Cedar Roof Preservation Guide

Cedar vs Asphalt — Different Biology, Different Risk

Cedar is organic: moss and lichen do not just sit on it, they consume the wood fibre. Rhizoid and hyphae penetration into cedar grain accelerates soft-rot onset. Unlike asphalt which fails through granule loss and delamination, cedar fails through fibre breakdown and splitting. High-concentration sodium hypochlorite bleach-based treatments strip natural cedar oils and accelerate degradation — wrong for cedar. Cedar contains phenolic compounds that provide natural rot and UV resistance. Aggressive biocides destroy these compounds, leaving the wood more vulnerable than before treatment.

Biological Threats to Cedar Shakes

Moss and lichen colonise cedar within 3–7 years without treatment in Victoria's climate. Soft-rot fungus (Serpula lacrymans and similar wet-rot species) establishes in persistently damp cedar where moisture content exceeds 25% for extended periods. Green algae films form on shaded slopes within 1–3 years. Iron tannin oxidation produces dark staining (normal — not biological, no treatment needed). The combination of a moss mat plus retained moisture creates the fastest degradation pathway. Each freeze-thaw cycle with moss-retained water penetrates deeper into the wood grain, accelerating soft-rot colonisation beneath the surface.

The Cedar Preservation Protocol

Cedar preservation follows a strict five-step sequence: (1) Low-pH biocide application at correct concentration for cedar, which kills biological growth without stripping oils; (2) 2–4 week dwell and die-off period — do not wash immediately or you interrupt the kill process; (3) Low-pressure rinse only, never high-pressure on cedar or you damage the grain; (4) Cedar oil or UV-stabilised preservative application after the surface has dried, which restores flexibility and water repellency; (5) Physical inspection for mechanical damage and fastener condition. Repeat this cycle every 3–5 years in Victoria's climate for sustained preservation.

Vancouver Island Climate Makes Cedar Challenging

With 150+ rain days, north-facing cedar shakes rarely fully dry in winter. Sustained moisture content above 19–22% allows fungal growth to establish and proliferate. This creates a critical assessment framework. At year 10, schedule your first preservation treatment — before colonisation reaches advanced stages. At year 20, assess oil condition and fibre integrity to determine if preservation will extend life or replacement is more economical. At year 30, begin replacement planning and assess decking condition. Cedar treated on schedule can last 50+ years. Untreated cedar in Victoria typically requires replacement at 20–25 years.

When to Preserve vs Replace Cedar

Preserve if: shakes retain flexibility when gently flexed, corner checking is under 30% per shake, no soft spots when pressed firmly, and moss has not penetrated to the decking level. Replace if: significant cupping beyond 10mm, widespread corner cracking, soft spots or spongy areas underfoot, consistent moss penetration with staining visible on decking, or fastener corrosion causing shake movement. A professional assessment distinguishes the two clearly and prevents investing preservation money in roofs that have already moved beyond the treatable window. Early diagnosis is the difference between a $600–$1,200 preservation and a $25,000–$40,000 replacement.