Friday, June 17, 2011

Conventional Construction: Related terms

Before moving on to a discussion of unconventional materials/methods, a side trip is needed here to define a couple of terms that relate to but are not quite the same as conventional. The two are standardized and prescriptive.


The first step toward use of a material or method in conventional construction is standardization. When you buy a 2x4 stud at the lumber yard, it has a stamp proving that it meets certain standards of suitability for the intended use. Common standards for lumber include wood species, quality grade, dimensions, and others. I don't plan on devoting too much blog space to discussion of standards - that's a whole different blog. For the inquiring mind, the conventional construction book published by the ICC does a pretty thorough job on standards.

Application of conventional construction provisions in modern building codes requires use of standardized materials, which is a change from the original definition of conventional. This is frequently an issue in Santa Cruz County, where many property owners would like to use on-site resources such as trees in proposed construction. In the old days, you might chop down a few trees, hand-saw some lumber and build yourself a house. It doesn't work that way anymore in this country. Native, site-prepared logs, lumber and/or heavy timber may be suitable for exterior construction of open structures like porches and trellises (if approved by building officials), but not allowed in conventional construction (or structural analysis) of a habitable building because the wood hasn’t been tested and standardized. Unless its physical properties are known, there’s no way to know where and how it can be safely used. On-site testing may be possible, but the extra time and expense means that such materials are not conventional.


Once a building component has been standardized, guidelines for its use can be published in a building code. These guidelines are prescriptive; giving designers and builders permission to use certain components in certain situations. For instance, to use that standardized 2x4 stud in framing a wall, building codes give us prescriptive limits on maximum height and spacing. Designers use those limits in drawing plans; builders use them in construction; plans examiners and building inspectors use them to verify construction quality. Prescriptive guidelines are also conventional, in the sense that we can use them in building structural design without further analysis.