Floor Framing Systems

You’ve reworked your floor plan, adjusted the orientation and window placement to take advantage of views and southern exposure, and tweaked roof lines to match the home you have envisioned for months and years. But how much thought have you given to your floor framing system? Unlike floor finishes, the floor framing system will become part of your structure for the life of your home. At Pacific Homes, in addition to multiple wall framing systems, including our Pacific SmartWall® System, we offer three major types of floor framing systems. A look at the advantages of each system will allow you to select a floor tailored to your design, and offer exceptional performance over the life of your home.

Dimensional Lumber

The original workhorse of wood frame construction!  There are plenty of good reasons to use this trusted standby in your home. Typically, 2×10 (9 ¼”) and 2×12 (11 ¼”), nominal joist depth lumber species are grouped by similar performance characteristics and graded by structural quality.  Allowing designers to size joists and beams, to specific loads and spans. The standard species category and grades vary by region. In BC, and in Pacific Homes packages, the base standard is Spruce/Pine/Fir grade #2 (SPF #2) or better, but when a specific beam or span for example requires, a Douglas Fir/Larch grade Select Structural (DFIR-L SS) for example may be used.

Dimensional lumber is often the most economical choice but that is not its only advantage.  It is also durable in shipping and installation.  Kiln drying and protection from the elements in transit and during construction helps reduce swelling across the wide grain and keep lumber dimensionally stable. Prefabrication is great for this. However, if the lumber is exposed to moisture, but then permitted to thoroughly dry out, the moisture will not have a significant detrimental effect on the structural integrity of the lumber.  Therefore pressure treated lumber is the go-to material for decks exposed to the weather.

Readily available at local lumber yards, dimensional lumber can be sized using tables in the local building codes and doesn’t require the loading analysis to be sealed by a professional engineer.  Rules for notching, drilling holes, and general installation, (while in some ways restrictive), are standardized in building codes across North America and are also familiar to carpenters and other trades.

I-joists

 

The same year Pacific Builders Supplies started manufacturing wood trusses (1969), a small company from Idaho, called Truss Joist Inc. created the first wood I-joist. Which is why I-joists are sometimes referred to as TJIs, the pioneering brand name. In an interesting British Columbian connection, TJI partnered with the Vancouver forest products company MacMillan Bloedel in the early 1990’s before both companies were eventually bought up by Weyerhaeuser. Now every major wood products supplier offers their own line of I-joists.

Similar in design to the steel I-beams used in heavy construction for decades prior, wood I-joists gain their mechanical advantage from their I shape profile. Two horizontal lumber flanges are joined by a vertical Oriented Strand Board (OSB) web. Standard I-joists depths vary from dimensional lumber at 9 ½ in., 11 7/8 in., and 14 in. For long spans, depths up to 24” are available at 2 in. increments.

To achieve longer clear spans and carry heavier loads, without increasing the depth of the I-joist, the size and structural grade of the flange is increased. For example, where a standard I-joist would use a 2×3 SPF #2 or better flange a higher end series in the same depth would use a 2×4 Machine Stress Rated (MSR) flange.

As Engineered Wood Products, I-joists are sized by an engineering technologist to meet specific requirements with regards to span and loading in a custom home. An analysis of the resulting design is reviewed and sealed by a professional engineer and supplied to the installer and building inspectors.

As there is some variation from dimensional lumber floors, installation instructions are provided with the floor material. This includes details on drilling holes in the OSB webs, both size, and placement of holes is very flexible in I-joists, some manufacturers even provide pre-pressed holes that only need to be punched out to run wires.

 

The I-joist manufacturing process allows for long continuous lengths to be produced which is only really restricted by shipping and site considerations. This along with the relatively light weight design of the I-joist allow for quick installation, reducing joints and blocking.

As a manufactured product, I-joists are dimensionally stable, consistently straight and any shrinkage is negligible. Defects such as crowning, twisting, cupping, and warping, seen in dimensional lumber, are not an issue. This results in high quality finished floors.

The smaller lumber and wood fibers comprising the I-joists allow smaller logs and a more use of raw logs, this coupled with sustainable tree farm and logging practices employed by the major manufacturers makes the I-joist an environmentally friendly product. Along with other components and considerations, incorporating I-joists into your house design may qualify your home for certifications such as the LEED Green Building Certification.

Floor Trusses

 

Similar to roof trusses, floor trusses utilize dimensional lumber, most commonly 2×4, for the top and bottom chords, and webs of the truss. All connections are made by steel plates pressed into the wood. Differing from roof trusses, floor trusses typically have the lumber oriented as 4×2 or ‘on the flat,’ this allows more room for the webs at a shallower depth than would be required with the lumber in the opposite orientation. This is why floor trusses are often called 4×2 trusses.

The length that a 4×2 truss can span is only really restricted by the depth of the truss. While depths as shallow as 10 inches are possible, 4×2 trusses really become the competitive choice around 14-16 inches depth. So, for some designs or areas with height restrictions they may not be the best option. Within a particular depth the species and grade of the lumber, the number of webs, and the size of the steel plates at connections can increase the span and load carrying capacity of the truss.

Floor trusses are similar to I-joists in that they are specifically designed by an engineering technologist, then reviewed and sealed by a professional engineer, with documentation provided to the site. They differ however in that they are produced in a truss plant. In our case, we produce them at our Cobble Hill facility. This provides the opportunity for flexibility in the design, such as top chord bearing trusses and sloped top chords for deck areas.

As the lumber can be spliced with steel connections, continuous spans across a building are typical. The trusses are precisely sized so there is no cutting required on site, speeding installation.

The open webs allow for electrical, plumbing, and mechanical to pass through, no drilling required. While webs can’t be cut, drilled, or modified, larger openings can be accommodated at the design stage, making floor trusses the most flexible system in this regard.

As they incorporate smaller dimensional lumber, trusses see similar dimensional stability and reduced defects as I-joists. Also, similar environmental benefits are realized.

While it may seem the consideration of options and decisions are endless at the design stage of your home, determining the appropriate floor framing system may achieve wide-open rooms or adding an additional bearing wall in the crawlspace, reducing a span, may make room in the budget for those gorgeous kitchen cabinets.

 

If you have questions on choosing the appropriate floor system for your project please contact us.

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