Like Oak, there’s not a lot Douglas Fir can’t be used for externally, it is naturally durable, but can be difficult to treat due to its density and resin content. Properly maintained the service life of structural timbers can be well over 100 years.
Douglas fir is more than just a good-looking wood. Its durability and resistance to rot and insects make it a favourite choice for outdoor projects like porch decking as well. ... It's also naturally resistant to rot, decay, and insects.
Due to its strength, Douglas-fir is primarily used for building and construction. It is hard and resistant to abrasion, making it suitable for uses where wear is a factor, such as wharves, trestles, bridge parts, log homes and commercial buildings.
Pines, firs, cedars—these are gymnosperms, or softwood trees. Douglas fir falls in the gymnosperm, or softwood category. ... However, if what you're really asking is whether or not Douglas fir is a tough, strong building material, the answer is yes—it's widely known as a very durable wood.
Douglas Fir is a type of species among softwoods that stays dimensionally stable without requiring any drying or another remedy. In a way, this highly valuable atmospheric moisture management property makes this lumber waterproof, although it is capable of holding water.
However, Douglas fir can be painted, stained and sealed if required. If purchasing green Douglas Fir Lumber, be sure the lumber is dry to the touch before finishing.
Sealing with wax and tar are other traditional methods for preserving and "waterproofing" wood that have been used with fir. In most home uses, a coat of oil bonding primer and two coats of latex paint, or two coats of exterior grade stain and clear finish, are sufficient to waterproof the surface.
How to Seal Douglas Fir for Exterior Use
- Sand the surface of the wood smooth with 180 grit sandpaper. Follow the grain of the wood when sanding. ...
- Apply a thin coat of a water-based latex exterior wood sealer to the Douglas fir with a nylon paintbrush. ...
- Let the sealer dry according to the package directions.
Extracts from a paper by THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION on the Performances of Douglas fir in real outdoor use conditions shows:
- The dimensional stability of Douglas Fir is particularly good. It displays unique behaviour in terms of response to moisture. It is well-known that it is a refractory species and that both its sapwood and heartwood are difficult to impregnate with water, even under pressure. Several studies have demonstrated that the water uptake of Douglas fir, when used outdoors and exposed to rain or high humidity rates, is very slow and that its moisture content remains low even after long exposure.
- A French study (Dirol & Déglise 2001) showed that the moisture content of Douglas fir samples never approached the minimum moisture content required for initiating fungal decay throughout the whole period of the test.
- In 2010, France Douglas, the French Douglas fir Association, performed a survey of about 30 existing wooden structures principally made of Douglas fir. Mainly 10- to 20-year-old buildings or structures located mostly in Central France were inspected (Fig. 3). Several sawmill and farm buildings constructed more that 50 years ago were also examined. A total of 30 buildings and 200 elements made of Douglas fir, mostly untreated and uncoated, were examined for any evidence of fungal decay.
- The survey demonstrated that of the 200 examined outdoor wooden elements only 6 showed partial decay, obviously due to poor design and wrong position in the structure. The resistance to fungal decay of Douglas fir's heartwood appears thus to be good enough to allow its use in Use Class 3.1 conditions for periods of time ranging from 50 to 100 years and in Use Class 3.2 conditions for periods of time ranging from 10 to 50 years.