• BLOG
• SITE MAP
• SITE INDEX
• SEARCH

Western Red Cedar Cladding

Literature and historic examples define western red cedar (Thuja Plicata) as a dimensionally stable "small movement," and highly weather resistant wood. Its characteristics have made it the most popular wood for cladding in North America over several centuries. Because of its durability it is often used untreated. In fact it is defined as "resistant to preservative treatments." It is relatively soft however, and should be used carefully in areas of high impact. It has a high tannin content which can be corrosive to iron, and is easily stained by ferrous metals. It should therefore be used with stainless steel or silicon bronze fasteners. Because of its tannin content, it sometimes has problems with "water-soluble extractives bleeding." This appears to be a problem only when the cladding is poorly ventilated.

Issues

Moisture trapped under cedar cladding will be forced to migrate through the wood, carrying with it the water-soluble extractives. This is often a problem with cladding that has been painted or sealed - a treatment which can trap moisture. End grain is also vulnerable to extractive bleeding. Because of its natural wicking capacity, water absorbed by end grain can carry extractives up the wood, leaving striated stains during a subsequent drying period. Proper detailing should address this issue. On untreated cedar water-soluble extractives can be cleaned, and are generally washed away by rainfall. This characteristic can be observed on untreated cladding where areas protected from precipitation, such as the space under eaves or overhangs will attain a rich reddish brown while exposed areas will weather to a lustrous silver-grey. Empirical observations from Klaas Armster (A reclaimed Lumber Co.) indicate that high quality cedars used in well ventilated applications will have little to no problems with staining. He also stated that his own belief is that wood like this should be either; completely sealed and encased (in which case the sealant must be maintained regularly,) or left completely natural. His observations suggest that penetrating stains and other oil products can cause problems because wood reacts differentially, allowing differential degrees of absorption of both the oil and rainwater and therefore making the weathering conditions less uniform. He commented that purchasing a high quality product like reclaimed old-growth cedar is a decision made in order to avoid the need for expensive and labor intensive maintenance.

(p.1, http://www.trada.co.uk/techinfo/asset/send/428/content/Cladapp/ar01s05.html)

Selected Documentation
http://www.shakertown.com/tech_bulletins/b_4001.html
Discoloration or extractive "Bleeding".
One characteristic of Western Red Cedar is the presence of water-soluble color extractives, which may "bleed" out onto the surface and discolor white or light-colored paints. This is neither a fault of the wood or paint formulation, but rather an indication of the presence of moisture in the wall. This moisture may come through the surface of the paint if the film has fractured; or it may enter through cracks, apertures or other defects in the construction; or it may arise within the building and be carried through the walls. No change in type of paint, or similar approach will be successful in dealing with the problem unless the source of the moisture is located and cut off. Insufficient ventilation of some homes is the major problem, as substantial quantities of moisture are produced and retained by improper venting of vapor-producing appliances, inadequately louvered attics and crawl spaces, poorly installed insulation and vapor barriers. If a small amount of bleeding occurs during the first year, it is worthwhile waiting through this "shakedown" period because these surface deposits usually either weather away or can be washed off by using soap and water, or a stronger detergent if needed.

"Prevention of Extractive Discoloration"
Joint Coatings/Forest Products Committee Chairman: Jeff Fantozzi, Pacific Lumber Inspection Bureau, Bellevue, WA (1-425-746-6542) Vernon Donegan, ICI Paints, Strongsville, OH Keith Kersell, Pacific Lumber Company, Mill Valley, CA Charles Jourdain, California Redwood Association, Novato, CA Alex Migdal, DeSoto, Inc., Des Plaines, IL Robert Springate, Tru Serv, Chicago, IL James Tooley, Weyerhaeuser Company, Tacoma, WA

A rundown or streaked type of extractive discoloration often occurs with lap siding when water gains access to the unfinished backside of the siding. Here, the water source can be from either the interior or the exterior. Interior conditions causing extractive discoloration include high moisture vapor levels, typical of new construction, and/or poor ventilation in high humidity areas. Exterior sources can be leaks in the exterior of the building that allow water to penetrate behind the siding or rain water driven behind lapped siding from winds or capillary action. Rundown extractive type stains can also occur where siding butts into window or door frames if caulking and end priming have been omitted.

Wood left to weather naturally with no protective finish may take on a myriad of color variations, depending upon exposure to sunlight and moisture. Condensation from high humidity can cause darkening of the wood due to the migration of water soluble extractives to the wood surface. When sufficient liquid water and UV are present, the wood may take on a bleached appearance due to the degradation of lignin and leaching of water soluble extractives near the surface.

Mild staining from water-soluble extractives is often washed away by rain over a period of several days or weeks.

Timber Cladding in Scotland - Scottish Executive

p.41
In contrast to the UK, Norwegian recommendations stress the importance of relatively tight cladding designs that seek to minimize the gaps where wind driven rain can penetrate the outer rain screen. It is believed open-jointed designs may put an undue reliance on the water shedding qualities of the breather membrane. This is because - even allowing for the best of modern breather membranes being very reliable - such designs are vulnerable to:

• Poor installation practice;
• Damage caused by vandals (or kids) pushing sharp objects through the cladding joints;
• In extreme cases, particularly with open jointed vertical cladding, sunlight may penetrate the joint and cause UV degradation of the breather membrane.
• Insect/pest infestations of ventilated cavity

p.54
Only austenitic stainless steels have sufficient corrosion resistance for use in cladding, with type 1.4301 (304) stainless steel recommended for most cladding applications, and the more durable type 1.4401 (316) recommended for coastal sites. Non-ferrous fixings such as silicon could also be used and are sometimes supplied with western red cedar cladding.

http://www.trada.co.uk/techinfo/asset/send/291/content/Clad4/ar01s03.html
4.3 surface protection

p.3
The durability of any wood is not affected by the bleaching process. There are many historical examples of unfinished softwood or hardwood cladding that have lasted many centuries without deterioration, often in areas exposed to a high level of ultraviolet attack such as the timber clad alpine buildings.

Iowa State University - University Extension
Pm-362 Electronic version November 1999

Natural weathering
The simplest and most economical way of finishing an exterior wood surface is to allow it to weather naturally. Wood exposed to the weather will undergo color changes during transition from the original to the final gray color. The time required to produce a fully weathered appearance depends on the severity of the exposure to sun, wind, and water and will not be the same on all sides of a building. Untreated wood may exhibit blotchy discolorations during the weathering process due to the action of microorganisms. Stable woods such as bald cypress, western red cedar, and redwood, are best suited for natural weathering.

An inclination to warp increases with the width of the lumber. The width of boards should not exceed eight times the thickness. The orientation of the annual rings also influences the tendency to warp. Flat-grain boards usually warp more than vertical-grain lumber.

"Water Repellents and Water-Repellent Preservatives for Wood"US Forest Service, R. Sam Williams and William C. Feist

p.3
If protected from excessive moisture, redwood and cedar are prone to weather to silver-grey.

p.4
The erosion rate for solid softwoods in temperate zones is on the order of ¨ù"- ¨ö" (6 to 12 mm) per century and depends mainly on the intensity of UV radiation and on the wood species.

Recomendations
The main concern surrounding the proposition for the use of western red cedar for the Sidwell project seems to be principally one of information, or architect-client communication. Considering the historic success of Western Red Cedar cladding, (there is no other zero-maintenance wooden cladding material that has had such long term or wide ranging popularity), the issue seems to be one of the aesthetic acceptance of the material rather than one of performance. If left untreated, the proposed cladding system can be expected to easily last 100 years based on the natural erosion rate of cedar heartwood. Furthermore, there are no currently available treatments that will prevent the weathering of cedar, or eliminate the possibility of extractive bleeding. There are some high-maintenance pigmented and bleaching stains which will produce a relatively uniform coloration if reapplied every few years. The essential issue seems to be the desire to assure the client of a uniform or predictable "appearance" of the cladding. Because of the unlimited variety of weathering conditions based on geography, site location, orientation of building elevations, and variable responses between wood specimens even within the same species it is impossible to predict exactly the long term visual appearance of a "zero maintenance" cedar cladding system, however, careful detailing can insure the maximum longevity of the cladding and a predictable appearance.Based on our research It is our recommendation that the cladding for Sidwell Middle School be left untreated and, as mentioned, that considerable attention be directed towards the proper detailing of this system in order to insure the longevity and durability of this precious material and to improve the predictability of a uniform weathered aesthetic.

© 1997– KieranTimberlake Associates LLP
[ 420 N 20 Street, Philadelphia PA 19130.3828   |   V 215 922 6600   F 215 922 4680 ]