Chimney Back Gutters in Lead: Design & Installation Guide
The back gutter (or chimney apron) is a common failure point if poorly detailed. Correctly designed chimney back gutters in lead prevent leaks at the rear of chimneys. This guide explains codes, bay sizing, falls and fixings in line with Leadwork Standards.
1. Selecting the Lead Code
Code 5 lead is the standard choice for chimney back gutters. For larger chimneys or exposed sites, use Code 6. Heavier codes resist distortion under water load.
2. Bay Size & Expansion
Back gutters should be laid in bays no longer than 1.5 m (Code 5) or 2.0 m (Code 6). Joints should be lapped 150 mm with welts. Expansion joints are required on wider chimneys.
3. Falls & Drainage
Provide a fall of at least 1:80 towards the side gutters or outlets. This prevents standing water, which is a leading cause of failure in back gutters.
4. Abutment Detailing
Back gutters must lap correctly with side flashings, soakers, and cover flashings. Always chase and fix into masonry at 25 mm depth. Ensure overlaps are sufficient to shed water safely.
5. Finishing & Maintenance
Apply patination oil to exposed leadwork. Regular inspection is advised, as back gutters often collect debris behind chimneys.
FAQs
What lead code should I use for a chimney back gutter?
Code 5 is standard, but Code 6 is recommended for large chimneys or exposed conditions.
How much fall should a back gutter have?
At least 1:80, ideally steeper where possible. Proper fall prevents ponding and early failure.
Do back gutters need expansion joints?
Yes, for bays over recommended lengths or on wide chimneys. Joints prevent cracking from thermal movement.
How are back gutters joined to side flashings?
With lapped and welted joints, correctly chased into masonry, and dressed over soakers where present.
How do I stop blockages in back gutters?
Inspect and clear debris regularly. Ensure outlets are clear and sized correctly to drain water away.
Next in the series (Post 10): Chimney Aprons & Front Gutters — correct detailing to protect the front face of chimneys.
