Conventional Construction: Selecting framing lumber

Wood is the most conventional of materials, and lumber is the wood used in conventional construction. Not just any lumber, though – as noted previously, lumber must be standardized and pre-approved in several ways. Also, allowable size and spacing of framing members are prescribed. The building code (CBC) simplifies the selection of conventional structural lumber by providing tables, organized by intended use. One thing you'll notice about the tables is that only a few wood species and sizes are included. For other choices, an online selection calculator from the American Wood Council is available here. If you use this tool, be sure to verify your selection with your inspector or building official.

The image at left shows the title of the first of two tables for selection of floor joists. The difference between the two tables is enclosed in parentheses below the title: (Residential sleeping areas, live load = 30psf…). The other table is for: (Residential living areas, live load = 40psf…). This split table is a change from the previous code; using different live load (mainly people and furniture) values for living and sleeping areas.

Just below the title is the header section of the table:
The header shows several important variables. The first is dead load, which is the weight of installed materials supported by the joists. The old code had a table giving weights of many different materials, but there’s really only one common situation that might call for using a dead load = 20psf – that’s heavy flooring like mortar-set tile. Another situation where using 20ps might be a good idea is if you know the room is going to be used for something like weight-lifting. In the following example, we'll use only the 10psf side of the table.

To use the table, start with the desired span, then move out to the edges to find size, wood species and spacing. For example, say you have a 14’-0” span. On the left half of the table (dead load = 10psf), find the span numbers closest to but not less than 14’-0”. For simplicity, we’ll stick to the most common (in Santa Cruz) wood species, Douglas fir (D.F.). Grade #3 is not commonly available, so we’ll ignore that also. As a designer, I would conclude that the three best conventional size/grade/spacing options are:

2x8 #2 @ 16” (rectangles added to highlight choices)

2x8 SS @ 19.2”

2x10 #2 @ 24”

At this point, you can plug these three into your estimator software along with current prices and go with the most cost-effective choice (note also that, for the contractor, current prices and/or labor costs might point to a choice other than these three). Cost aside, sometimes there are design reasons for preferring one size or spacing over another, but that’s a topic for another time.

Conventional Construction: When Does It Become Unconventional?

Designers and contractors wishing to keep projects conventional obviously must avoid the unconventional. That's one of the main objectives of the pre-proposal feasibility research. It’s no fun explaining to clients the need for expensive structural analysis after they agreed to a proposal based only on conventional design (of course, proposals that don’t allow for unexpected discoveries later on should also be avoided).

Neither is it any fun for a contractor to make what seems like a minor client-requested design change in the field, only to be told by the Building Inspector that the project must now be engineered. Something about that minor change caused the project to cross an invisible line into the unconventional zone.

It can be a very fine line indeed between conventional and unconventional, so a thorough understanding of the determining factors is important. Previous posts have discussed a few of the general factors. Also, some situations are more perilous than others, so we'll try to cover as many of those as possible. 'Dividing line' situations fall into three categories: materials, methods and conditions.

Conventional Construction: One More Term

I should mention one other important term - approved. It’s a term you often hear, but it has many different meanings, depending on what’s being approved and who’s doing the approving. Our favorite, as designers and builders, is the one that appears on that long-awaited letter from the Planning Department – ‘all agencies have approved your building permit application’. Two slightly different meanings are also important in project design and construction.

Pre-approved

A material or method not normally considered conventional may be used prescriptively (without further analysis) if it is pre-approved. The pre-approval process involves extensive testing by a testing lab, which must itself be pre-approved by the ICC. The lab then prepares a report describing the approved uses of the product tested. A familiar example of a testing lab is the Underwriters Laboratory (UL). For conventional construction, the ICC Evaluation Service (ES) is probably the most important. Metal framing connectors, engineered wood products and gravity retaining wall systems are examples of products that are pre-engineered and pre-approved.

Reviewed and approved

Everything on a set of plans and everything built from those plans must be reviewed and approved by one or more representatives of the local authority. Also, despite the complexity of building codes, local regulations and all the other hoops that must be jumped through to get a building permit, there are still gray areas not clearly defined and/or understood. When these situations arise, the local Building Plans Examiner, Building Official or Building Inspector must review the proposed solution and decide whether or not to approve it.

For the project designer, the big hurdle is getting that approval letter so the contractor can begin work. For the contractor, there’s always the fear that the Building Inspector will look at your ‘conventional’ work and say ‘you can’t do that’ or 'that's not on the plans' - and his word is the final word. However, a thorough understanding of the rules will often allow a designer or contractor to convince the inspector to see things your way. Don’t relax yet, though – especially if you get a different inspector for the next visit (when scheduling an inspection, always make sure your regular inspector is not sick or on vacation). The project team can’t really breathe a collective sigh of relief until that final permit sign-off.