Joists, I-joists or floor trusses?
You have reworked the plan, adjusted the orientation, tweaked the rooflines. How much thought have you given to what holds the floor up? Unlike floor finishes, this becomes part of the structure for the life of the house.
The three systems compared
All three are offered in Pacific Homes packages. Which one suits depends mostly on your spans, your available depth, and how much you care about running services through the floor.
| Dimensional lumber | I-joists | Floor trusses | |
|---|---|---|---|
| Standard depths | 9 1/4" (2x10), 11 1/4" (2x12) | 9 1/2", 11 7/8", 14" | 10" possible, competitive at 14 to 16" |
| Deep options | Limited by lumber sizes | To 24" in 2" increments | Limited by design depth |
| Clear spans | Shortest of the three | Long, restricted mainly by shipping | Continuous spans typical, spliced with steel |
| Running services | Notch and drill within code rules | Drill the OSB web, some pre-pressed knockouts | Open webs, no drilling at all |
| Engineering | Code tables, no sealed analysis needed | Sealed by a professional engineer | Sealed by a professional engineer |
| Dimensional stability | Can crown, twist, cup and warp | Stable, shrinkage negligible | Stable, similar to I-joists |
| Moisture tolerance | Recovers if allowed to dry thoroughly | Keep protected | Keep protected |
| Cost | Usually the most economical | Mid | Mid to high, depth dependent |
| Made where | Local lumber yards | Engineered wood manufacturers | Our own plant in Cobble Hill |
Bold marks where a system has the clear advantage on that row. No system wins outright, which is why all three are still in use.
Only one of these lets services pass through untouched.
Floor trusses have open webs, so electrical, plumbing and mechanical run straight through with no drilling. On a house with significant mechanical runs that is the difference between a straightforward rough-in and a difficult one.
The catch is that the webs cannot be cut, drilled or modified afterwards. Larger openings can be accommodated, but they have to be designed in rather than found on site. That makes floor trusses simultaneously the most flexible system and the one that most rewards planning.
I-joists come second: the OSB web can be drilled fairly generously within the manufacturer's rules, and some come with pre-pressed knockouts that punch out for wiring. Dimensional lumber is last, since notching and drilling are restricted by code.
The same logic applies in the walls. How services are routed through a SmartWall covers that side.
What each one actually is
Dimensional lumber
The original workhorseTypically 2x10 and 2x12 nominal joist depths, with species grouped by similar performance characteristics and graded by structural quality, letting designers size joists and beams to specific loads and spans.
It is often the most economical choice, but that is not its only advantage. It is durable in shipping and installation, and if it gets wet and is then allowed to dry out thoroughly, the moisture does not significantly affect structural integrity. That resilience is why pressure treated dimensional lumber remains the go-to for decks exposed to weather.
It can also be sized from code tables without a sealed engineering analysis, and the rules for notching, drilling and installation are standardised across North America and familiar to every carpenter on site.
I-joists
Sometimes called TJIsIn the same year Pacific Builders Supplies started manufacturing wood trusses, 1969, a small Idaho company called Truss Joist Inc. created the first wood I-joist. That is why they are still sometimes called TJIs, after the pioneering brand. There is a British Columbian footnote too: TJI partnered with Vancouver forest products company MacMillan Bloedel in the early 1990s before both were bought by Weyerhaeuser.
The design borrows from the steel I-beams used in heavy construction, taking its mechanical advantage from the I profile. Two horizontal lumber flanges are joined by a vertical OSB web.
Long continuous lengths can be produced, restricted mainly by shipping and site access, and the relatively light weight makes for quick installation with fewer joints and less blocking. As a manufactured product they are dimensionally stable and consistently straight, with negligible shrinkage and none of the crowning, twisting, cupping or warping seen in dimensional lumber.
The smaller lumber and wood fibres allow smaller logs to be used and more of each log, which combined with sustainable forestry makes I-joists an environmentally sound choice. Alongside other measures, they can contribute toward certifications such as LEED.
Floor trusses
Made in our own plantSimilar to roof trusses, using dimensional lumber, most commonly 2x4, for the top and bottom chords and the webs, with every connection made by steel plates pressed into the wood.
The difference from a roof truss is orientation: the lumber is laid 4x2, or on the flat, which leaves more room for the webs at a shallower depth than the opposite orientation would need. That is why they are often called 4x2 trusses.
Like I-joists they are designed by an engineering technologist and sealed by a professional engineer, with documentation supplied to site. Unlike I-joists they are produced in a truss plant, in our case at Cobble Hill, which allows design flexibility such as top chord bearing trusses and sloped top chords for deck areas.
Because the lumber can be spliced with steel connections, continuous spans across a building are typical, and the trusses are sized precisely enough that no cutting is required on site.
This decision has a budget consequence.
It can seem like the options at design stage are endless, and floor framing is rarely the one people want to spend time on. But choosing the right system might get you the wide open rooms you wanted.
Equally, adding one additional bearing wall in the crawlspace to reduce a span might free enough budget for the kitchen cabinets you actually care about. It is worth asking the question rather than accepting whatever comes as standard. More decisions like this one.
Floor framing questions
An I-joist has two lumber flanges joined by a solid vertical OSB web, taking its strength from the I shape in the same way a steel I-beam does. A floor truss uses dimensional lumber, usually 2x4 laid on the flat, with open webs between the chords connected by pressed steel plates. The practical consequence is that services pass straight through a floor truss without drilling, while an I-joist has to be drilled within the manufacturer's rules.
Floor trusses, clearly. The open webs let electrical, plumbing and mechanical pass through with no drilling at all, and larger openings can be accommodated at the design stage. The webs themselves cannot be cut, drilled or modified afterwards, so anything unusual has to be planned rather than improvised. I-joists are next best, since the OSB web can be drilled generously within the manufacturer's rules and some come with pre-pressed knockouts for wiring.
Standard depths are 9 1/2, 11 7/8 and 14 inches, which differ slightly from dimensional lumber. For long spans, depths up to 24 inches are available in 2 inch increments. To carry longer clear spans or heavier loads without increasing depth, the flange size and grade is increased instead: a standard I-joist might use a 2x3 SPF #2 flange where a higher series in the same depth uses a 2x4 machine stress rated flange.
Depths as shallow as 10 inches are possible, but 4x2 trusses only really become competitive around 14 to 16 inches. For designs with height restrictions they may not be the best option. Within a given depth, span and load capacity are increased by changing the lumber species and grade, the number of webs, and the size of the steel connector plates.
No, which is one of its advantages. Dimensional lumber can be sized using tables in the local building code and does not require the loading analysis to be sealed by a professional engineer. Rules for notching, drilling and installation are standardised in codes across North America and familiar to every carpenter. I-joists and floor trusses are both engineered products, designed by an engineering technologist and sealed by a professional engineer, with documentation supplied to the installer and the building inspector.
Less than people assume, for dimensional lumber specifically. If it is exposed to moisture and then permitted to dry out thoroughly, there is no significant detrimental effect on structural integrity. That durability is why pressure treated dimensional lumber remains the standard for decks. Kiln drying and protection in transit still matter, because they keep the lumber dimensionally stable.
Ask us which floor suits your spans.
Send the plan and we will tell you what each system does to the depth, the spans and the number. Floor trusses are made in our own plant at Cobble Hill, and estimating is included in the package.