It is important to understand and communicate the material’s character, behaviour and limitations and communicate these to stakeholders. Surface quality may be defined by local standards,¹ but manufacturers each use differing terms to describe various finishes, as Table 5.1 shows.²

Manufacturers typically provide benchmark samples or indicative images but the importance of mock-ups cannot be stressed enough given the potential variety in appearance. In some markets, there may be a choice of species used to form CLT, or decorative outer layers, with differing performance and visual characteristics, as Figure 5.3.

FIG 5.3 Timber species, availability and cost vary by region. Manufacturers typically offer a default (economic) option but may offer a range of timbers with differing characteristics and surface qualities. From left: Douglas Fir, Pine, UC3 pressure treated Pine, Spruce – as available from French manufacturer Piveteaubois.

For a more detailed summary of the range of surface characteristics by grade, refer to ‘CLT Panel Surface Quality’ within the Appendix.

Viewing panels in-situ

The highest grade (often termed domestic visual quality) may be appropriate when exposed adjacent to occupied zones; however, other areas may suffice with more cost effective lower grade panel surfaces. Viewed from further away, a lower grade of panel may be sufficient visual quality and for overhead applications, only a modest increase in height, as Figure 5.4, can double the effective viewing distance.

FIG 5.4 Soffit heights and effective viewing distances.

Aging of exposed timber

Yellowing and aging, which may be referred to as ‘suntanning’, of spruce and other whitewoods (other timbers typically darken) is the visible sign of lignin degradation over time. This is caused by ultraviolet (UV) light from both natural and artificial sources although it will be most apparent in areas receiving direct sunlight.⁵ This effect, often seen within the interiors of older alpine chalets, has been described as ‘aging gracefully, mellowing to a rich honeyed tone over time’.⁶ Some may view it less favourably, particularly when it results in contrasting light and dark ‘shadows’ being created by areas protected from exposure by furniture, fittings or picture frames. Owners and occupiers should be made aware of the issue and although affected areas can be re-sanded to expose paler wood this can be disruptive to occupants.

CLT surfaces and applied finishes

Some manufacturers offer factory-applied surface finishes such as clear or UV-protecting lacquers, UV stable silicate finishes, tinted or opaque finishes with or without fire retardant for limiting surface spread of flame but this is not typical. Most are reluctant to apply liquid coatings in their (dusty) factories to avoid slowing down production and taking up valuable space.⁷ Such finishes are typically applied on site once panels are installed, for similar reasons and although generally not conspicuous, clear finishes may change the light reflectivity of the surface slightly.

Figure 5.6 illustrates a freshly sanded spruce panel edge. By way of contrast, Figure 5.7 indicates a section of wall of untreated spruce, exposed to internal artificial light for around three years.

FIG 5.7 Aged spruce panel.

FIG 5.6 Freshly sanded spruce sample.

Decorative or protective waxes or oils may be applied to well-finished (domestic quality) panels to areas within reach of occupants (i.e. where most likely to show staining or dirt and oil build-up from occupant contact). Note that any stain or wax/oil treatment may highlight different parts of a panel depending upon whether the end grain is exposed or not (highlighting cut grain at panel edges). Figure 5.8A illustrates the effect of two coats of white wax/oil stain – the effect is marginal due to the pale colour of the stain and the timber but UV retardant additives ensure that this tone would be maintained. Figure 5.8B shows the effect of two coats of black wax/oil stain. The dramatic effect on the end grain (with radial rings seen) reflects the more absorbent nature of this section whereas the impact on the long grain sections is still limited.⁸ Lye is a surface treatment that mitigates the yellowing of softwoods (Figure 5.8C), enhancing the natural grain of wood and creating a light whitewashed effect when applied to spruce.

FIG 5.8 Coatings on spruce, from top: Two coats of white wax oil; Two coats of black wax oil; Softwood lye treated.

Surface boards may move or shrink and small gaps will likely open up – this is likely within the first year after installation and possibly beyond. Figure 5.9 illustrates gaps and cracking to vertical wall panels (as well as a visible finger joint to the outer board). Such cracks typically remain visible.

FIG 5.9 Vertical splits to outer lamella of vertical wall panel (beside finger joint in surface board).

Consider allowing a heating season before painting if possible. Figure 5.10 illustrates multiple fissures in the surface of a panel coated with two coats of white tinted fire-retardant finish shortly after the panels were installed. Despite each crack being only quite small, the colour contrast is quite marked. Such splits may be compounded above heat sources where the moisture content is reduced below that elsewhere in the building (Figure 5.11). Painting over knots, and surface deviations can be challenging; they will show through most light-coloured finishes being perceived as defects or marks. Consider a non-opaque colour tint to soften variations in the surface rather than trying to conceal them.

FIG 5.10 Panel with two coats of opaque white finish, knots and splits clearly visible.

FIG 5.11 Wall panel above radiator (with clear flame retardant finish) with disproportionate number of vertical splits due to localised heating and drying effects. NB: a vertical panel joint can be seen to the right of radiator.

Many common areas and commercial or larger buildings will require a liquid applied fire-retardant coating to limit surface spread of flame to exposed panels. Such coatings may be transparent, tinted or opaque and coloured as required, with varying degrees of gloss or reflectance. All will however modify the matt finish of timber and may appear worn over time in highly trafficked areas where occupants come into regular contact with the surface. All finishes used should be compatible with treatments such as fire retardants or edge sealants.

Panel connections and joint details

Panel connection design is frequently overlooked and can impact design time as well as cost. Exposed panels might be readily fixed with concealed fixings (fixings are typically long wood screws), screwed from back faces if access is readily available but will be more of a challenge when visible on both sides.

Panel connections may be considered as follows:

• Exposed, utility. The cheapest, most robust fixings are typically not attractive (lots of screws and/or galvanised brackets are used unless specified otherwise).
  
  
  
• Exposed, refined. May be more expensive and challenging to coordinate and deliver. These may include rebated, slotted or lapped joints.
  
• Concealed (e.g. plugged or covered afterwards, possibly with cover plates).
  
• Not visible (fully hidden connections and brackets).

Although factory finish and panel quality can be high with extremely tight tolerances, very large and cumbersome panels can be easily mishandled and edges to be exposed are not readily repaired. Panels need to be manoeuvred on site and edges and corners can be easily damaged. Placing structural panels is not like crafting furniture so consider ‘aiming to miss’ – avoiding unrealistic details and outcomes by staggering joints, offsetting datum lines from prominent features or introducing chamfers, rebates or shadow gaps to panel edges to help ensure that installations are not compromised by matters beyond your control. Coordinating exposed panel corners such as at the intersection of four panels at a single point, for example, may be a challenge due to cumulative tolerances, however small.

Surface board or grain direction should always be checked if it is being exposed to ensure it runs the intended direction and the visual quality of panel edges can be variable (they are typically not controlled), so it is important to discuss these with suppliers early.

Appearance after exposure and wetting

Panels are not intended to be exposed to moisture for significant periods. Figure 5.13 illustrates the effects of medium term exposure (nine months) on panel samples of spruce. Colour effects aside (samples exposed turned a glorious silver colour), samples exposed to water and air typically cracked and began to delaminate in part. The sample fully submerged displayed deformation but no decay (which would likely require exposure to air).

FIG 5.13 Weathered samples, from left: Top surface fully exposed to weather; Fully submerged (no air).

FIG 5.14 Example of a particularly convincing pre-construction render, of a proposed modular CLT scheme for developer Urban Splash.

Realistic rendering

Many visualisations deliver an unrealistic impression of how exposed panels will look. Given the importance of managing stakeholder expectations and the increasing reliance on digital models and rendered outputs to inform decision making and sign-off from early stages (as well as potentially sales), Table 5.2 outlines what may be considered realistic when developing and presenting proposals.

Such details may seem minor but will help avoid surprises, disappointment or extra work, ensuring proposals are acceptable, realistic and affordable.⁹ Consider reviewing outputs against images of any completed local precedents for comparison once these issues have been considered.