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