White Fir Lurnber For Small House Construction

By R. P. A. Johnson, Engineer in Forest Products, Forest Products Laboratoryl, Forest Service U.

S. Department of Agriculture.

A considerable portion of the cut of white fir lumber goes into small house construction. The species is used largely in the form of dimension and common boards. In some localities the species has proved satisfactory and has established a desirable reputation as a construction material. In other localities satisfactory results have not been obtained and it is regarded as an inferior species, unsuited.for construction purposes. Thus there exists conflicting opinion as to the suitability of the species for small house construction.

The difierences of opinion concerning the merits of white fir for construction purposes are not due to differences in the inherent properties of the wood. Ratlrer they are due to differences in the character and the condition of the lumber as it is received for use. Where dissatisfaction has arisen because of the lumbir, it can generally be traced to wet lumber, substandard sizes, poor storage conditions, or other factors in manufacturing and marketing that may be corrected; they may easily be prohibited by specifications. Some dissatisfaction, for which the tumber apparently has been blamed, has been the result of poor design or poor construction of the house. Such a trouble can hardly be corrected.

The inherent properties of White Fir are fixed, they will be the same next year as they are now, and they are the same in the San Joaquin Valley of California as they are in the Middle West. Hence, comparison of the properties pf white fir with those of a number of species that have been and are being successfully used in house construction will indicate its suitability for small house construction when the lumber is properly manufactured, seasoned, and used.

The properties desired of building material may be divided into t$'o groups,-one containing those properties that tend to insure satisfactory service when the wood is in place, and the other containing those properties that tend to facilitate construction and thus reduce costs. Strength, stiffness, nail-holding power, and small shrinkage are the important properties of the first group; light weight, softness, and ease of working are those of the second. A species can not excel in the properties of both groups, since, for instance, if it has high strength it is comparatively heavy and hard. I'he desirable properties of white fir are those of the second group.

The adequacy of the strength of white fir for structural purposes is shown by a comparison of recommended working stresses for white fir with those for other species; Table l makes such a comparison. The stresses shown in the table are those recommended by the Forest Products Laboratory of the U. S. Forest Service for material meeting the basic requirements of American Lumber Standards for structural materiat of the common grade. They have been approved by the American Railway Engineering Association and are published by the American Society for Testing Materiats.

Table I shows that the working stresses for white fir compare favorably with those for a number of other species. .They are the same throughout as those recommended for eastern hemlock, a wood which for years has been extensively and successfully used for buitding in the Lake States. The working stresses for eastern spruce do not difier greatly from those for white fir. Eastern s-pruce has for years been a- standard construction material in New England and is stitl regarded as an excellent wood for tha,t purposE. Working stresseq for white fir, eastern hemloch and east-lrn spruce are- all lower than those for Douglas fir (Coast tyqe) and southern yellow pine. The lower working stresses of -white fir, hemlock and the spruces do not mean they are unsuited for structural work but indicate the necessity for larger sizes to obtain strength and stiffness equal to that obtained with strong-er woods. For example, a 2 by 8 inch white fir is stronger and stiffer than a 2 by 6 incir o? a co-pat"ble grade of any species shown in the table. It is evident, therefore, thaf where white fir has given trouble-it was not because adequate strength could not be obtained with the species, for the strength and stiffness of a structure is dependent far more upon the design of the structur€ than upon the species of wood used for lumber.

The nail-holding power is an important requirement of framing. It depends, howevei, more upon the moisture content of the wood at the time the nails are drivin than upon the species used. This comes from the fact that nails driven into wet wood lose holding power as the wood dries.

The nail-holding power of white fir is lower than that of the woods commonly usCd in building. (Table 2.) The difterence in nailholding power of white fir and of other species. however, is small as co-paied to the loss of from 60 to 90 per cent in nail-holdinq pgYe-r that results when nails are driven into wet framing. Even the high nail-holding power of Douglas fir and of southern yellow pine-under unfavorable ionditions will drop below the value for dry white fir.

Well seasoned white fir has sufficient nail-holding power to meet the requirements of house framing. Framing put up when wet will usually resutt in unsatisfactory service. regardless of whether the lumber is white fir or some olher species. The elimination of wet or partly wet lumber will assure good nail-holding power. .Wh-..n ttecissary the difference in nail-holding power between white fir and such species as Dougfas fir can be compensated for by the use of additional or larger nails.

The shrinkage of white fir is intermediate between that of the heavy, strong species, such as Douglas fir and the southern yellow pines, and the light-weight, soft species, such as western yellow ind northern white pine. (Table 3.) Objectional shrinkage, however, is commonly the reiult of the use of green lumber and is not very dependent upon species. Wet wood of any species will cause trouble, evin when it is only partly wet, and regardless of the cause of the lack of dryness.

The decay hazard of framing, subfloors, and sheathing of properly constructed and maintained buildings is comparatively small' Wood kept constantly dry does not decay. Subftoors, sheathing, and framing in houses are normally too dry to permit the growth of wooddestroying fungi. Non-decay resistant species can therefore be used satisfactorily for these items.