead flashing at the base of the dormer window. Its purpose is to bridge the junction between the window sill and the tile field below, preventing water from tracking into the dormer wall and soaking the lintel or sill timber.

The front apron typically consists of:

  • A lead tray (Code 4) dressed under the window sill, tucked under the window frame and dressed over the tiles below
  • The lead must extend a minimum of 150 mm under the tiles to provide adequate weather protection
  • The lead must lap over the tiles below by a minimum of 75 mm and be dressed flat against the tile face

Where the dormer window is set in a reveal (not flush with the cheeks), the apron must also be returned at each side to meet the cheek flashings. These side returns are typically bossed or welded.

Thermal Movement Provision

Every element of a dormer lead installation must be designed to allow free thermal movement independently of adjacent elements.

  • Back gutter: maximum 1,500 mm bay (Code 4) or 2,000 mm (Code 5) with drip detail at each bay junction
  • Cheek cover flashings: maximum 1,500 mm bay; do not continue a single piece around the corner from the cheek to the front apron — the corner changes the plane of movement
  • Front apron: maximum 1,500 mm per bay; typically a single piece unless the dormer is wider than 1,500 mm
  • Junction pieces (corners): these must allow the adjacent bays to move independently; the junction piece should not be rigid-fixed to both adjacent elements

A common error is to continue the back gutter liner as a single L-shaped piece that turns around the cheek corner and becomes the cover flashing upstand. This forces the back gutter and the cheek detail to move as one element; when the thermal movement of the gutter acts perpendicular to the movement of the cheek detail, the L-bend is forced to flex and eventually cracks.

The correct approach is to treat each face of the dormer as an independent lead zone, with properly overlapped lap joints at every transition point.

Relationship to Roof Window Installations

Many modern dormers incorporate proprietary roof windows (VELUX or equivalent). Roof window manufacturers supply integrated flashing kits that are designed to work with their specific window frame geometry. These kits typically include:

  • Sill flashing (apron)
  • Side flashings
  • Top flashing

Factory kits are engineered for the specific pitch range of the window installation and include allowances for thermal movement in the plastic/aluminium profiles. They are acceptable under Building Regulations and carry manufacturer warranties.

However, the back gutter behind the dormer wall above the roof window is not covered by the window flashing kit — it is a separate leadwork element and must be designed and installed to BS 6915 requirements regardless of whether a proprietary window kit is used.

On heritage or listed building work, lead-only details are typically specified in preference to proprietary kits, and an LCA-qualified leadworker is expected to form all details in lead to the appropriate standard.

Frequently Asked Questions

How wide does a back gutter need to be if the main roof above the dormer is very steep and large?

Use the Lead Sheet Manual valley width table to calculate the required gutter width. Measure the catchment area of the main roof slope above the dormer (plan area in m²), then cross-reference with the roof pitch to get minimum gutter width. For a steep 45° main roof with a 20 m² catchment, the required gutter width will be at the higher end of the range (200–250 mm clear minimum). When in doubt, go wider: the cost of an extra 50 mm of lead is trivial compared to the cost of a callback.

Can I use a single-piece lead liner for a long back gutter without drips?

No. The maximum bay length for Code 4 is 1,500 mm. A 3 m dormer needs at least two bays with one drip between them. A leadworker who fits a 3 m single-piece liner is creating a known failure point. The lead will develop creep wrinkles and eventually crack within a few years.

Do I need planning permission to replace dormer lead flashings on a listed building?

Replacing like-for-like lead flashings on a listed building is generally considered routine maintenance and may not require formal listed building consent. However, any change to the materials (e.g. switching from lead to a synthetic alternative) or to the profile of the flashings would require consent. Always advise the client to check with their local authority conservation officer before work begins on a listed building.

What causes a back gutter to pond?

The two most common causes are: insufficient fall toward the outlet (the gutter was fitted level or, worse, tilted slightly toward the dormer wall rather than toward the outlet), and a blocked or undersized outlet. The back gutter must have a positive fall to the outlet of at least 1:60 (approximately 17 mm per metre). In practice, a visual fall is preferable — the gutter should drain visibly when wetted with a hose.

How do I deal with an existing dormer where the cheek wall is out of plumb or the back gutter is at an irregular angle?

Template before cutting. Use a cardboard template to capture the exact geometry of the existing back gutter and corner junction before cutting any lead. Cut the lead oversize and dress to fit on site. For complex three-dimensional corners, lead is forgiving of irregular geometry when bossed correctly — the malleability of the material is its main advantage over rigid sheet alternatives.

Regulations & Standards

  • BS 6915:2001 (with amendments) — Design and construction of fully supported lead sheet roof and wall coverings; back gutter dimensions, bay lengths, and drip details

  • BS EN 12588:1999 — Rolled lead sheet standard; Code 4 and 5 specifications for dormer elements

  • Building Regulations Approved Document C — Resistance to moisture; dormer flashings as part of the building weathering envelope

  • Building Regulations Approved Document L — Conservation of fuel and power; relevant where dormer is being insulated or upgraded as part of the work

  • NHBC Standards Chapter 7.2 — Pitched roofs; dormer flashing requirements for new-build warranty

  • Lead Sheet Association Lead Sheet Manual — Dormer back gutter, cheek flashing, and thermal movement guidance

  • Lead Contractor Association — LEADSKILLS Level 2 covers dormer flashings; Level 3 covers complex heritage dormer details

  • Historic England Practical Building Conservation: Roofing — Heritage dormer flashing guidance

  • bs en 12588 lead sheet — Code numbers 3–8, thicknesses and colour coding

  • lead soakers and cover flashings — Step-and-cover details at cheek abutments

  • lead valley gutters — Valley gutters adjacent to dormers and catchment calculations

  • lca leadskills qualification — LEADSKILLS Level 2 and 3 covering dormer leadwork