External timber cladding: the book

Langley Academy, Slough. Foster + Partners. Western red cedar

Timber facades have long been used on low-rise housing in North America and in Scandinavia. Most recently timber cladding is becoming popular in some other countries, Austria and the UK among them. Moreover, timber is nowadays being used as an external finish on medium-rise and non-domestic buildings.

But it is not easy. The main uncertainties related with using timber in facades involve: durability, weathering, dimensional change, corrosion, wind resistance and fire safety. Now, considering this long list, does it mean that timber is unsuitable as an external finish? Far from it. If design intent and construction details are in tune with its characteristics, timber can be a versatile facade material with a unique combination of performance benefits.

Architects in search of guidelines on how to use timber in facades have reasons to congratulate. This post is devoted to a recent book (released in April 2011) whose title says it all: 'External timber cladding: Design, Installation and Performance'. Its authors are Ivor Davies, a researcher from Edinburgh Napier University, and John Wood, professor of engineering at the same Scottish university. The book is more than its authors' baby. It is one of the outputs of a trans-national, EU financed project titled 'External timber cladding in exposed maritime conditions'. The project had inputs from Scotland, Iceland and Norway. More info about the project can be found here.

But this book is much more than the summary of an international study, and it is worth down to the last page if you are interested in timber for facades. In fact, it can be considered as the first true guidelines for timber facade engineering. The authors note very rightly that, during the past decade, facade engineers have tended to ignore timber in favour of more conventional - or more à la mode - materials like concrete, steel and glass. Timber exteriors have been left to architects (general practitioners) and timber specialist suppliers. This has proven risky sometimes, and reductive in most cases. Even more, the main technical standard for facades in the UK (the standard from CWCT) largerly ignores timber, whilst the existing guidance on timber cladding only covers a limited range of topics. This book comes to fill the gap between timber facade construction and facade engineering. It was about time!

The book is structured in six parts, each one dealing with the answer to six fundamental questions, exposed in a sort of 'ignorance pyramid':

What is wood?

How wet does it get?

What effects does it have?

How are these effects controlled?

How do the controls relate to fire safety?

What does all of this mean for facade engineering?

Chapter 1 describes what performance-based design means for timber facades, with a fundamental section on service life. Chapert 2, the top of the pyramid, deals with timber as a facade material, describing its main parameters. Chapter 3 covers moisture conditions in timber facades, and how to predict and to prevent them.

Western red cedar facade with pronounced staining

Chapter 4, as an outcome of the trans-national study, presents the results of site tests conducted on Sitka spruce as a timber cladding. Chapter 5 continues down the pyramid with fungal decay and insect attack. Chapter 6 goes for weathering, and how can we anticipate or respond to weathering in exposed timber cladding. In this chapter we understand why virtually all timber facades in Scandinavia are given an opaque surface coating - good to remember.

Chapter 7 adds to our limited knowledge in dimensional change on wood, and on how / why timber shrinks and moves. Good news for us: movements can be limited and estimated. Chapter 8 goes for corrosion - yes, that of metal fastenings, flashings and brackets embedded in timber. Chapter 9 describes in more depth the structural performance of timber facades - not of structures - which is an often ignored issue. Windloads, robustness of connections, dowel type fasteners and strenght grading are discussed here.

Selection process of timber design

Chapter 10 contains some of the most innovative pages: design for durability. It starts with a decision sequence to aid selection of a timber cladding design from a durability point of view - a must. It then goes down the sequence using the relevant EN standards on timber durability and preservation. It finally relates service life with timber class and use / exposure. Interestingly the authors don't take a side in the discussion pro / against wood preservatives: they present us the arguments in favour and against, so that we can decide case by case - as it should be.

Chapters 11 to 15 deal with fire and timber buildings. The fire triangle, fire testing, fire performance of timber, how to limit external fire spread, the role of air cavities, and a summary of fire regulations in the UK. Finally, a long chapter 16 is devoted to construction details for timber facades. This is an issue largerly discussed in other manuals, but again the authors bring novelty to the case, aided by clear and well drawn details. One of the good points is the treatment given to the junction between heavy (brick) and lightweight (timber) cladding.

The book ends with an updated and interesting list of appendices and references, among them the British and European standards on timber for panelling and external cladding.

Horizontal timber cladding details

In summary: if you are tired of simple and often repeated statements about how timber facades work, and want to know what is really going on and how this should inform your design decisions, this is your book. The authors challenge some of the prevailing assumtions about moisture, its effects and how they are best controlled. New light is shed on how moisture issues affect, and are affected by, the need to ensure that fire safety is fully addressed. And the construction details are based on a combination of new experimental data and a fresh appraisal and synthesis of existing information - they deserve a look and some thought, not just a copy-paste!

Go and buy it. You'll find the link to the publisher at the title of this post. It's not cheap, but it's worth every penny.