FIG 5.0 (chapter opener) The main prayer hall of Cambridge Mosque by Marks Barfield (2019), with white painted CLT wall and roof enclosure beyond tree form glulam structures. Exposing CLT reflects the increased expression of pattern, texture and natural materials in architecture and interiors and an apparent awareness and acceptance, or even celebration, of what the Japanese term wabi-sabi.1 However, CLT does not need to be visible to be advantageous in use. Limitations on expressing structural elements may be due to issues of perception, taste or important performance aspects, typically fire or acoustic issues. As such, typical CLT apartment buildings in the UK, including the highest profile examples, have nearly all (if not all) CLT panels concealed rather than exposed. As a composite formed from multiple sections of a natural material with inherent, sometimes significant variations, deviations in appearance must be expected. Some aspects can be controlled; others are less predictable or unavoidable. Other features may become apparent only over time, e.g. with changes to moisture content and resultant movement or exposure to light. Managing expectations from the outset is critical. Teams and clients should remember that all materials will move to some degree and even small cracks or tolerances may be noticeable when viewed across large panels and that structural elements (concrete, steel or timber) should not be assessed as one might a piece of furniture. Such detail is often overlooked leading to surprises on site and difficult conversations between teams and clients. Figure 5.1 shows AHMM’s Cobalt Place (102 apartments, completed 2015). CLT was reluctantly concealed to suit the sales agents’ idea of what would suit the local market (that included wooden flooring). Doing so did however prove a cost-effective and easily understood and conventional means of providing fire protection and appropriate acoustic performance. FIG 5.1 Typical interior view, full CLT structure concealed. 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. 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.
CHAPTER 5
VISUAL ASPECTS
MANAGING EXPECTATIONS
DESIGN STAGE ISSUES
Surface appearance
Manufacturer
Lowest grade
Medium grade
Highest grade
According to European standard4
Visible surface not covered by standard
Planed, may be lightly sanded to Class B/C
Planed and sanded to Class A/B
Binderholz
Non-visible (NSI)
Industrial areas (class BC)
Living areas (AB)
KLH
Non-visual Quality (Nsi)
Industrial Visual Quality (Isi)
Domestic Visual Quality (Wsi)
Stora Enso
Non-visible guality (NVI)
Industrial Visible Quality (IVI)
Visible guality (VI)
Viewing panels in-situ
Aging of exposed timber