The Evolution of Wall, Floor and Roof Structure
The stats remain fairly stable: 93% of new homes in the U.S. are wood-framed; concrete homes make up about 7%, and steel-framed homes less than half a percent of the market share, according to NAHB analysis of Census Bureau data tracked over the past five years.
In spite of the challenges that wood framing has faced—declining quality of the lumber, tariffs on lumber, the threat of moisture in the wall system, potential for termite infestation, VOC emission liabilities— many builders and their subs are knowledgeable and comfortable with wood-stud construction; this makes them less likely to use other building methods. However, what used to be considered “advanced framing” techniques have been adapted into the mainstream due to increasingly stringent energy and structural building code requirements.
“Advanced” Framing for Walls Becoming the Norm
Wall frames provide the most opportunities for material savings when advanced framing techniques are used in place of conventional framing methods. Advanced framing uses fewer framing materials, yet boosts structural strength and energy efficiency, lessens environmental impact, and lowers labor costs.
It’s now widely accepted to build walls with 2×6 wood studs spaced 24 inches on center. Sheathing them with OSB or plywood allows compression and tension loads to be directly transferred to the vertical framing members under the roof trusses or rafters. Wood structural panels furthermore allow for greater architectural flexibility in the number and location of door and window openings, and the fact that the sheathing serves as a base for fastener attachment between the studs supports the cost-effective use of virtually any type of siding product.
Single, rather than traditional double top plates and headers, insulated two- or three-stud corner junctions, minimal use of jack studs and cripples, elimination of redundant studs and unnecessary blocking and bridging, and ladder junctions at interior wall intersections are other advanced framing techniques. All of these advanced framing techniques reduce the potential for insulation voids and create more space for cavity insulation.
Floor Framing: More Than Just Initial Material Costs to Consider
For floor systems, the Engineered Wood Association recommends wood floor joists (such as I-joists), structural composite lumber (SCL) and/or glulam at 24 inches on center and encourages engineers to maximize member spans between supports. Specifying I-joist floor systems between finished floors will typically allow for the installation of plumbing, electrical and mechanical services within the floor frame cavity, eliminating the need for dropped ceilings.
Thicker floor panels are recommended for a stiffer, more solid floor structure. OSB (oriented strand board) or plywood subfloors are typically a 3/4-inch tongue and groove panel, nailed or preferably screwed down. The glues and processes used to make both OSB and plywood, however, make the panels prone to soaking up moisture when it rains on the wood structure before it’s died in. Up-and-down floor movement is caused when edges swell, leaving gaps beneath the floor underlayment, leading to a creaky floor and a callback. Plywood and OSB manufacturers try to improve this by sealing the panel edges, but the sealant, whether it is a wax or a paint, tends to get scuffed off. Higher-density, more water-resistant OSB products, like AdvanTech panels are made to specifically address this problem. AdvanTech panels use an advanced liquid coating during the manufacturing process to make sure the panel is thoroughly protected.
Prefabricated insulated structural components that simplify construction such as insulated headers and insulated corners, are becoming commonly available. The Tstud™ is a newly engineered building product that uses two lumber members, an internal truss system, and a frothed-in-place closed-cell foam.
When creating their latest roof hangers, Simpson Strong-Tie’s goal was to help installers and designers meet new load path requirements.
Roofs and Codes Related to Them Become More Complex
As roof designs have become more complicated with a lot of valleys, dormers and other features, the code requirements for stick framed roofs have become more complex over the years, too. Truss roofs, constructed at a factory and delivered to the site, outnumber stick-frame roofs two to one by some estimates. But there are regions of the country where builders still prefer stick-frame roofing in order for the roof to be more customized—for example, when a large attic space or high, vaulted or cathedral ceilings are desired. To meet current IRC roof framing requirements, the bottom of each roof triangle (the ceiling joists) must be fastened securely to the rafters on each end and must continue across the entire width of the ceiling so they keep the ends of the rafters from spreading out when loaded.
On vaulted ceilings, where there is no tie at the bottom, the rafters must be supported at their upper end to prevent the rafter thrust at the lower end. This means that half the load is now supported by a load-carrying ridge beam designed to bridge the span between the supports, which carry the vertical load to the ground. If the vaulted roof is constructed as a hip roof, the ridge beam as well as the top ends of the hips must also be supported, and the rafters must have a secure connection to the hips. Simpson Strong-Tie recently developed three products designed to provide strong, simple connections at the three points required when building these types of roofs.
What Building Science Has Taught About Advancements Versus Structural Integrity
So modern practices and principles still support that our industry will be constructing with wood well into the foreseeable future. Let’s talk about what else is involved in building a sound structure. Once upon a time, we built walls with wood board sheathing, and they were generally uninsulated. Then plywood walls insulated with fiberglass batts became the norm in the 1950s. The wood surfaces breathed when moisture content was introduced; so did the “kraft paper facing” traditionally found attached to the interior side of fiberglass batt cavity insulation. Most of the facing materials inside and out also had breathability as a natural characteristic. This means that a wall built of plywood insulated with a kraft-faced batt was able to dry outwards in the winter and inwards in the summer and in both directions if a window leaked or a missed flashing causes a cavity to get wet. Fast forward to the late 20th century, when OSB, plastic housewraps, and foam and other new types of insulation helped us build a tighter home than ever before. We got us into a whole set of new problems–walls with little to no breathability at all.
Building science taught us that we needed an air gap between the cladding and the housewrap/OSB interface. Weather protection barrier manufacturers introduced products designed to act as an air and moisture barrier while allowing moisture vapor to escape from the wall cavity to the outside. Building regulations required roofs to be ventilated by the use of soffit or ridge vents for the same reasons. So, we now have the breathability back along with superior insulation.
But we still have installation training to do. Factors such as improper fastening, incomplete coverage and poor flashing, caulking or taping can all affect energy efficiency and indoor air quality. Installers have had to learn that a house wrap should cover completely and be properly lapped in order to shed water. And here’s a new one—material compatibility with sealants can be an issue. Many water-resistant barriers, sealants, and building tapes used to keep water out of wall and roof assemblies have the potential to react badly when brought together. Some window manufacturers are now suggesting that asphalt-based sealing tapes not be used on PVC nailing flanges because the adhesives may liquefy and ooze as they react with the plasticizers in the PVC. It is now necessary to check which product a window or housewrap manufacturer has tested with its products and approved for use with them.
The Bottom Line:
Savvy builders will agree that as with most material choices and best practices, the elimination of problems means less cost—both from the perspective of bottom line and reputation. Building science has advanced the common principles of framing design, structural integrity and energy efficiency significantly over the past couple of decades. But take care; tight is good, impermeable is not. Plastics and chemicals are not always bad; they have their place in modern building products, but beware about pairing products from different manufacturers unless they have been approved for contact with each other. Overall, our industry continues to take major leaps forward with everybody elevating their game and paying more attention to detail, execution and building science.