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.

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.

Standardized

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.

Prescriptive

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.

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.

References

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?

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.