NBS National Building Specification Building technologies Wood shingle and shake roofing

NBS National Building Specification Building technologies Wood shingle and shake roofing

Wood shingle and shake roofing

The NBS Technical Team introduce NBS work section H64, published in NBS Building update 47.

With todays advanced technology and building methods, using wood shingles and shakes for roofing (or cladding) might seem strange. Indeed, you may ask: Why use them? A principal reason is that, whether you choose cedar, oak or chestnut shingles or shakes, they are all recyclable and all use renewable materials so their use helps designers achieve sustainable or green architecture. With this in mind, the wood used should originate from forests that follow established principles on forest management, taking into account environmental, social and economic factors.

There are other benefits too.

The Forest Industries Development Council believes that Buildings constructed of wood benefit from woods superior insulating properties. They are more efficient than concrete or steel, which translates into long-term energy savings and a reduced load on our environment.

The texture and colour of the wood shingles and shakes has the ability to create an individual building and enhance unusual properties. They are also light in weight, have a low rate of expansion and contraction, and have the added benefit of being easy to install and maintain.

Wood shingles and shakes can have high natural durability. Resistance to rot is one of the most important properties to consider when using wood shingles and shakes. However, erosion may occur due to weathering, and therefore it is necessary to protect them for effectiveness and longevity. But using good quality wood shingles and shakes, and installing them correctly, should result in a roof covering that will last over 20 years. In some cases, with preservative treated shingles and shakes, this can be increased up to 40 years.

Preservative treatment

BS 5534 recommends that wood shingles and shakes are pressure-treated with a suitable preservative to provide improved durability, irrespective of species. It is important that the wood has acclimatized before applying such finishes or treatments, which are generally applied pre-delivery,

by either immersion or spray.

There are three main types of preservatives and finishes: film forming, penetrating and preservatives.

    NBS National Building Specification Building technologies Wood shingle and shake roofing
  • Film forming: These are paints or solid colour stains which can be applied to smooth, saw textured or split surface shingles and shakes. Latex based paints and stains are continually evolving and several years ago it would have been unthinkable to use them on roofs as they had little tolerance to shrinking and swelling. However, coating technology is advancing fast with the result that film forming finishes now provide reasonable mould resistance and colour retention, usually for 12 to 15 years.
  • Penetrating: Semi-transparent penetrating oil and latex-based stains usually provide colour but do not conceal the grain and texture of the wood. They can be used on saw textured and split surface shakes. Oil based stains are suitable for cooler climates, whilst latex should be used in hot and humid conditions. The advantage of using semi-transparent stains is that they contain both a water repellent and a wood preservative, often with UV protection.
  • Preservatives: Water repellent preservatives are not as effective as semi-transparent stains and can only be used on saw textured and split surface shakes. Their life span is short and maintenance is high.

The environment and the desired life of the shingles and shakes should be taken into consideration when specifying the preservative treatment. This should be in accordance with BS 8417:2003 Preservative of timber. Recommendations, which covers traditional, boron based and water soluble preservatives.

Maintenance

However much you treat a shingle or shake roof with preservatives, for increased longevity it is important to limit the amount of debris that falls on it. Organic matter, such as moss and lichens that develop on the roof, will retain water and dust which will promote fungi and eventually rot the shingles or shakes.

To prevent the growth of moss and lichens, zinc or copper flashings can be nailed at the top of the roof pitch. Rainfall corrodes such metals and the resulting solution, being toxic to moss and lichens, can restrict growth.

Conclusion

Wood shingles and shakes are a traditional form of roof covering and are the only form that is from a renewable resource. Inevitably this means that they are biodegradable but, with appropriate care in design and construction, a shingle or shake roof should last a very long time.


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