Composition Asphalt Shingle

What a shingle is actually made of

Modern composition shingle is a layered product, and the layers are worth knowing because they explain everything that follows.

At the core is a fibreglass mat, which provides the tear strength and dimensional stability. That mat is saturated and coated in asphalt, which is the waterproofing. On top, pressed into the asphalt while it is hot, is a layer of ceramic-coated mineral granules.

The granules are not decoration. They block UV from reaching the asphalt, they provide fire resistance, and they carry the colour — the material underneath is black whatever the roof looks like from the street. When asphalt is exposed to sunlight it dries, loses its volatiles, becomes brittle and cracks. The granule layer is the only thing standing between the binder and that process.

So the shingle’s service life is, functionally, the life of its granule coverage. Everything below follows from that one sentence.

Three-tab shingles are a single flat layer with cutouts that make three tabs per strip — the older, thinner, cheaper product, common on housing stock here from the 1980s and 1990s. Architectural or dimensional shingles are laminated from two or more layers, which makes them heavier, thicker, longer-lived and visually textured. Most re-roofs here are architectural now.

How the roof keeps water out

Nothing on a composition roof is sealed. The waterproofing is geometry.

Each course overlaps the one below, so water arriving from above runs down the surface and off the eave without reaching a joint. Adhesive strips bond each course to the next once the sun has warmed them, which is what stops wind lifting tabs — but the adhesive is a wind detail, not a water detail.

This matters for two reasons. Anything that lifts a course breaks the geometry and lets wind-driven rain past it. And anything that pushes water upward — a pressure washer, a hose held wrong, a brush worked uphill — is sending water in the one direction the entire system was designed not to handle.

What grows on it here

Algae, most often Gloeocapsa magma, is the dark vertical streaking almost every older comp roof in this region carries on its shaded slopes. It is a cyanobacterium, not dirt and not mould; part of what sustains it is limestone filler within the shingle itself, and it spreads downward with the water, which is why the streaks start high and run to the eave. It is a surface organism and it responds to chemistry.

Moss is the more serious one, because it does not stay on the surface. It establishes at the lower edge of a course where debris collects, grows into the lap and lifts the shingle above it. That lifting is the damage.

Lichen bonds tightly to the granule surface and is the most stubborn of the three, because removing it aggressively removes granules with it.

Organic debris — conifer needles above all — is upstream of all of this. It holds moisture against the surface, collects in valleys and at flashings, and gives moss the damp organic bed it needs to start.

Why north slopes fail first

The same rule that governs walls governs roofs, and on a roof it is starker because the two planes are identical in every other respect.

At this latitude a north-facing slope receives effectively no direct sun for months. Through a wet season running October into May it does not dry between rains. The south slope on the same house, laid the same day from the same bundles, is frequently clear.

Add shade from conifers and the effect compounds — less light, more debris, and needles that mat and hold water. A roof under mature Douglas fir on a north aspect is working in the hardest conditions this region offers.

What cleaning can and cannot do

Comes off: algae streaking, moss, lichen to a large degree, needle and leaf debris, and general atmospheric film.

Improves: the colour reads truer once the darkening film is gone, and a roof often looks years newer than it did.

Does not change: granule loss, bare patches, curling or cupping tabs, cracked or brittle shingles, failed flashings, or a roof at the end of its life. None of those are dirt, and a clean will make them more visible rather than less — which is uncomfortable but useful, because it is a truer picture of the roof.

The method is chemistry and patience: the growth is killed, it dies, dries, loosens and weathers off over the following weeks. The roof does not finish looking clean on the day. That slow release is the point — it is what lets the material leave without anything mechanical crossing the granule surface.

When a roof should not be cleaned

There are roofs we decline, and it is worth knowing what they look like, because the alternative is paying for a clean that shortens the roof.

Visible bare patches. Where the granule layer is already gone in areas, the asphalt underneath is exposed and brittle. Treating it will not restore anything and walking it will make the bare areas larger.

Curling, cupping or clawing tabs. Edges lifting away from the plane mean the mat has shrunk and the shingle has lost flexibility. Those edges break underfoot and they break under water pressure.

Widespread cracking. Fine map-cracking across the surface is the binder telling you it has lost its volatiles. That roof is near the end, and the useful spend is a roofer’s opinion rather than a wash.

A roof somebody has already pressure washed. It is usually identifiable — an unnaturally uniform light patch, or fan-shaped striping where a wand was swung. There is nothing left to clean off; what remains is the mat.

In each case the honest recommendation is to clear the debris, keep the gutters running and spend the money on getting a roofer up there instead.

The part of the roof nobody sees

One thing that materially shortens a shingle roof here has nothing to do with what is growing on top of it.

A composition shingle sheds heat upward and downward. If the attic below is poorly ventilated, heat builds under the deck and cooks the shingle from underneath, ageing the binder from the side nobody inspects. Blocked soffit vents — often blocked by insulation pushed into them — and an inadequate ridge vent are common, and the symptom on the roof surface is premature brittleness across a whole plane rather than in the wet, shaded places you would expect.

It is outside what a cleaning visit addresses, and it is worth mentioning when a roof looks older than its age, because cleaning a roof that is failing from below solves the wrong problem.

At a glance

Signs worth acting on:

• Dark vertical streaking on the shaded slope — algae, treatable
• Raised green growth at course edges — moss, and it is lifting the shingle
• Coarse grit accumulating in the gutter year after year on a roof that is not new
• Tab edges lifting away from the plane
• One slope visibly worse than the other on the same house — aspect, not neglect

The two cheap things that extend it

Keep the debris off. Needles in valleys and at flashings are where the damp starts and where moss establishes. Clearing them is a blower-and-hands job, not a cleaning job, and it is the highest-return maintenance on a shingle roof in this climate.

Keep the gutters clear, especially after any roof treatment — everything killed up there comes down over the following two months, and a treatment followed by no gutter clearing produces a blocked system going into winter.

Method, chemistry and access is on the roof cleaning page. If the growth is raised and green rather than flat and dark, the relevant page is moss growing under shingles.