Timber

Timber is one of the oldest and most versatile building materials in the world. It’s a key part of almost all of Scotland’s historic buildings.

Wood is said to be either hardwood or softwood. Hardwoods are mostly used for interior woodwork as well as furniture. Softwoods are mostly used for structural beams or framing in walls. Good quality pine, a softwood, was used for sash and case windows and many other areas of finishing joinery.

Main hardwoods used historically in Scotland:

• silver birch
• ash (tools and equipment)
• beech (furniture only)
• elm (furniture and water resistant uses)
• european oak (structural work)

Main softwoods used:

• Douglas fir
• Scots pine
• Baltic pine
• Scandinavian pine (European redwood)

Durability

Timber is easily worked and is strong and light. Its strength to weight ratio is 20% greater than that of steel. The grain structure of wood means it has different qualities in three different directions. Used carefully, it can be suited to areas where compressive strength or flexibility is required.

Structural timber was, on the whole, carefully selected for quality on every level. It was close grained and free of knots. Shakes (drying cracks and splits along the grain) were generally permitted if below a certain size. Older timber is generally superior to modern timber in terms of quality and durability.

Risks

Various factors can lead to the decay and breakdown of timber. The most serious of these is rot, but attack by insect pests is another risk.

Rot is caused when damp wood is attacked by one or more types of fungi. The two most common forms of rot found in Scotland are dry rot (Serpula lacrymans) and wet rot (Coniophora puteana).

Despite its name, dry rot can’t progress if no moisture is present, although the spores can remain dormant. The term comes from the state in which it leaves timber: a dry, crumbly condition. Where dry rot exists, there’s usually some underlying failure in a building’s structure that is allowing excessive moisture to get into the wood. This might be a leaking pipe, gutter or roof.

Dry rot can survive in a range of conditions but will thrive when humidity is over 90% and the temperature is around 23°C.

For rot to take hold in wood, all of the below must be present:

• the fungal spore that introduces the rot
• a food source – i.e. the wood in which the rot takes hold
• moisture

Dry rot fungus can produce vein-like strands (rhizomorphs) through which food and water can flow. These can enter and pass through cracks in building materials – e.g. masonry, brick and plaster as well as wood – letting dry rot spread quickly.

The final stage of the spread of rot is when the fungus develops a fruiting body. This large, fleshy body releases spores into the air to spread the rot further.

Find out how wet rot differs.

It’s vital to deal with rot urgently.

Spotting dry rot

It can be hard to spot dry rot in its early stages, particularly as it usually develops out of sight, e.g. behind panelling or under floors. It favours damp areas with little air movement.

When it is taking hold, dry rot can look either:

• white or greyish, with the texture of cotton wool
• yellowish or mauve

In later stages, you may spot dry rot by:

• its large fruit body (sporophore) – this is circular or semicircular and usually reddish in colour with a white edge
• fine, reddish dust on the surface of wood
• ‘cuboidal cracking’, i.e. right-angled cracks, light brown in colour

General signs of dry rot include:

• wood that is spongy or soft to the touch
• warping of skirtings or panels caused by shrinkage
• discolouration of wood – darker or lighter depending on the type of rot
• a mushroom-like odour

Wet rot comes in a variety of forms. It requires timber to have a higher moisture content (between 50% and 60%) than dry rot needs. In many ways, wet rot can be seen as the natural decay process of wood.

Where wet rot exists, there’s usually some underlying failure in a building’s structure that is allowing excessive moisture to get into the wood. This might be a leaking pipe, gutter or roof.

For rot to take hold in wood, all of the below must be present:

• the fungal spore that introduces the rot
• a food source – i.e. the wood in which the rot takes hold
• moisture

The final stage of the spread of rot is when the fungus develops a fruit body. This large, fleshy body releases spores into the air to spread the rot further.

Find out how dry rot differs.

It’s vital to deal with rot urgently.

Spotting wet rot

There’s often no outward growth where wet rot is present. But some forms – like cellar rot – will cause dark brown strands to appear on nearby wood or plaster.

The fruit bodies of wet rot are similar in form to those of dry rot, though they have a wider range of colours – from green to red. You’re unlikely to see a fruit body indoors.

If wet rot is present in wood, you may be able to lift paint from its surface, exposing the damp and degraded timber behind.

General signs of wet rot include:

• wood that is spongy or soft to the touch
• warping of skirtings or panels caused by shrinkage
• discolouration of wood – darker or lighter depending on the type of rot
• a mushroom-like odour

It’s vital to treat rot as early as possible. Once established in timber, a rot fungus can spread throughout the wood as long as conditions remain right.

The long-term solution to avoiding rot in a building is to rid timber of moisture. But drying out an area of a building could take some time. You can follow the steps below to stop the spread of rot while drying is taking place.

Proper diagnosis of the kind of rot is vital to the effective treatment of both its causes and its symptoms.

Control moisture levels

Rot and fungi need water to live. Removing the source of moisture that caused the rot to develop is the first step to tackling such problems.

Decay will cease if the level of moisture in the timber falls below 20%. Rot shouldn’t develop in a well maintained building, unless any areas are poorly ventilated or suffer from excess moisture. A detailed survey could help to uncover the source of a problem.

Excess moisture inside may result from:

• poor chimney condition
• faulty plumbing
• blocked rainwater goods
• damage to the roof structure
• masonry decay or damaged render
• saturated foundations

Remove affected timber

You must remove and replace any timber weakened by rot to reinstate the building’s structural integrity, which may have been affected.

You don’t have to remove large areas of timber if the rot only affected a small area. For example, there’s no need to remove the floorboards next to a board that has suffered an attack, unless they too show signs of rot.

Dry out affected areas

Let any affected area dry out as quickly as possible. You may need to use dehumidifiers or heaters for this. But be careful not to heat wood too quickly as cracking may occur.

Ventilation is also vital to the drying process. Reopen any existing vents that are blocked up. In severely damp areas, you may need to add new ventilation for a time, e.g. by lifting floorboards next to a damp wall.

Use of fungicidal fluid

This is a simple, short-term treatment for areas next to removed rotten timber. You should apply the fluid using a brush or coarse spray. Avoid using atomised sprays: these are less targeted and can introduce harmful agents to a building.

The larvae of wood-boring insects can infest structural timber and feed on the wood until they mature. The holes that are often the first sign of insect attack in timber are the flight holes made by the emerging winged adults.

Wood-boring insects have three stages of development:

1. Egg – laid in timber.
2. Larva – which feeds on timber.
3. Winged insect – which emerges from the timber to lay more eggs.

Larvae can exist in wood for up to 10 years. Treatment must target not only the winged insect but also the larvae and eggs. The most effective long-term cure is to remove the conditions that allow eggs, larvae and insects to survive by ensuring the building is properly heated and ventilated.

Once moisture is removed from wood, and any decay dealt with, the insect infestation will usually end without the need to resort to using chemicals.

Learn more about the treatment of insect attack.

Insects most likely to attack timber

Woodworm

The common furniture beetle, or woodworm, is the most likely insect to threaten timber in Scotland. A serious outbreak of woodworm larvae can cause severe damage, and the larval phase can last up to three years. When coupled with dry rot or wet rot, timbers may need to be replaced.

It’s hard for larvae to survive if the moisture content of timber is below 12%. The sapwood (outer section) of softwoods, like pine, spruce and fir, is especially prone to infestation.

Signs of woodworm:

• tiny round holes in timber
• egg-shaped pellets of bore dust
• fine dust behind furniture

Where woodworm is suspected in rafters or other structural timbers, a small drill hole can be made in the centre of the wood to examine the drill residue. Clean-cut fragments with a rich resin smell indicate sound, strong timber. Dusty residue with no smell suggests decayed and unsound material.

Death watch beetle

Though uncommon in Scotland, this beetle is still a threat. A severe infestation can cause structural failure in timber. The dangerous larval phase can last as long as 10 years.

Larvae attack wood that has had some form of decay or rot in the past. They are more likely to attack built-in sections of timber, where ventilation is poor, and so may not be spotted for some time.

The holes left in timber by the emerging death watch beetle are slightly larger (around 3mm across) than those left by woodworm.

Wood-boring weevil

This insect attacks damp timbers especially where there is also decay. Wood-boring weevils leave oval-shaped holes, rather than round holes, and finer bore dust than woodworm.

Powder post beetle

These beetles usually attack new timber, including hardwoods and building elements such as flooring. Existing building elements are also at risk if conditions are right for the insect. Powder post beetles make small holes (around 2mm across) in timber and leave a very fine bore dust.

Environmental control and chemical treatment are the two main ways to get rid of an insect pest. With either option, post-treatment actions are important to prevent re-infestation.

Knowing which pest is present will help you to tackle the problem. Discover the insects most likely to attack timber.

Environmental control

The best long-term solution to dealing with an attack is to remove the conditions that allow insects to thrive in the first place. The easiest way to do this is to reduce the moisture content of the timber and areas next to it.

You can reduce moisture content by:

• adding or reopening sources of ventilation
• removing sources of moisture such as leaks from gutters, roofs or pipes
• using artificial drying methods

Another environmental control is to remove and replace any timber affected by dry rot or wet rot. Partially decayed timber is far more likely to be attacked by insects than sound wood.

Chemical treatment

Environmental controls are the most effective way to rid a building of insect pests in the long term. But you may need to use chemical treatments to help stop an outbreak and to stop established insect pests causing more damage.

Whichever treatment and method of application is used:

• know which species of pest you’re dealing with to ensure that the correct treatment is used
• only areas affected by an infestation should be treated
• the treatment must have enough contact with the insect to ensure that most are killed and the colony becomes unviable
• you should use a suitably trained contractor to carry out the work
• check what chemicals will be used and if they pose any risks to health – all chemicals should be Health and Safety Executive-approved
• find out how long the treated area must remain vacated afterwards
• ensure water tanks are sealed to avoid contamination

Methods of application

Your contractor is likely to use one or a combination of the following methods, depending on the situation.

• Spraying onto the surface – unlikely to be fully effective against some insects, but is more effective against woodworm.
• Injecting into the wood – more likely to ensure that the chemical agent penetrates deep enough to kill most insects.
• Applying paste to the surface – likely to ensure deeper penetration of the chemical agent and be more effective than spraying.
• Smoke treatment – only likely to be successful against emerging adult insects, so must be repeated over several years to get rid of the pest.
• Fogging of chemicals – used for large areas and targets eggs, but is unlikely to destroy all eggs in one go as hard-to-reach areas.

Post-treatment actions

To prevent re-infestation, you should:

• remove and replace all severely damaged or decayed timber – decayed timber is more likely to encourage future insect attacks
• remove any sources of moisture and damp – without this, pest control is unlikely to be successful in the long run
• monitor previously affected areas to make sure the pest is completely gone – more treatment may be needed if there’s evidence of activity

Swelling

Seasonal changes in humidity can result in timber swelling, which can cause minor issues to do with movement. For example, swelling can cause ‘binding’ in a door, which is where the door and its frame rub together. This prevents the door from opening and shutting smoothly and may cause it to stick or jam. This can happen in the summer when central heating is switched off and internal relative humidity rises.

Reduction in humidity can also cause timber to contract. This may lead to timber joints becoming loose, for example.

Splitting

Wetting and drying cycles can lead to shrinkage, which can cause wood to split or crack. Door and shutter panels and other elements made of thin timber are especially prone to splitting. Small cracks should be filled with wood filler. Wider cracks can be filled with slivers of timber and sanded smooth before redecoration.

Impact damage

This can be caused by a variety of hazards such as moving furniture or decorative work. A small scratch or gouge caused by impact damage can be repaired with wood filler or a small wood indent. More serious damage may affect the structural stability of a building component, requiring the damaged elements to be replaced.

Wear and tear

Wooden floors and staircases will inevitably suffer from some wear. In old floors, physical wear can be quite obvious, especially in areas of high traffic. This can add character, but trip hazards or weakness may also result from the thinning of the timber. Badly worn timber floorboards or stair treads should be replaced.

Wood can suffer from decay and damage in buildings that aren’t properly maintained. One of the most serious culprits is rot. Keeping your property watertight is the best way to avoid timber decay.

When decay is found, it’s not enough to simply replace rotted timbers – any cause of damp must also be addressed. Regular maintenance can help to prevent such problems arising in the first place.

Keep moisture levels low

Rot and fungi that attack wood in historic buildings need water to live. But rot has little chance to develop in a well maintained building, as the moisture level in timber is unlikely to rise above 20%.

Similarly, if decay has already begun, it will cease when the moisture level in timber is reduced to below 20%.

Be aware that excess moisture may arise from many sources, including:

• faulty plumbing
• blocked rainwater goods
• damage to roof structure
• masonry decay or damaged render
• raised ground levels

Problems in joinery elements are often caused by small but persistent defects. Even a small crack in a lead gutter can lead to saturation over time.

Ensure adequate ventilation

Make sure that your property is well ventilated throughout, especially those areas particularly prone to damp or moisture. These areas include cellars, voids in walls and the area beneath floors. This doesn’t mean that you have to live with draughts, simply that moist air has a chance to escape.

Look out for timber decay on:

• the foot of a door – where it comes into contact with damp stonework or rainwater splashback, especially if paintwork is in a poor state
• rafter feet, wall plates and timberwork that are part of or next to a roof valley
• the northern pitch of a roof – there’s less warming and drying by the sun, and water (condensation or leaks) lingers longer
• a timber staircase affected by damp – moisture can easily travel from a masonry wall next to the string course
• shutters – dry rot can result if water gets in to the timber

Thick, rubberised carpet underlay can prevent moisture movement and create the right conditions for woodworm and decay to thrive. In the past, laying linoleum made the problem worse. Today, vinyl and laminate floors can cause similar issues.

Repaint timber regularly

Keeping paintwork (or varnish) in good order will help to continue to protect timber windows, doors and shutters. Take care when removing past layers as older paints often contained lead.

View detailed advice on painting sash and case windows.

You should check whether you must first apply for planning permission or any other type of consent before making any changes to historic timberwork of any kind, in particular sash and case windows. Contact your planning authority to find out more.

Listed building consent

Plans to carry out any work that may change the appearance of windows in listed buildings will require listed building consent. You may also require listed building consent to replace or change, in terms of design or materials, other areas of historic timberwork.

Plans to install secondary glazing or special ventilation arrangements are also likely to require listed building consent.

Other consents

Your home may be unlisted but within a conservation area where planning controls over external door and window alterations are in effect. Your planning authority may also have planning policies controlling alterations to windows and doors in other locations such as flatted properties or properties along important routes. These may include acceptable paint colours for external joinery.

Find out more about listed building consent and conservation area consent.

Timber is one of the oldest and most versatile building materials in the world. Examples of early timber construction can be seen across Scotland – from the simple cruck framing of Lewis blackhouses to the grand hammer-beam roof of Edinburgh Castle's Great Hall.

Wooden dwellings

It’s believed that around half of Scotland was covered in forest in prehistoric times. Timber was probably first used as a building material by hunter-gatherers in the Mesolithic era (10,000 BC to 5000 BC) to support tent-like structures. At Balbradie, in Aberdeenshire, a timber hall measuring 24.6m by 13.4m and dating to 4000 BC to 3500 BC has been excavated.

Crannogs were built from about 500 BC through to AD 400. These dwellings were built in lochs and rivers, above the water level. A large number of timber poles were driven into the ground beneath the water to support the floor.

In the 12th century, motte and bailey castles – a style imported from the continent – were made mainly of timber. Over time, stone largely replaced timber, but wood continued to be used to create ever more complex roof structures and interior fittings.

In many rural parts of Scotland, homes were built using small lengths of timber set into a turf or masonry plinth. You can still see the remains of such houses today.

Darnaway Castle has one of the earliest surviving hammer-beam roofs, from the 15th century. Stirling Castle’s great hall, built around 1500, had one of the largest hammer-beam roofs of the time. It has since been re-instated with new timber.

By the 1500s, Scotland’s forests couldn’t provide enough timber to meet demand and wood began to be imported from the Baltic. From the 1600s, town councils pushed stone construction over timber, to reduce the risk of fire. Many of the nation’s timber buildings were lost as a result.

Timber has, however, remained to the present day an important material for constructing roofs, floors and finishings in houses and other buildings.

Other uses of timber

Timber played a huge part in the Industrial Revolution, as both a fuel and a structural material. Trains still cross the Aultnaslanch trestle viaduct, built from timber in 1897, though today a concrete structure supports the original build.

Traditional buildings in Scotland have featured timber components since the earliest times. Timber can be found in almost all elements, including floors, walls, windows, doors and roofs. At the close of the 20th century, timber began to make a comeback as a structural material, thanks to its sustainable nature.

Get in touch if you have any questions. We will be happy to help.

• Technical Research Team
• 0131 668 8951
• technicalresearch@hes.scot

Professional institutions

Scottish Timber Trade Association
Timber Trade Federation
Forestry Commission Scotland
British Woodworking Association
British Woodworking Federation
Association of Scottish Hardwood Sawmillers

Publications

Inform guides

An introduction to conservation and maintenance of traditional buildings for complete beginners.

External Timber Doors
Insect Attack to Timber
Maintaining Sash and Case Windows
Rot in Timber
Structural Joinery
Timber Floors
Timber Staircases
Timber Window Shutters  

Short guides

Information on building conservation topics for home owners, trades people and building professionals.

Sash and Case Windows

Research reports 

Detailed reports of research carried out by Historic Environment Scotland on a variety of building conservation topics.

Technical Advice Notes

In-depth technical information for building conservation professionals.

24 – The Environmental Control of Dry Rot

Conference Proceedings

Papers and abstracts from building conservation and archaeology conferences.

Timber and the Built Environment Conference 2002

Other resources

Building Scotland – Celebrating Scotland’s Traditional Building Materials
Timber Decay in Buildings – The Conservation Approach to Treatment
The Building Conservation Directory