Cedar Singles #5
Treating and staining a cedar shingle roof

old cedar shingles before
and after treatment

Cedar shingles are an excellent roofing material for garden buildings, sheds, and summerhouses which not only look very classy and natural, but will often outlast most other options such as roofing felt and corrugated sheet materials.

To get the most out of cedar shingles they need maintaining and protecting. Cedar is naturally a very durable timber, but it deteriorates if constantly exposed to damp shade and a build up of organic material.

The Problem

Under ideal conditions cedar shingles dry out quickly after rain, but in countries with long wet winters (such as the UK) or where roofs are north facing or shaded by trees, they may not dry quickly enough allowing problems such as rot to set in.

There are many practical measures that can be taken to help a cedar roof avoid this fate, most of which involve protecting it from excess water ingress and helping it dry out quickly. I have outlined these in Post #3: Identifying and preventing damage, and suggested regular maintenance to help avoid problems arising.

Solution 1: Coatings that Reduce water absorption

A suitable water-repellant stain applied to cedar should help prevent the amount of water penetrating the shingle in the first place.

In my experience, treated timber that I have used in fences and on my garden buildings lasts much longer and shows much less weathering when it is coated (painted or stained) than where it is left bare. This has been so striking in my garden that I intend to coat all my fences as I replace them in the next few years. Commercial pressure preservative treatment systems (such as Tanilith E) recognise various use categories which represent increasing levels of vulnerability. The highest category (4) represents situations where timber is most likely to rot, and therefore, needing the highest level of treatment:

• Use Class 1 – internal building timbers – no risk of wetting.
• Use Class 2 – internal building timbers – risk of wetting.
• Use Class 3a coated – external timbers used above ground contact and coated.
• Use Class 3b uncoated – external timbers used above ground contact and uncoated.
• Use Class 4 – external timbers used in ground or fresh water contact.

Notice that category 3 is split into coated and uncoated timber. Quite simply, coated timber is better protected. Here are some examples from my garden to make the point:

The top two photos show examples of heavily weathered timber that was uncoated (despite being pressure treated). The lower two examples are coated timber: the cedar greenhouse shows no sign of deterioration whatsoever, likewise, the pressure treated weatherboards show no signs of deterioration. Whilst I recognise that these are not entirely comparable situations, I believe the point stands that coated timber stands up to the elements far better than uncoated. And the main reason for this is because it keeps the wood dryer.

Microporous stains and coatings

Stains (which are somewhat transparent) and opaque coatings (which are not) should be microporous so any water that does get in is able to escape by evaporation. These modern coatings allow wood to breathe. Traditional gloss and acrylic paints on the other hand, form a sealed skin which can cause problems by trapping moisture in the wood.

Not only do modern microporous coatings act as a physical barrier to water penetration, but many also contain water repellant waxes which cause water to bead on the surface and thereby run off more easily.

I rate Osmo paints and stains for their excellent coverage and compatibility with exterior woodwork. The white and black painted timber you see throughout my garden are Osmo products. I have also used Osmo decking oil on our decking steps.

These products are superb to use, repel water remarkably well and never peel, blister or crack. They do, however, weather slowly by gradual erosion so after a few years need touching up. They are very easy to maintain as you can just slap another coat on top at any time — they do not need rubbing down first! Below are some examples of where we have used these products.

The only problem with these coatings is that they do not contain fungicides so they are prone to discolouring with algae – especially in the shade. That said, Osmo do produce a preservative/anti-fungal/insecticidal undercoat for bare softwoods which may reduce this problem if applied first. Also, they recently introduced Natural Oil Wood Stains which contain an anti-fungal and come in a range of semi-transparent natural wood shades. I would think they would be an ideal for a recently installed cedar shingle roof, but for an older one, like ours, the focus needs to be on the preservative function.

NOTE: Oily woods like cedar need to weather for a minimum of 12 weeks before they can be treated.

Solution 2: UV protection

Along with water penetration, part of the weathering of timber is caused by ultraviolet (UV) rays degrading the top layer of the timber. It is the UV that turns uncoated timber grey. What exactly is going on when wood turns grey?

“During this process, the UV rays damage the polymer bonds within the wood substrate (cellulose, hemicelluloses, and lignin). Lignin is especially prone to UV degradation, and if not protected, the wood cells will become faded and loose, subsequently eroding the wood surface.”

‘Weathered’ Wood and How to Protect Wood From Weathering, Silvatimber.co.uk

During the UV greying process, the timber loses its natural oils from the surface, making it more water-absorbent. Cells close to the surface break down and wash away leaving indentations. Mosses and algae can more easily get a foot hold in weathered timber speeding up the decline and trapping degraded surface cells which begin to form simple soil for these plants.

UV degradation can be slowed by using a pigmented stain. The pigments absorb the UV before the rays reach the wood. Furthermore, most of these stains include oils and waxes that protect the wood further slowing the loss of natural oils. This can be important in the case of timber such as cedar which depends on these natural oils for its resistance to decay organisms.

Solution 3: Preservative treatment

It is commonly believed that cedar shingles do not need preservative treatment because they are highly durable with their natural oils acting as an effective preservative. However, pressure treated cedar shingles are increasingly available with claims that preservative treatment increases their longevity.

Another situation where preservative treatment seems warranted to me is as a maintence treatment, perhaps every 5 to 10 years, or where there has been some evidence of deterioration.

So where did the idea that cedar roofs don’t need preservative treatment come from? It appears that it was mainly an economic consideration. In the 1997 edition of Troubleshooting Guide to Residential Construction, editor Steven Bliss points out that “Until recently it made little sense to preservative-treat an existing wood roof because of the low cost to replace. But as replacement costs have increased, the economics of preservative treatment look much better”

See: Troubleshooting Guide to Residential Construction (Google Books) for information on the effectiveness of cedar shingle preservatives, power washing and more

Pressure treated shingles

It is now possible to order pressure treated cedar shingles which adds about 5% to the cost, yet pressure treatment is claimed to increase the life expectancy by 20% (Marley Red Cedar Roof Shingles) If I were building a new cedar shingle roof I would definitely use pressure treated shingles. The small additional cost looks like a good long-term investment to me.

Renovation treatment

Where an old cedar shingle roof has begun to show signs of decay (especially if it has not been well maintained) a case can be made for treating it with preservative.

Such preservative treatment should only be done after the causes of decay have been identified and remedied: algae, lichens and mosses removed, overhanging branches cut back, gutters cleared and downpipes checked, split and damaged shingles replaced. Once these have been completed and the roof cleaned, then a preservative treatment can be applied.

Many traditional treatments for cedar shingles used copper-based preservatives which were considered the best, but most of these have been phased out, and I could not find any copper-based preservative in Europe. In the US copper naphthenate products are still available and are recommended for cedar roofs.

My roofs are all 15+ years old and had started suffering from weathering and lack of maintenance. I wanted to significantly slow the deterioration in the hope that they might last for another 20+ years. I decided that both a preservative and a water repellant stain was prudent, but I didn’t want to have to apply two separate products, so Osmo was out of the question.

Oil-based or water-based treatments?

I prefer spirit-based (oil-based) products over water based products for exterior woodwork. Whether that is a stain, a paint, a preservative or a waterproofing agent. Oil is superior in almost every way as far as I can see. Being used outdoors fears about fumes are overblown, and most modern formulas are low-aroma anyway. Cleaning up brushes with white spirit is only marginally less convenient than soap and water. What matters to me is performance as a treatment.

There has been a move away from solvent-based (oil-based) products in the last decade with many popular brands of fence stains now only being available as water based formulations. While these products claim to “penetrate deeply into the wood” this is mostly marketing hype than reality. Any side-by-side comparison will demonstrate that oil based products are far better absorbed, penetrating quickly and deeply into the timber. For all the hype, water based products tend to smear over the surface after soaking just the surface fibres, especially when applied to naturally oily timber such as cedar.

Steven Bliss points out another benefit of oil based products over water-based ones when applied to cedar roofs: they reduce cupping and splitting because they do more to seal the natural oils in the shingles than water-based treatments.

Part of the excellence of Osmo products is because they are solvent-based, which helps them go on so smoothly and easily. The preservative stain we ended up choosing for our cedar shingle roofs (Barrettine wood preserver) is also oil-based and that proved a wise choice.

Combined Preservative and Stain

In the end we chose an oil-based combination wood preserver and wood stain: Barrettine Wood Preserve, Light Brown. The reasons behind this choice were:

• Oil based – high penetration
• Three preservatives (permethrin – against wood boring insect; Tebuconazole and Iodopropynyl Butyl Carbamate – fungicides protecting against wet rot and decay)
• UV fade resistant colous – Reasonably natural looking (always a concern with wood stains)
• Good water-repellant properties

Application

We applied the preservative-stain with a good brush, working on a section of the roof that could be reached comfortably from the ladder. Starting at the top and working down, the coating was applied to saturation, ensuring that plenty was able to get into the gaps between shingles.

The shingles absolutely drank it up, so It was not possible to ‘paint it on’ as you might expect with a normal paint. Rather, the brush was used to load each shingle with preservative repeatedly until it was nearly saturated. Only then could the excess be brushed out!

It took a good three hours to treat each side. The total area was about 20 square yards (17 m²) and used 7 litres of preservative-stain, which is about 70% higher than stated on the tin, but that was anticipated as this was the first coat on very weathered rough timber. And we applied it very liberally! The tin recommends two to three coats. I’m not sure its getting even a second one. We will see.

It claims it is effective against wood destroying fungi, blue stain and wood boring insects at 180 ml/m². By my calculation this roof has received more than double that amount, so we should be good for a few years.

The resultant colour is darker than anticipated (it being classed as ‘light’ brown). On the plus side, darker colours should provide better UV resistance and last for longer.

Here is a close up two days later when it was fully dry. I sprayed it with water:

The treatment shows off its water-repelling properties making the water sprayed on it form beads on the surface of the shingle. Last week that same water would have been absorbed instantly, like ink on tissue paper.

[TO BE CONTINUED WHEN ONCE STAIN THE CABIN ROOF!]