A detailed annual survey and analysis of timber sash windows is helpful to determine the extent and precise  causes of any deterioration, and the category of renovation involved. You can do this yourself or employ a professional. If you choose to do this yourself, it is best to record your observations on a sketch elevation of the window.

Look for the following key points:

  • any signs of structural movement which is deforming the opening and damaging the window. (Remember that some signs of movemenet are so old that they have long since been stabilised or repaired, leaving the window in working order; its deformation expresses its age and character.)
  • evidence that the pointing of the frame to the wall opening is cracked, loose, or missing, allowing moisture to penetrate the sash-box timbers.
  • sashes that do not move properly, or at all. This may be due to overpainting of the joinery, stop beads that have been fitted too tightly, pulley wheels that have seized up because of overpainting or lack of lubrication, broken sash cords, swelling due to water absorption, or ineadequate lubrication between the sash and the pulley linings.
  • evidence of water absorption, indicating possible wood decay (wet rot). The signs to look for are:
    • interior paint failure caused by condensation
    • exterior paint failure
    • opening of the frame joints
    • degradation of the wood surfaces (the paint has flaked off) or depressions in the wood surface
    • cracked, loose, or missing putty
    • standing water, especially on the sills

It is important to ensure that water does not enter crucial joints, such as the lower parts of sills or jambs, where deterioration most often occurs. Joints should be kept tightly closed. In addition it is helpful to seal end grains with paint before assembly. You should also watch for any putty failure (which encourages water to sit on flat surfaces of the glazing bars and meeting rails) and for deterioration in the protective paint finish.

If the timber has been affected by rot, the underlying surface will be soft and fibrous. The wood’s moisture content can be measured with a meter; repeated readings of over 20% indicate that wood decay is likely unless steps are taken to dry out the area at risk and to eliminate the cause of the dampness.

It is easy to repair affected areas by cutting out the rotting wood and replacing it with a piece of sound, treated timber. Epoxy resins are sometimes used as a substitute for treated wood in these patch patch repairs.  However, it is important to paint over the repaired area as soon as possible, as resin degrades in ultra-violet light.

A recent survey indicated that, on average, only 5% of timber in windows that were being replaced was affected by decay. Yet a 1991 Gallup Poll revealed that 46% of home owners replaced wood windows because of ‘rotten timber’, and only 20% to reduce draughts and heating bills.

There are no inherent defects in the original design of the sash window, an extremely sophisticated piece of technology that has lasted, with minor modifications, for 350 years. It is also quite feasible to apply modern repair and maintenance techniques to our stock of existing sash windows. Therefore whenever possible original sash windows should be repaired rather than replaced. Permanent repair of a window may be less expensive than wholesale replacement, and no facsimile can be no other than new work. It is also possible to bring original windows up to modern enviromental standards without harming any features of historical value.

It is important to remember that where necessary:

  • permanent repairs can be made using appropriate materials and timber preservatives
  • the use of modern paints and methods can lengthen the time between redecorations
  • draughtproofing, secondary glazing, and even traditional shutters and heavy curtains improve energy efficiency and reduce noise transmission
  • it is possible to hinge a bottom sash to allow easy and safe cleaning
  • modern locking devices are available to deter intruders and to restrict opening

If sash windows need repair, it may well be sensible to upgrade them at the same time.

The science of timber preservation is relatively new. However, many timber sash windows of the 18th and 19th centuries continue to provide excellent service. In contrast, some of the new timber windows of the 1960s and 70s are already deteriorating.

Part of the reason for this situation lies in the choice of timber. Heartwood, from the centre of the tree, is naturally durable, and should always be preferred. Since 1945 it has been common practice to use poor quality wood for many joinery tasks. It is therefore important to retain old joinery wherever it is sound, or, if repair or replacement is necessary, to ensure that the wood chosen is heartwood or, at the very least, well-treated sapwood. It is not true that all modern softwood is low-grade: British, Scandinavian, and North American softwood from farmed, properly managed sources can still last for decades and even centuries, particularly if treated before use.

When repairing window joinery, it is always a good idea to deal with the source of the problem, such as water ingress first. If  you need to apply preservative treatments, these can be brushed onto the affected area after the decayed wood is has been cut out. A more sophisticated method is pressure-inject organic, solvent-based preservative into the timber through non-return valves, which are later filled. This is best done by a specialist and is not really economical for fewer than 5 windows.

The insertion of preservative rods containing water soluble chemicals (usually boric acid), which diffuse into the surrounding timber is also highly effective, but again is best carried out by an experienced person.

For more detailed advice on timber improvement services, please contact Timeless Wood Windows.

Useful information on the Sash Window History by the English Heritage.

Single-hung sash windows (where only the bottom frame moves and is supported by wooden gates or pins when open) were probably copied by the British from France some time in the mid-seventeenth century. The double-hung sash, however, appears to have been an British invention. This was an ingenious technological breakthrough that enabled a far more subtle and sophisticated system of ventilation to be achieved than was possible with the old, side-hung casement. It used a system of hidden, counterbalanced weights to allow both top and bottom sash frames to be moved independently. The earliest double-hung sash discovered so far dates from 1701; however, by 1720 double-hung sashes had spread only as far as Holland and the British and Dutch colonies.Until the early eighteenth century sash frames were usually made of native oak or a similar hardwood. By 1700, oak was becoming rarer and thus more expensive, while softwoods from Scotland, the Baltic, and North America were more widely available. From the 1720′s most sash joinery used deal, a generic term for pine or fir softwood.

As a precautionary measure against fire the 1709 Building Act required that the corners of a sash box frame be hidden behind the face of the brick or stone masonry and that ‘no door or window frame of wood… shall be set nearer to the outside face of the wall than four inches’. In 1774 this distance was increased to nine inches, and nearly all of the frame had to be hidden behind the face of the wall. While this legislation was initially only applicable to London, within a few years its provisions were being taken up throughout Britain and America.

By 1730 segmentally arched windows had largely been replaced by square-headed varieties that were cheaper to make. The glazing patterns inserted into these frames often took the form of six panes over six, although this was by no means the rule. Nor were the dimensions of each pane necessarily dependent on the principle of the golden section: in some cases, individual panes were broader than they were tall. The overall size of the window was, nevertheless, always kept in strict proportional harmony with the rest of the facade, in accordance with Palladian theory.

Early glazing bars were thick and robust, to support and protect the fragile glass. Most seventeenth – and early eighteenth-century examples were based on the ovolo, or quarter-circle moulding. As glass technology improved and glass became stronger, glazing bars became increasingly slender, with pointed (gothic) and lamb’s tongue mouldings becoming very popular. By 1820 some glazing bars were only 12.5mm wide (although, to provide lateral strength, these could be up to 38mm deep).

As with so many elements of the Georgian house, glazing-bar patterns and profiles varied according to the social status of the window. Thus, for example, basement or attic windows, used only by servants, were often fitted with old-fashioned, obtrusive ovolo glazing bars and inferior-quality glass.

With the introduction of cheap, strong plate glass in the 1830′s glazing bars became less necessary and views became accordingly less cluttered. By 1850 many sash-window frames had no internal glazing at all. However, the great weight of these frames and the absence of any internal supports necessitated the introduction on the upper frame of sash horns, extensions of the stiles which helped to strengthen the vulnerable joints at either end of the meeting rail.

Accoya® wood is currently being used for the following applications:

  • Decking
  • doors and windows
  •  ground and waterworks
  •  cladding, siding and facades
  •  garden furniture
  •  Conservatories
  •  civil engineering including glulam beams
  •  velodromes

Accoya® can be used in other applications where strength, durability and environmental protection are important.

Wood modification opens a new and broad range of innovative applications for timber, for which architects and designers have hitherto only considered using steel, synthetic materials or concrete. For example, a heavy loadbearing traffic bridge in the Netherlands, measuring twenty metres high by forty metres long, was completed in 2008 (see Figure 8). The structure comprises three- dimensional double bent glue-laminated beams, with exceptional dimensions of 1400 x 1080 mm. The minimal required life expectancy of the bridge is 80 years, with minimal maintenance and high safety level. Accoya® is the first ever modified wood used for heavy traffic bridges, finding its place in civil engineering projects.

The properties of wood acetylated by Titan Wood’s process have been investigated using both standard and non-standard test methods conducted by many independent test laboratories.

Extensive long-term field testing allows performance of Accoya® wood in service to be predicted with confidence. In many cases reference timbers, whose properties are well understood, were included in tests for comparison. In addition, there is a body of information available regarding performance of acetylated wood exposed in exterior trials for up to 18 years. The following sections consider specific properties of Accoya® wood and their significance.


Durability is the inherent resistance of wood to attack by wood destroying organisms. Use classes provide levels of risk for wood products used for different applications in the UK. These use classes and types of wood commodity included within each class are given in BS EN 335-2.

Fungi are the most important pests in the UK and, since they require a wood moisture content in excess of 20% to decay wood, wood that is exposed to wetting is at risk.

Insect pests are less important in the UK, the most common being death-watch beetle, house longhorn, powder post beetle and common furniture beetle. Further information on common types of insects and fungi which can affect timber is covered in TRADA WIS 2/3-32.

Durability of the heartwood of a species is assessed using BS EN 350-1. This involves laboratory standard testing to BS EN 113 and field testing to DS DS/EN 252. Table 1 shows the five durability classes from BS EN 350-1 with examples of species in each class from BS EN 350-2.

Tests to BS EN 350-1 with radiata pine have shown that acetylation increases its durability against fungi from the least durable (Class 5) to the most durable (Class 1). Class 1 species can be used in ground contact conditions such as fence posts, railway sleepers, structural timber in fresh water (e.g. bridge foundations) and sole plates below damp-proof courses. They achieve a 60 year service life in above ground applications such as windows, doors, cladding and decking.

Independently conducted tests to BS EN 330 compared the durability of window joinery manufactured from radiata pine that was untreated, treated with alkenyl succinic anhydride (ASA) and acetylated radiata pine, and showed the acetylated L-joints (see Figure 5) performed significantly better than the others. After nine years of exposure, the acetylated timber showed slight discolouration but no significant softening or decay, while untreated and ASA-treated timber were both severely decayed. This trial indicates that Accoya® wood meets requirements for long-life exterior joinery applications in the UK. It also demonstrates that the product continues to perform following coatings failure (which is induced at the start of this test).

Dimensional stability

Timber is a hygroscopic material. This means that its moisture content changes in response to the temperature and humidity of its surroundings and causes dimensional changes (shrinkage or swelling), known as ‘movement’. The degree of movement exhibited by timbers varies with species.

Accoya® offers much better dimensional stability (resistance to movement) in both radial and tangential directions than unmodified radiata pine. Tests have shown that dimensional changes can be reduced by up to 80% (see Figure 6).

Since the movement of Accoya® is low, it is ideal for exterior applications where dimensional stability is important, such as doors, window frames, garden decking, cladding, boat decks and garden furniture that are exposed to wetting and drying. Other specialist applications such as guitars, bridges and large dimension mouldings also benefit from its improved dimensional stability.

The benefits of using the more dimensionally stable Accoya® wood can be seen in decking and cladding applications. The maximum nominal widths of decking or cladding boards should not exceed 150mm to reduce effects of moisture movement and risk of ‘cupping’. Cupping occurs in flat sawn boards because of the differences in the amount of tangential and radial shrinkage across upper and lower faces. Since Accoya® is more dimensionally stable, flat sawn boards up to 325mm wide may be used without risk of cupping. This simplifies fixing and improves performance in service.


For many applications the frequency and cost of maintenance is a critical factor in determining the choice of material that is used.

Accoya® wood can be coated in the same way as other wood. Its better dimensional and UV stability means coatings last longer and require less frequent maintenance.

For designers, architects, builders and property owners, the extended coating performance and guarantees reinforce the excellent natural performance attributes of Accoya®.

Independent tests (conducted as shown in Figure 7) have shown that coatings perform significantly better on acetylated wood than on untreated wood. After 91⁄2 years’ outdoor exposure, coatings on untreated samples were failing with cracking, flaking and complete erosion in some cases. In contrast, the same coating on Accoya® remained serviceable.

Based on these results, it is predicted that the maintenance frequency would be as low as once every ten years. Independent modelling studies into whole-life costing of window joinery manufactured from Accoya®, sapele, laminated European oak and European redwood showed that the Accoya® performed best over a simulated sixty-year period. Better performance was a direct result of the increased time intervals between coating maintenance cycles for Accoya® joinery.

Close collaboration with Teknos, Sikkens and GORI (three of Europe’s leading wood coatings companies) has resulted in a package of coatings guarantees for windows and doors:

  • A 10-year guarantee for a translucent (wood stain), fully factory-applied maintenance coating system, to first brush applied maintenance.
  • A 12-year guarantee for opaque fully factory applied maintenance coating system, to first brush applied maintenance.

It is important to highlight that guarantees for these coatings on Accoya® are affected by the level of exposure of the site. Site exposure will also influence performance of coatings on unmodified wood.

Weathering performance

In outdoor weathering tests, Accoya® exposed to the sun changes colour in the same way as unmodified softwood, although rate of colour change is slower. The improved photostability of Accoya® allows for a broader range of translucent and lighter coloured coatings to be used, without the risk of the underlying wood darkening in service on exposure to sunlight.

Density and machining properties

Density has an important influence on the ease of wood machining. The rate at which a higher density timber can be cut or planed is reduced as density increases and the blunting effect is more severe.

The slight increase in the density of radiata pine as a result of modification using the Accoya® process has been shown to have little influence on ease of machining. Accoya® can be machined in the same way as unmodified wood. Cross cutting, ripping, planing and profiling result in a smooth surface comparable to other commonly used species in the joinery industry. In addition, the timber is relatively light and therefore easy to handle. These properties make Accoya® very suitable for joinery.


The strength of timber refers to its ability to resist applied forces that could lead to its failure. While some modification processes (eg thermal modification) weaken wood, acetylation does not reduce its strength.

The bending strength of both unmodified, non stress-graded radiata pine and Accoya® is 80N/mm2. This is towards the middle of the range for cladding and joinery timbers, with Western red cedar being 55N/mm2 and meranti 90N/mm2.

These values are obtained from standard tests on straightgrained timbers free from defects.


Hardness is the resistance of timber to abrasion or indentation. Hardness provides useful information on how timber is likely to wear. Timber hardness is normally measured using the Janka test.

The higher density and lower moisture content of radiata pine modified according to the Accoya® process results in higher hardness values when tested to ASTM D143. Results show that acetylation increases hardness of radiata pine by 47% in the radial and 52% in tangential orientation.

Hardness values for Accoya® are similar to those of mahogany, black cherry or African walnut, wood species often used for flooring.

Thermal properties

Thermal conductivity of construction materials is increasingly important when considering performance of the building envelope, in particular windows and doors. Where wood is used for joinery (doors and windows) reduced thermal conductivity is beneficial.

Tests with Accoya® show this material has a lower thermal conductivity (0.108W/m2/K) than that normally presented for softwoods and hardwoods (0.150 and 0.160) respectively.

For this reason Accoya® joinery will give improved thermal performance.

Interaction with metal fixings

Since low concentrations of acetic acid may remain in the wood after acetylation, risk of corrosion of metal components is increased to a level comparable with many durable hardwoods.

For this reason, and the fact that Accoya® wood may be exposed in applications that promote corrosion, only A2/A4 grade stainless steel and specifically tested alternatives should be used when wood contact or its interaction with the component is likely.


Accoya® wood can be glued using many commonly used wood adhesive systems. However, it is imperative that the product’s modified properties are taken into account. TRADA recommends that Titan Wood be contacted in order to confirm compatibility of specific adhesives with this modified wood.


At the end of its serviceable life, Accoya® may be recycled, disposed of responsibly and with ease (unlike many building products) because it contains only naturally occurring wood components. It can be composted or incinerated without release of toxic materials into the environment. It could also be used as a raw material for particleboard, MDF or other reconstituted wood.

This is the second in our series explaining Accoya, one of the latest timber products, that we proudly use in our materials.

Accoya® is currently manufactured from radiata pine that is supplied from sustainable sources in the southern hemisphere including sources certified by the Forest Stewardship Council (FSC) or the Programme for the Endorsement of Forest Certification (PEFC). Certified timber is harvested from well-managed forests and full ‘chain of custody’ ensures that all wood used to manufacture Accoya® is sustainably sourced to the highest standards.

More information on the process of timber certification as a tool for ensuring supply of sustainable timber products is covered in TRADA WIS 2/3-58. Both FSC and PEFC certified woods meet requirements of the UK Government timber procurement policy via The Central Point of Expertise on Timber Procurement (CPET). Therefore, Accoya® is suitable for use in UK government buildings, both national and regional.

Radiata pine (also known as Monterey pine) is a native of North America, but is widely grown around the world. The largest areas of plantation are in New Zealand, Chile and Australia. Radiata pine was selected for the manufacture of Accoya® because pruning of the trees results in large volumes of clear grade wood. Modification takes place throughout its section. This allows for conversion of

Accoya® post modification without risk of exposing untreated wood, which is a real benefit over preservative treated wood. As part of the Accoya® quality control processes, sample cores

are taken from the centres of boards to confirm modification has taken place throughout the section. Figure 2 shows a sample being taken from the centre of a section of Accoya® wood.

The modification process involves impregnating wood with acetic anhydride, a colourless liquid used for the manufacture of photographic films and in the production of aspirin and other medicinal drugs. Accoya® is produced by placing dried timber in large autoclaves and then introducing and removing acetic anhydride from these vessels.

The finished Accoya® wood has a very low residual acetic acid (vinegar) level and slight associated odour. This odour dissipates naturally or can be immediately blocked by wood coatings. Acetylation causes no significant change to wood colour. Although there is darkening of the outer surface, this is insignificant compared to wood treated by thermal modification processes which considerably darken wood.

Accoya® wood is presently manufactured by Titan Wood in the Netherlands.

We strive to offer the latest advances in timber window, door and shutter technology and materials. We are pleased to provide offer the Accoya® modified wood as part of our timber materials.

In this and the following 3 posts, we will outline the principles of the Accoya® wood modification process, properties of Accoya® wood and its increasing number of applications.

Modified wood is a term used to describe wood whose molecular composition has been altered, normally by chemical or thermal processes. This differs from a conventional preservative treatment where wood is impregnated with a biocide. An increasing range of modified wood products are available in the UK and it is essential, when specifying, that they are considered individually because their properties vary considerably.

Accoya® is the brand name of wood that has been modified by acetylation. This modification process replaces the water binding sites in the wood cell wall with acetyl groups that do not bind with water. These acetyl groups occur naturally in wood, albeit at much lower levels than in Accoya® wood.

Although Accoya® wood has only been on the EU market since April 2007, research on acetylated wood began as early as 1928. For this reason there is a wealth of knowledge on the performance of acetylated wood.

Acetylation significantly improves several wood properties. It increases durability of both sapwood and heartwood, and makes timber more dimensionally stable. For these reasons, Accoya® wood offers many advantages over unmodified wood. Some properties, such as ease of processing, strength, adhesion and ease of fixing, are relatively unaffected.

Radiata pine is the wood species presently used to manufacture Accoya®, although the process has proven effective on a range of other softwoods and hardwoods.The radiata pine is sourced sustainably and, since acetylated wood may be re-used, recycled and disposed of without the risks of environmental damage, Accoya® offers an alternative to using preservative treated wood (or increasingly scarce naturally durable timbers) for applications where there is a risk from decay.



Accoya is becoming widely recognised as the number one timber in the production of high quality timber windowsdoors and shutters.

Whilst we have been working with this timber for some time, we asked the UK management team to come and see the staff at our factory for a training day. This covered (and confirmed) best practice on all aspects of using Accoya.

We can tell you in great detail why Accoya is ideal for use in external joinery, but in summary:

Durability: Class one durability, comes with a 50 year anti rot warranty

Dimensional stability: reduces selling and shrinkage by up to 75%, meaning paint finishes last 3-4 times longer than on conventional timbers

Environmental credentials: FSC certified, improved thermal performance, can be safely recycled and long product life

It may not be the cheapest of timber and this is why many companies will not use it. The result is invariably a product that is still susceptible to rot. At Timeless we look to offer the best possible performance from our products – this means using the best possible materials and that is why Accoya is used for the majority of our timber window and products.

At Timeless we can manufacture all types of folding timber shutters to compliment your windows. We take pride in the fact that all our internal shutters are to bespoke designs and are custom-made. Where necessary, we can create exact replicas of shutter doors and panels, matching materials and profiles as required. We construct and install shutters using traditional methods, creating a stunning feature for any room, as beautiful as any piece of furniture and enhancing the look of your timber windows or doors.

As well as providing a detail that is very pleasing to the eye there are other benefits of fitting internal timber shutters to your sash or casement window:

  • Shutters offer tremendous improvements to the thermal and acoustic performance of the existing window.
  • Timber shutters will typically replace the need for curtains in a room, giving privacy to the room or blocking the light out completely.

See some photos of numerous shutter projects completed by us.