#Proper Leadwork: Chimney Packages—Aprons, Back Gutters & Soakers that Don’t Fail
The most common roof leak? The chimney. Success depends on a complete lead chimney flashing package—soakers that step with the covering, an apron that sheds water to the roof, and a back gutter (with saddles/secret gutters as required) that moves water around the stack. In this guide we specify codes, geometry, sequencing and fixings for durable, tidy details aligned to best practice.
Copper & Stainless Fixings
Lead Underlay & Support
The Lead Lads: Services & Prices
Lead Lads Standards Hub
GNR Roofing: Leadwork Authority
Regulatory Standards (BS 6915)
Part 4 of our “Proper Leadwork: Technical Series”. Pair this with Part 1 (code selection), Part 2 (movement), and Part 3 (step vs cover flashings).
1) The complete chimney package
A compliant lead chimney flashing is a system, not a single strip. It includes:
- Soakers under each course of slate/tile, turning up the abutment and out over the covering.
- Side flashings (stepped or cover) dressed over the soakers and into a chase.
- Apron flashing to the downslope face, dressed over the covering with a formed drip.
- Back gutter to the upslope face, with saddles turning water around the stack to each side.
- Secret gutters or crickets where geometry demands additional diversion.
2) Codes & thickness selection
Code selection balances stiffness, formability and movement. For most chimney packages:
| Component | Typical Code | Why |
|---|---|---|
| Soakers | Code 3–4 | Light, discrete, easy to step with courses |
| Step/Cover flashings (sides) | Code 4 (5 in exposure) | Flexible enough to boss; durable in wind-driven rain |
| Apron | Code 4–5 | Holds a tidy drip; resists rippling on wider spans |
| Back gutter (incl. saddles) | Code 5–6 | Stiffer for capacity and splash; tolerates debris |
In coastal/upland sites or on very wide stacks, step one code heavier for aprons/back gutters, and tighten fixings/centres.
3) Geometry: upstands, laps, falls & clearances
Geometry drives performance—height, lap and fall work together.
| Parameter | Indicative Value | Notes |
|---|---|---|
| Upstand height (flashings) | 75–150 mm above finished surface | Increase with exposure; keep render/cladding off the lap |
| Chase depth | ~25 mm | Clean, continuous; no feathering; don’t overpack |
| Apron drip | ~20–25 mm projection | Throw water clear of the covering; even line |
| Head lap (flashings) | 75–100 mm | Increase on lower pitches or higher exposure |
| Back gutter fall | ≥ 1:80 | Provide sumps/outlets; avoid dead-flat pockets |
| Saddle upstand | ≥ 75 mm | Turn water into side channels; smooth transitions |
4) Soakers: sizing, pitch rules & installation
Soakers are the primary defence at the side cheeks. They step with each course, guiding water onto the covering below the visible flashing.
Sizing & material
- Material: Lead Code 3–4.
- Width: Sized to slate/tile gauge; ensure generous cover over the course below.
- Upstand: Turn up against the stack sufficiently to meet the flashing lap.
- Length: To suit course length with acceptable headlap to the next soaker.
Fitting & sequencing
- Install with the covering, one per course.
- Maintain consistent headlap; avoid capillary traps at edges.
- Dress side flashings over completed soaker runs.
Discreet, regular soakers plus a correctly clipped side flashing outperform any heavy cover strip alone.
5) Aprons: shape, drip & chase details
The apron carries water away from the downslope face. It must present a clean drip, sit flat without ponding, and remain free to move.
- Patterning: Template the roof contour; avoid dog-ears and stress raisers at corners.
- Drip: Form a smooth, even drip ~20–25 mm; ensure it clears the surface below.
- Chase: Cut ~25 mm deep; insert and wedge; point with compatible mortar or neutral-cure sealant.
- Movement: Keep the lap free of fixings; clip at ~300–450 mm along the edge.
6) Back gutters: capacity, saddles & secret gutters
The upslope face must not trap water. A well-formed back gutter gathers flow from above and pushes it around the stack via saddles or secret gutters.
Capacity & falls
- Use Code 5–6 for stiffness and longevity.
- Set a visible, measurable fall (≥1:80).
- Where runs are long, add intermediate sumps/outlets.
Saddles & secret gutters
- Saddles: Turn water around the stack; upstand ≥75 mm; smooth corner radii.
- Secret gutters: Concealed channels where geometry channels flow against a wall; size generously and keep clear of debris.
- Avoid sharp folds; soften to reduce fatigue at corners.
Back gutters are debris magnets—design a safe overflow route so temporary blockage doesn’t drive water inside.
7) Sequencing & workmanship
- Prepare chases on all faces; confirm substrate integrity.
- Install soakers course-by-course with the roof covering.
- Dress side flashings (stepped or cover) over soakers; clip @ 300–450 mm.
- Template & fit apron with formed drip; insert to chase; point after set.
- Form back gutter with fall, saddles/secret gutters; clip/strap as specified.
- Finish chases neatly; do not overpack; confirm movement paths free.
- Quality control: Photograph laps, corners, drips and outlet paths for the handover record.
8) Fixings & restraint without pinching movement
Fixings resist uplift; they must not lock sheets against thermal movement. Keep laps clear of nails. Use the right materials and spacing.
| Detail | Fixings | Spacing | Notes |
|---|---|---|---|
| Side flashings | Clips + chase fix | 300–450 mm | Do not overdrive; allow longitudinal slide |
| Apron | Clips + chase fix | 300–450 mm | Keep drip free; no pinning within laps |
| Back gutter | Straps + clips | 300–400 mm | Increase in exposed sites; protect outlets |
| Saddles/secret gutters | Clips/straps | As designed | Maintain movement at transitions and corners |
9) Masonry, flaunching & lining considerations
Flashings succeed when the substrate is sound. Weak mortar, crumbling stone, or defective flaunching can undermine perfect metalwork.
- Masonry: Sound, clean chases; avoid feathered edges; respect coursing for stepped flashings.
- Flaunching: Sound at the pot; repair cracks; prevent water entry from above that bypasses flashings.
- Chimney liners: Ensure terminal and tray details do not discharge onto the back gutter.
- Render: If rendering, coordinate beads to avoid trapping water above flashings.
10) Common defects & remedies
- Leaks at back gutter: No fall/overflow; ponding at corners. Remedy: Re-form with fall; add saddles/secret gutters; clear path to outlets.
- Loose flashings: Shallow chases or brittle pointing. Remedy: Re-cut chase ~25 mm; repoint with compatible materials; reset clips.
- Creep/ripples at apron: Laps pinned or insufficient movement. Remedy: Free lap; re-space clips; consider heavier code.
- Capillary ingress at sides: Tight seams; no welt or poor soaker laps. Remedy: Welt long edges; correct soaker sizing.
- Debris blockage: Leaves/soot in back gutter. Remedy: Maintenance plan; overflow provision.
- Staining: Drip too short or buried under render. Remedy: Re-form drip; keep clear of finishes.
11) Worked examples
Example A: Brick chimney on slated 35° roof (urban exposure)
Goal: Build a robust, tidy lead chimney flashing with easy maintenance.
- Soakers: Code 3–4 each course; consistent headlap.
- Side flashings: Stepped, Code 4; chase ~25 mm; clips @ 300–450 mm.
- Apron: Code 4 with 20–25 mm drip; lap free of fixings.
- Back gutter: Code 5, ≥1:80 fall; saddles to divert flow; outlet protection.
- QC: Photograph corners, laps, drip and outlet path.
Example B: Large random stone stack on profiled tiles (exposed site)
Challenge: Irregular substrate, wind-driven rain, splash risk.
- Soakers: Code 4; adjust sizes to tile gauge and profile.
- Side flashings: Stepped (adapted to random courses) or cover flashing where render is introduced.
- Apron: Code 5 for stiffness over wider span.
- Back gutter: Code 6 with secret gutters; generous saddles; defined overflow route.
- Fixings: Shorter clip centres; no pinning at laps.
Example C: Narrow stack near valley intersection
Risk: Concentrated flows from adjacent valley overload the back gutter.
- Design: Integrate a secret gutter/deflector to redirect valley discharge.
- Material: Code 6 back gutter; Code 5 saddles; welted edges.
- Drainage: Confirm clear falls; add overflow path outboard.
12) On-site checklist (printable)
- Chases pre-cut (~25 mm), substrate sound ✅
- Soakers sized to gauge/pitch; installed per course ✅
- Side flashings (stepped/cover) Code 4; clips 300–450 mm; laps free of fixings ✅
- Apron Code 4–5; drip 20–25 mm; uniform line ✅
- Back gutter Code 5–6; fall ≥1:80; saddles/secret gutters included ✅
- Overflow route designed; outlet protected ✅
- Mortar/sealant compatible; no overpacking ✅
- Photographic record of corners/laps/outlets ✅
13) Useful links & where to buy
