Flat Roofs in Lead: Design, Bay Sizing & Installation Guide
Lead is one of the most durable coverings for flat and low-pitch roofs, but only if bays, joints and fixings are detailed correctly. This guide explains how to design flat roofs in lead to ensure long service life, aligned with Leadwork Standards.
1. Selecting the Correct Lead Code
Flat roofs require heavy codes. Code 6 is the minimum; Code 7 or 8 are often specified for large spans. Heavier codes resist fatigue and buckling under thermal movement.
2. Bay Sizing & Expansion Joints
Flat lead roof coverings must be divided into bays. Maximum bay length: 1.5 m for Code 6, 2.0 m for Code 7. Use wood-cored rolls between bays for expansion, and introduce expansion joints on wide areas or where roof penetrations occur.
3. Falls & Drainage
Flat roofs should be laid to a minimum fall of 1:80 to avoid ponding. Outlet positions should be carefully planned, with sufficient capacity for rainfall intensity.
4. Substrate & Underlay
Boards should be ventilated and free from defects. Always use a suitable underlay such as geotextile to prevent abrasion and reduce condensation beneath the lead sheet.
5. Fixings & Finishing
Fix at the head of each bay only. Avoid restraining the lead along its length. Free edges should be clipped with stainless or copper clips. Apply patination oil to prevent staining and ensure even weathering.
FAQs
What lead code is used for flat roofs?
Code 6 minimum, with Code 7 or 8 often recommended for large or exposed flat roofs.
How large can a bay be on a flat roof?
1.5 m maximum for Code 6, 2.0 m maximum for Code 7. Always use wood-cored rolls or expansion joints.
Do flat lead roofs need a fall?
Yes. Minimum fall of 1:80 is essential to prevent ponding and prolong service life.
What underlay should be used?
Use a geotextile or felt underlay designed for lead. It reduces condensation and prevents wear-through.
Should I fix along the length of bays?
No. Fix only at the head. Clips should restrain free edges without restricting thermal movement.
Next in the series (Post 7): Lead Expansion Joints — why they matter, how to design them, and where to use them.
Can you help by pointing me in the right direction? I need a definitive guide as to the maximum size of a sheet (of lead) on a flat roof, for any given thickness please. Essentially a reliable reference for designing a flat roof from lead. Thanks
Good Afternoon
Please see: https://shop.theleadlads.co.uk/correct-lead-bay-sizes/
This provides a guide based on codes 4-8, both lengths and widths.