Cedar Shake and Shingle Roof

What the material is doing

Western red cedar is naturally durable — it contains extractives that resist decay, which is why it was used on roofs in a rain-forest climate in the first place. Those extractives are also finite. They leach out over decades of weathering, and an old cedar roof is progressively less protected than a new one by exactly the mechanism that made it viable.

Three things happen to the wood as it ages, and all three matter for cleaning.

It silvers. UV breaks down lignin at the surface and rain washes the products away, leaving the grey. This is weathering of the material, not a deposit on it, and it is permanent.

It checks. Cedar shrinks and swells with moisture cycling, and the surface develops splits along the grain. Those checks are where moss and lichen get a foothold that no surface treatment fully reaches, and they deepen over time.

It erodes. Rain and UV wear the softer summerwood faster than the harder latewood, so an old shake develops a ridged, furrowed surface. Anything abrasive accelerates this enormously.

Shakes, shingles, and how they are laid

Shakes are split rather than sawn — thicker, irregular, textured, with a deep shadow line and considerable surface relief. Shingles are sawn, thinner and flat-faced.

Both are traditionally laid over spaced sheathing — battens with gaps rather than solid deck — so air can reach the underside and the roof dries from both faces. That detail is the reason a cedar roof works in this climate, and the reason a cedar roof over solid sheathing with no ventilation gap fails early.

Knowing which you have changes the job. Shakes have more relief and therefore more places for needles and moss to lodge; shingles are flatter and shed somewhat better but are thinner, so there is less material to lose before a split goes through.

Why moss on cedar is the hard case

On a composition roof the moss is on the surface and in the lap. On cedar it is in three places at once: on the surface, in the lap between courses, and down inside the checks of individual shakes.

That third location is what makes it different. It is inside an absorbent material that is itself holding moisture, which means the growth has a permanently damp substrate and a physical refuge. Surface treatment reaches what is on top and in the joints. What is deep in a check keeps some foothold, which is why cedar regrows faster than composition on the same property with the same exposure.

It also means the removal method cannot be mechanical. Scraping or brushing breaks shake edges, opens checking, and removes the weathered surface layer that is now doing the protecting. Levering material out from between courses widens the gap that admitted it.

The approach that works is gentle, chemical and slow: treat, let it die, let it dry, let weather take it out over the following months, and come back on a cycle rather than waiting for it to look bad again.

What honest expectations look like

Cedar is the surface where the gap between the marketing photo and the real outcome is widest, so it is worth being blunt.

Achievable: moss, lichen and algae removed or killed; needles and debris cleared from courses, valleys and around penetrations; a roof that reads as maintained from the ground and that stops actively deteriorating from trapped organic material.

Not achievable: original colour; a uniform appearance across a roof that has weathered unevenly; removal of every trace of lichen, which bonds into the wood itself; reversal of erosion, cupping, curling or splits.

Not a cleaning question at all: shakes that have cupped away from the plane, courses with through-splits, missing shakes, and any roof where daylight is visible from the attic. Those want a cedar roofer, and cleaning a roof in that condition spends money in the wrong place.

Treated cedar, and why it changes the job

Not all cedar roofing left the mill in the same condition, and it is worth establishing which you have before anything is applied.

Untreated. The traditional product, relying on the wood’s own extractives. Most older cedar roofs in this region are this, and by the time they are thirty years old the extractives are largely spent.

Preservative-treated. Pressure-treated with a fungicide or a water repellent before installation. It resists decay and growth noticeably longer, and the treatment depletes from the surface outward, so an old treated roof behaves more like an untreated one at the face while still being protected deeper in.

Fire-retardant treated. Required in some jurisdictions and on some assemblies, identifiable by a stamp on the butt if any original shakes are accessible. The retardant chemistry is water-soluble in some older formulations, which is a genuine reason to be conservative with volume and dwell time rather than a theoretical one.

Previously coated or stained. A cedar roof that has been oiled, stained or treated with a surface product at some point in its life has a partial film on it, usually failing unevenly. Cleaning removes the loose remnants and reveals how patchy it had become, which is an honest result and not always a welcome one.

Where the history is unknown — which is most of the time on a house that has changed hands — the method defaults to the gentlest thing that works.

Where cedar turns up here

Cedar is not evenly distributed across the territory, and knowing that shapes what a crew expects to find.

It concentrates in the older and more wooded parts of Bellingham — Edgemoor, Fairhaven and South Hill — and around Lake Whatcom, where mature conifer canopy, lakeside humidity and mid-century housing stock combine into exactly the conditions cedar finds hardest. Sudden Valley has a substantial share of it, on lots often deeply shaded by second growth.

Chuckanut is the concentration. The stretch running south from Edgemoor along Chuckanut Drive, down past Chuckanut Bay and up into the wooded lots on the mountain side, is where this region’s cedar problem is at its most acute — and it is not one thing causing it, it is four stacked on each other. Mature conifer canopy over most of the lots, so the roofs barely see sun. Marine air off the bay holding humidity against everything. Steep, wooded ground that keeps the air still where a flatter site would get a breeze through. And a housing stock with a high share of architect-designed mid-century and later homes, which is precisely the era and the taste that put cedar on roofs in the first place.

A cedar roof in Chuckanut is working harder than the same roof three miles north on an open lot, and it wants a shorter cycle and a gentler hand. It is also where getting the method wrong is most expensive, because it is where there is the most cedar left to ruin.

That geography is the geography of the hardest moss pressure in the county, which is not a coincidence — the same shade, damp and tree cover that made these addresses desirable is what the roof is up against.

At a glance

Signs a cedar roof needs attention:

• Green swelling between courses in the wet months, invisible by August
• Moss down inside the checks of individual shakes, not just on the surface
• Gutters filling faster than they used to with fine wood debris
• Cupped shakes standing proud of the plane, especially on the north slope
• Daylight visible from the attic — a roofer question, not a cleaning one

What actually extends a cedar roof

Shade is the variable. A cedar roof in sun with airflow lasts; a cedar roof that never dries does not — and nothing applied to the surface changes that arithmetic as much as opening the roof to light and air does.

So the highest-return maintenance on a cedar roof in this region is unglamorous: trim back what overhangs it, and keep the debris off. Needles held against cedar are worse than needles held against asphalt, because the wood absorbs what sits on it. Clearing valleys, the ridge line and the area around chimneys and skylights two or three times a year does more than any product.

Keeping the gutters running matters for the same reason — a backed-up gutter keeps the bottom courses wet permanently, and the eave is where cedar roofs fail first.

Method, chemistry and access is on the roof cleaning page. The general case for how moss damages a roof is on moss growing under shingles.