Finishing Cedar Decking Techniques
Although Western Red Cedar is a naturally durable wood ideal for decks, its performance is enhanced when protected by an appropriate ﬁnish. Decks are subjected to the most challenging exposure conditions. The combination of a horizontal surface, the abrasive effects of foot traffic, pounding rain and full exposure to sun make deck finishing more demanding than other finishing applications such as siding. It is important to ensure that the finish you apply to your Western Red Cedar decking boards has been specifically formulated for this end use.
Unseasoned decking should be air dried (not exposed to direct sunlight) and finished on all sides prior to installation.
Choice of Deck Finishes
A full range of both penetrating and film-forming finishes are available for cedar decks. Since they do not form a film, the penetrating finishes provide better overall performance and are the easiest to maintain and refinish.
Penetrating finishes do not flake, crack, or peel. These treatments help repel water and if formulated with a mildewcide, slow down the growth of micro-organisms like mildew. The penetrating finishes are often used as a natural finish for cedar decks. There are three types of penetrating finishes: water repellents, water-repellent preservatives, and solvent-borne semi-transparent stains. The best of these finishes are solvent-borne semi-transparent ones.
Semi Transparent Stains
Semi-transparent stains have pigments which provide color and greatly increase the durability of the finish. The pigments help block sunlight and protect the cedar surface, thus increasing service-life. The better products contain water repellents and mildewcides or fungicides. The solvent-borne, semi-transparent stains penetrate into the cedar without forming a film and do not blister or peel even if excessive moisture enters the cedar. However, latex semi-transparent stains can sometimes build up a film, especially on recoating, which can lead to peeling. Latex stain formulations currently being developed may correct this problem.
It is possible to switch to a semi-transparent stain when the deck needs to be refinished. Even if the deck has been coated with a clear or transparent stain for many years, an oil-based semi-transparent stain can be applied, provided the wood surface is cleaned.
Note: Film-forming finishes are normally not recommended for use on Western Red Cedar decks because they can fail by cracking, flaking and peeling and can be very difficult to refinish and maintain.
Application of Transparent Stains and Water-Repellent Preservatives
These are essentially colorless, penetrating finishes that are commonly used for cedar decks. These natural finishes enhance the appearance and service-life of cedar decks. The water-repellent preservatives are preferable to the water-repellents because they contain a mildewcide and often other fungicides that help inhibit surface mildew growth. Many of these water-repellants contain ultraviolet light blockers or absorbers that help protect the Western Red Cedar from damaging sunlight. To date, the best of them are solvent-borne. The water repellent preservative finishes are very effective in stopping the absorption of liquid water thus reducing swelling and shrinking of the wood. Water-repellents should be used only on newly manufactured bare cedar, on restored bare cedar, or on cedar previously treated with the same type of product. When used as a natural finish, the service life of a water repellent is only one to two years depending on the wood and exposure conditions.
Western Red Cedar performs satisfactorily as a decking and siding product if it is left unfinished to weather naturally.
You may choose this option due to:
- A preference for the silvery grey look of naturally weathered Western Red Cedar;
- A desire not to invest the time and effort required to maintain the finish on a cedar product;
- Project inaccessibility, causing difficulty in maintaining the finish on a Western Red Cedar project, for example, siding on very tall buildings; or,
- A concern about the environmental impact of applying a finish on your Western Red Cedar.
These are all valid considerations and the reality is that Western Red Cedar siding and decking can have a long service life without a finish coating in many environments – as long as good design, installation and maintenance practices are followed.
It is important to understand that the choice not to apply a finish to your cedar product has long-term implications. This choice requires advance consideration before you allow natural weathering to begin. As Western Red Cedar weathers, it will lose its natural color and become grey. In very dry climates, it weathers to a silvery grey color, but in most other climates, because of varying moisture and sun exposure conditions, the Western Red Cedar does not weather uniformly and is likely to develop a dark, blotchy, grey appearance.
Should you wish to restore weathered cedar to a state where it can be painted or stained however; significantly more effort will be required to prepare the surface of the wood for coating. If a truly uniform grey appearance is desired, then this effect can be achieved by the use of a commercially prepared bleaching oil or grey weathering stain. This is essentially a water-repellent finish containing pigments and other additives which is most effective on textured cedar. To maintain this look, the bleaching stain may have to be re-applied periodically.
Although the natural weathering effect is usually only “skin deep” (less than 1-2 mm), with the cedar largely unchanged beneath, extra care must be given to the design of the project, the installation of the cedar and routine maintenance. Without the protection provided by a coating against moisture intrusion, (especially end grain sealing), steps must be taken during the installation process to allow Western Red Cedar to readily dry following exposure to moisture, otherwise the risk of decay increases.
All cedar products used in exterior applications require a degree of maintenance to keep them looking their best. Even if the choice is made not to apply a finish to Western Red Cedar, contaminants such as dirt and mildew should be regularly removed to maintain its beautiful, natural appearance.
Life Cycle Assessment (LCA)† of Western Red Cedar siding and decking shows it has the most benign environmental impact of all the alternative product choices for those building applications. Coatings, especially the solvent borne finishes and the frequency of their application negatively impact the LCA of cedar siding and decking. This impact may influence a decision not to apply a finish. Fortunately, a new generation of more environmentally friendly, water borne, coatings are emerging and these products should be considered.
†Life Cycle Assessment (LCA) is used to assess the environmental performance of a product from cradle to grave.