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.

Monday, June 6, 2011

Conventional Construction: What is it good for?

Current building codes describe three methods for structural design of buildings:

1. Conventional construction is the oldest, simplest design system and the main focus of this series. It is often adequate for structural design of small projects, but limited to light-frame, one-or-two-story wood buildings. The utility of conventional construction can be extended by use of:

2. Pre-engineered components and systems. Many of these are described in building codes, such as conventional concrete perimeter foundation systems. Some pre-engineered components are found in manufacturers' catalogs, such as metal framing connectors.

3. Structural analysis. Performed, in most cases, by a professional structural engineer, structural engineering analysis is outside the scope of this discussion. Many design situations require structural analysis, and designers have the option to employ it for any design. If there’s a choice, however, conventional solutions are usually more cost-effective. One of the goals of this discussion will be to find and discuss those situations where the designer and builder have conventional options which might reduce the cost of a project without compromising quality.

Successful application of conventional construction requires the builder or designer to know when you can't use it, so it might be easier to start with a discussion of what it’s not good for. One category of situations where conventional construction often won't work is with un-conventional materials and methods. That’s our subject for next time.

Conventional Construction: What Is It?

History and Definition

Builders, designers and city/county plans examiners are all familiar with the term 'conventional construction', but what exactly is it? Long before the first building codes were written, builders had accumulated a lot of knowledge about light-frame wood construction. Buildings that stood the test of time (or didn't) demonstrated what worked and what didn't. The first building codes incorporated the materials and methods that worked best into a section on 'wood-joisted dwelling construction'. That term evolved into 'conventional construction'. Looked at another way, conventional construction is prescriptive structural design; "if you follow these rules, your building won't fall down". The review and update cycle continues today. In California, for instance, the 1989 Loma Prieta earthquake was a real-world test of conventional construction. Study of failed structures led to changes in the next code edition's structural design rules.


Reference works on conventional construction have been hard to find. The ICC (formerly ICBO) has published one book, updated with each new code version. The new edition, called Conventional Construction Provisions of the 2009 IRC: An Illustrated Guide, won't be available until July so we don't yet have an up-to-date guide to current codes. The latest published edition of the book is titled Conventional Construction Provisions of the 2006 IBC: An Illustrated Guide. It's still useful on most topics, but I'm hoping the new edition will be more complete. This book, and its predecessors, have been my primary guides to conventional construction. Conversations with G.C. Scott Milrod and Jim Heaney, building plans examiner at Santa Cruz County, have also been helpful.

Study on the subject of conventional construction is further complicated by the fact that so-called "model codes" such as the old Uniform Building Code (UBC) and the current Uniform Residential Code (IRC) comprise only the base layer of local requirements. California has its own rules, contained in the California Building Code (CBC). Although based on the model codes, the CBC contains many differences, so the ICC books must be used with caution. Thanks to the Internet Archive website, you don't have to buy a copy of the CBC-modified IRC. A viewable/downloadable copy of the document is available, titled Title 24, Part 2.5, 2010 California Residential Code.

Three Layers of Codes

More layers, sitting on top of the CBC, are local building codes which vary from one city or county to another. These take the form of amendments to the CBC, and are contained in documents such as the Santa Cruz County Code. The County Code is viewable online, but is currently not very helpful. The online version has not yet been updated to the current (2010) CBC.

This series of posts will attempt to provide some answers to questions about conventional construction. Next: What is it good for?

Wednesday, June 1, 2011

How to install corrugated metal roofing

For roofing a storage shed or other small utility building, the most cost-effective choice is good old galvanized corrugated sheet metal. Though maybe not the most attractive option, this product gives the best combination of low cost and durability. Readily available panels are approx. 26" wide (for 24" o.c. rafter spacing) x 120" long. I found them at Home Depot for around $14 each. It's harder, however, to find accessory pieces and installation instructions.

On-line sources advise that, to avoid leaks, exposed panel fasteners should be installed at panel ridges - never in the valleys. But what about the space under the ridge? Some kind of support must be provided to avoid deformation of the metal. The cross-section detail below illustrates one easy solution.

The 1/4 round wood molding strips are perfect for this job for many reasons:
  • They're cheap and readily available,
  • They provide solid support for the roof panels along the entire length of the panel overlap,
  • The shape of the panel holds the strips in place during installation,
  • Splitting is not a problem because the screws pass between the two wood pieces.
Panels with deeper corrugations may need molding strips with a larger radius. The drive screws shown are another standard item. The neoprene washers provide a weather seal around the penetration. For extra leak protection, run a fat bead of silicone caulk between the panel overlap surfaces. Shaped neoprene strips are available to seal panel ends under the eaves above exterior walls. Because the metal roof is lightweight, 2x4 rafters at 24" spacing are sufficient for spans up to eight feet.

Welcome to the Casey Building Design blog

This is a place for me to post random musings on the business of building design, and my part in it. I plan to share tidbits of design advice, tips on dealing with local permitting agencies, construction details, portfolio photos and anything else that may occur to me in lucid moments. Please also visit my page at the Santa Cruz Construction Guild.