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.

Open | Close: the new Scale series by Birkhauser

The Birkhauser construction books are a source of never-ending information, that grows larger every year. Some may criticize the fact that authors and themes are too German-oriented, understandable for a Basel-Berlin located publisher. But truth is, in my opinion, that if Birkhauser did not exist, we would miss it - and a lot!

There are many more books there apart from construction. In architecture the list is long and with some big names (Le Corbusier complete works to name just one). But that doesn't make Birkhauser unique: their uniqueness in the world publishing scene is their capacity to push the best known specialists in construction to write, to draw and to expand the knowledge of world readers - in spite of the German touch, or maybe because of it?

A friend has brought to my attention a book from 2010, titled 'Open | Close. Windows, doors, gates, loggias, filters' This is the first book of a collection called Scale. The second book of the series will be released shortly, 'Enclose I Build'. According to the Editors' foreword, 'The Scale series (...) provides illustrations at various different scales and with various degrees of abstraction, wich demonstrate the interrelation of space, design and construction' Judging by the first book of the collection, I would say that the degree of abstraction is a bit too high, and the technical scale is somehow lost in translation.

Open | Close examines architectural openings, from idea to implementation. The authors did not see the need to have a Contents page, which I see as a bad decision, so here it goes: Introduction - Windows - Filters - Doors and gates - Case studies - Appendix. I was intrigued because Loggias, one of the promises of the title, are not a chapter: in fact, loggia is a word almost non-existing along the book, apart from the title. A real pity.

The Introduction is poetic to say the least. Issues covered here range from 'Atmosphere' to 'Passageway, threshold and entrance' to 'Spatial openings and intermediate spaces' to 'Ambience and materials'. Luckily it's not too long. The second chapter, Windows, is the longest and at least to me the most disappointing. Aluminium windows and plastic windows share one page of the chapter. Enough. Window hardware (that is, fittings and the like) deals with old drill-in hinges, cremones and espagnolettes used in ancient timber windows, but tilt and turn fittings (covering 85% of all windows installed in Germany, as we learn) don't have a simple illustration or a technical description. Another pity.

The third chapter, Filters, covers sun and glare control systems, shutters, blinds, curtains and screens. To say 'covers' is a figure of speech: it runs short and passing through all these points. Chapter four is devoted to Doors and gates. Again: fire rated doors and emergency exits (both) can be dealt with in one page, one page meaning a short column of text and one big sketch. Chapter 5 brings us nine Case studies. We had been promised at the Introduction that the examples would be both practical and generally applicable. Maybe, but at least that's not the case with the conversion of the Moritzburg castle in Halle, by Nieto Sobejano. The project is one of the more interesting ones, the problem is that no openings are brought to our attention apart from one small section of a skylight in a nice roof construction - clearly not an opening in itself.

The book ends with an Appendix that includes several tables and information pages. If your project is in Germany and you don't speak German, it will be of help. There is a list of standards, most of them DIN and EN but not complete and maybe not too reliable either. DIN EN 12208, dealing with watertightness of windows and doors, comes under the heading 'Doors - Thermal insulation'. Would you say DIN EN 14351-1, the product standard for windows and external doors, the standard on which CE mark for windows is given, should be in the list, maybe under the heading 'Windows - Planning in general'? You got it: it's not there - nor anywhere else, but you can enjoy DIN 107 instead, titled 'Left and right designation in construction engineering'. A pity once again.

Then there is an 'Associations and manufacturers list'. All associations are German. No problem with that, but couldn't the authors (three architects from TU Darmstadt) do some Google digging and add the equivalent British, French and maybe US counterparts? Manufacturers are from... yes. Reynaers is in the list because they have an address in Gladbeck. Technal is not in the list - OK, too French. But Wicona, a great supplier from Ulm providing aluminium window systems all around Europe, is not in the list either! Why?

My friend paid 49,90€ for this book. I arrived too late to tell him that he should have invested less than half that quantity in buying another Birkhauser book, a much humbler one: Facade Apertures from the Basics series. Its cost? 12,90€. The amount of valuable information? Quite the same, with less nice colour images for sure. This - having arrived late with my advice - is the biggest pity indeed.

Introduction to Architectural Science

Introduction to Architectural Science
Steven V Szokolay is an Australian architect and energy / environmental consultant who has worked in Sydney and London, and teached in Liverpool, London and finally Queensland. In this university he was the founding director of the Architectural Science Unit, as well as Head of the Department of Architecture. He is now retired.

This book is a great introduction to the facts, concepts and numbers of heat, light, sound and energy applied to architecture. The second edition from 2008 is the most recent one. The reader can be a practising designer or an undergraduate student; both will benefit from a clear presentation of facts, examples and data sheets. But the best in my opinion is how difficult issues are treated in a way that sounds as if they were being taught for the first time. Just one example: the description of glare (part 2.2.4, page 148). Szokolay aptly describes the differences between glare due to saturation and glare due to contrast, as well as discomfort vs disability glare. Another clear definition is that of daylight factor (part 2.4.2), also belonging to the lighting chapter. But probably the most useful part of the book is Part 4, Resources. Here we can find the basics of energy, renewable energy, energy use in buildings, water, waste, and sustainibility issues. Short and to the point.

Szokolay has also co-authored with Andris Auliciems a 60 pages booklet on thermal confort. The thesis is very attractive: nowadays lifestyles, clothing, technology in building construction and microclimate controls have tended towards homogenizing indoor environments to which humans are exposed. These developments may be driven by market forces, but the result is that humans are becoming adapted to a very narrow band of conditions. This may be a threat to our survival as species: within a changing environment, survivability is greater among the adaptable than the adapted. Which trend is being favoured by technological development and thermal design?

The booklet is available on the net. Do yourself a favour and have a look at it here: Thermal comfort

Metal curtain walls

One more example of a great old document, this time from 1955 and related to a then start-up facade system, the metal curtain walls in the United States.

This time it's a book (fully available to us thanks to the 'make no evil' Google guys) with the title 'Metal curtain walls'. It is a compilation of the papers and discussions presented at a conference in Washington in September 1955, organized by the Building Research Institute, then a division of the National Academy of Sciences.


Let's start with the obvious: this conference seems to have been a well organized event, the presenters were among the best available specialists at the time, and the response of the public was overwhelming, judging by the number of companies and experts that attended the conference. It was definitely a good time to talk about the matter. Ten years after the end of the war, and just one year after the Lever House opened at 5th Avenue, there was a lot to say and to learn about curtain walls.

The presenters at the conference came from different grounds. There were architects, some big firms and some from academia. Among the former was Max Abramovitz, partner at Harrison Abramovitz in New York and responsible for the planning process of the United Nations headquarters in Manhattan between 1947 and 1952. There was also a partner from SOM, describing the firm's design method for curtain walls at the Ford Motor headquarter in Detroit. The result of an investigation from the School of Architecture at Princeton University about stainless steel curtain walls (conducted in 1954) was also presented.

There were several specialists from different manufacturing companies and some other experts on thermal and acoustic issues. One presentation discussed the role of curtain wall erectors. It is surprising to see how little things have changed in this regard: the poor chaps who installed facades were experiencing the same problems and showing the same clear logic as their equals do today.

Poorly insulated curtain walls have a two times better U-value than non-insulated brick facades. It all depends on the comparison you select to do...

The general impression among the panelists was that curtain walls were going to be the next big thing in facades design and construction for the years to come. They were damn right. One might expect here a lot of naive comments on the advantages of the new technology. Instead, there was surprisingly very few self-praising; quite the opposite, presenters with different technical backgrounds were rather clear in assessing the many problems still unresolved for lightweight cladding.

It seems that at the time architects and owners had fallen in love with glazed facades, whilst contractors and specialists seemed already worried about the limitations of the technology in terms of thermal & acoustic insulation, air tightness, water leakage, metal corrosion and coating durability. The summary of a survey on metal cladding panels, conducted that same years among owners and contractors, shows it cristal clear. Only 2.5% of building owners were dissatisfied, while 13% of contractors would not recommend using curtain walls again. See the details here below.

Things were bound to change in the years to come, with owners becoming less and less interested in curtain walls - but this was still 1955. The Seagram Building details were at Mies' desk by then, and Tom Wolfe had not the least idea that 25 years later he would be writing 'From Bauhaus to our house'.

The architect's view
Max Abramovitz's text is very interesting. We are literally attending to an architect's explanation of modern architecture provided to an audience of builders and manufacturers. He could be outspoken and not too academical. He started saying that the idea of curtain wall was not new, but a development of wooden skeletons and non-bearing stone veneers or other lightweight walls. He then described the factors that made curtain walls so appealing to architects. The list deserves a quotation:

1. The dry wall, allowing facade construction to proceed even in wet and cold weather.
2. Lightweight, saving in construction manpower and load to support.
3. Larger units, thus reducing the number of joints in the facade. For him joints were an architect's headache: the less of them the better.
4. Non-corrosive and fire resistant materials, that is, metals. A curtain wall wasn't more fire safe then than today, but architects have always found it difficult to distinguish between fire reaction and fire resistance.
5. Prefabrication, very neatly expressed: 'We will get more construction for our money'.

The facade contractor's view

My favourite presentation, though, is the one given by the erectors representative, a such Mr. Collier, president of a facade contracting company. Among many interesting things in his text, there is a lateral comment that struck my attention. Remember, we are in 1955. Weather proofing concepts such as rain screen or pressure equalization had not been identified yet, and do not appear along the conference proceedings. On the other hand, selants based on silicone had not been developed at the time. Facade joints were caulked with Thiokol at best. Mr Collier, though, was not happy with the then predominant solution of open joints in lightweight facade panels to allow for movement, with an internal air and water sealed barrier. He prefered, based on his personal experience, an externally sealed wall to prevent water to come into the building in the first place. Well, of all the predictions made in this interesting book, the preference of Mr. Collier for externally sealed facades was going to become the mainstream solution in the US, as soon as silicone became available, and lasting until today. Dow Corning was going to be more convincing to American builders and architects than the rain screen principle. 

We Europeans laugh at the Americans love for sealed joints in stone or aluminium veneer facades. But apparently Americans prefered the reasoning of Mr. Collier, an entrepreneur, to that of G.K. Garden in 1963, a researcher from a Canadian building institution and the father of the open joint design in modern facades. And they still do...

Roof selection and details - Carles Broto

Roof selection and details
The author's name is Carles Broto from Barcelona, but sometimes he signs his books as Charles Broto.

The book is basicly a description of roof systems with many images, some sections, a projects selection and a short introduction to each chapter. It doesn't seem bad as a first introduction to the topic, but I would not rely too much on its accuracy.

Chapter 1 deals with types of roofs - nice details and clear graphic descriptions of each element. For those of us not English native, a good help indeed! Pitched roofs are described in chapter 2, covering monopitch, gable end, hipped and polyhedral roofs. Chapter 3 covers curved roofs, while chapter 4 goes for flat ones, both the walk-on and the ones with gravel finish. Chapter 5 explains the roof/facade systems, so common lately. Other types of roof (such as simmingpool, parking and green roofs) are described in chapter 6. Next one covers auxiliary elements as skylights, louvres, eaves, vents, chimney pots, gutters and drains. The last chapter describes the most common roofing materials: ceramic and slate tiles, bituminous and synthetic membranes, wood, glass and metal.

In summary: a simple writing style, no profound findings, good quality images. The price ($55) looks like a bit high to me though.

Facades: principles of construction - Ulrich Knaack

Is there a need for yet another book about facades? Yes, if you are looking for a basic understanding of the facade and its technical realization in a more fundamental manner. Facades: Principles of construction is a very simple and well organized introduction about facade design. If you are a student and are interested in facade design and construction - that is, in how a facade works - this is the book to start with.

Professor Ulrich Knaack, the main author, is head of the Chair Design of Construction at the Faculty of Architecture, TU Delft. In 2005 he founded the Facade Research Group and is iniciator of the conference series 'The Future Envelope'. The other three authors are Auer, Klein and Billow. Thomas Auer is a partner at Transsolar in Stuttgart. He specialises in the field of integrated building services. Tillman Klein is an architect and heads the Facade Research Group at TU Delft. Marcel Bilow is a research associate with Professor Knaack at the University of Applied Sciences in Detmold.

The book presents the design process of a facade as a sum of progressive steps. Chapter 2, From wall to facade, discusses the development of today's facades and their typological classification. Chapter 3, Principles of construction, explains the interrelation between the building structure and the facade system. Chapter 4, Principles of detailing and tolerances, covers the generation of technical details for the general solutions defined previously. Topics such as integrated design and building physics aspects of the facade are discussed in chapter 5, Climate and energy. Chapter 6, Adaptative facades, analyses how facades can adapt to changing parametres. Chapter 7, Case studies, illustrates typical and special facade solutions on the basis of selected projects. In closing chapter 8, A look into the future, the authors provide an outlook into possible developments in facade technology.

Mike Davies, the concept of polyvalent wall as drawn in 1981. The layers between the two glass panes were supposed to have a thickness of a few microns.

The problem with facades design and construction, in relation to past eras, is that envelopes have become a complex structure with numerous functions and a difficult technical realization. Thus, the prevailing trend in facade technology is its increased complexity. Two paths of design have tried to answer this complexity: one, the separation of each performance requirement in a specific layer and / or material; the other, the search for integration via adaptability. The real message of this book - and quite relevant for us, regardless we are students or not - is the authors' preference for an integrated envelope instead of a multi-functional facade concept.

In 1981 Mike Davies - while working for Richard Rogers and Partners - formulated the idea of a polyvalent wall in an article signed with Rogers and titled 'A wall for all seasons'. Here, several functional layers within a glass element were to provide sun and heat protection, and to regulate the functions automatically according to current conditions. The wall itself (see the image above) was to generate the necessary energy. As a matter of fact, the label 'Intelligent wall' derives from Davies' concept of the polyvalent wall. His idea, not yet realized, still acts as a driving force for new facade technologies, and many researchers have been engaged in this topic over the last two decades.

Capricorn House, Düsseldorf. Gatermann + Schlossig, 2006

Knaack and his colleagues, following Davies' steps, start by presenting the concept of an integrated facade in a more macroscopic - and realistic - way (see page 100 of the book). It seems reasonable - they argue - to integrate heating elements, air-conditioning and ventilation units, movable shading and other appliances into a facade module. These functions can be combined on the basis of a modular design principle. We could cite some examples where this has been already achieved, as in the Capricorn House in Düsseldorf (image to the right), where decentralized ventilation components and lighting are integrated into the enclosed facade elements.

Functional concept of the integrated envelope according to Knaack: loadbearing, insulation, water and air tightness, ventilation, energy generation, radiation control and transparency, all in one element.

But later in page 130, when discussing the future facade trends, the authors come up with their search for the almighty facade: a further integration with the building services and the structure of the building, all in one. This is the Holy Grial of tomorrow's facade, but it won't be easy to achieve. In their own words:

The future facade has to be based on an integrated concept, i. e. it has to combine and regulate functions that in some cases might contradict each other. (..) This leads to the question of how the facade should be structured - all in one layer or stacked on top of each other. The smaller the components, the easier and better they can be spatially arranged in the facade system. (...) The integrated facade is a vision that will materialise progressively throught the development of new components and technologies.

Amen.