When Newer Is Not Better

Q: When we built our house in 1985, we asked the architect to design it so it could be built in two stages: first as a one-story house (so we would not exceed our budget), then as a two-story house, built on top of the original walls. After nine years, we can now afford the second floor but the local building department will not accept our original approved framing plans. What is going on?

A: Over the past forty years, American lumber suppliers have harvested most of the original growth trees on their forested lands. The seedlings they planted were selected for their fast growth characteristics, reducing the number of years foresters had to wait until the next harvest. This led to a gradual decrease in the average strength of visually graded wood members because the new, faster growing trees (with wider growth rings) are not as strong per unit area as the original trees. Adjustments in the commonly accepted design values for wood were called for.

The American Forest & Paper Association ( in cooperation with ANSI, the Forest Products Laboratories and the Department of Agriculture ) provided these when they published the latest version of the National Design Specification for Wood Construction (NDS 1991 and Supplement). Not only have allowable stresses been reduced, but member strength is now linked to the width and depth of the cross-section. In this instance, if your plans called for 2 x 10's made from either Douglas Fir or Larch, the material you receive from the lumber yard today would only posess 73% of the calculated strength versus the same members in 1985. Good thing the building department did not accept your approved drawings.

The required modifications to your plans are not extreme. You can expect an increase in floor member depths (2 x 10's become 2 x 12's) or your architect may propose the use of fabricated floor member (glulams, "I" joists, etc.) if sawn members are not available. Wall studs dimensions may be kept the same, but you should anticipate the additional costs of obtaining material of a higher grade (better wood) to offset the NDS reductions. The amount of reduction also depends of the wood species to be used. Your architect should consult the NDS for wood species in your region.

Your To-Do List

A. Take the plans back to the original architect and ask him to review the plans for current building code requirements. There may be other updates which are not related to the structural framing which should be considered.

B. If the Architect has moved, closed up shop or "made the final revision," you can take your plans to another local professional, but expect to pay more as there are liability issues involved when plans are altered by someone other than the original designer.

Scott McVicker, S.E.

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