Consider the highest grade if you wish to space your rafters wider. For instance, thicker plywood can carry more load, thus allowing wider rafter spacing.įinally, roof sheathing materials are graded depending on their quality. However, the thickness changes everything. OSB is stronger, accommodating wider rafter spacing. The two main roof sheathing materials are OSB and plywood. Three sheathing properties are important when calculating rafter spacing – material, size (thickness), and grade. Type and Properties of the Roof Sheathing Meanwhile, speak with your structural engineer about truss design. You can find roof joist spacing requirements from the IRC or IBC.

Otherwise, you may need thick rafters spaced closely.

Thus the design, size, spacing, and physical properties of your trusses and joists are critical.įor instance, you need steel trusses for industrial applications or extra-heavy roof systems. The best roof trusses and joists help to evenly distribute the roof load to the walls and foundation for greater structural stability. Therefore, you need a closer rafter spacing. Naturally, the greater the load, the more support you need to prevent a sinking roof. Meanwhile, dead load refers to permanent roof load factors, like the shingle and decking weight. Live lead refers to temporary weight factors affecting the roof, including wind, snow, and a contractor on the roof. The two main roof load categories are live load and dead load. Instead, you can only adjust the rafter spacing to match the span. Unfortunately, you cannot change the span after building the structure. Generally, the shorter the span, the wider the maximum allowable rafter spacing. The span directly impacts rafter spacing. Instead, it’s measured parallel to the ceiling or floor. So, it’s not the rafter’s diagonal length.

Rafter span is the rafter projection horizontally from the ridge to the wall. Meanwhile, 2×4 and 2×6 rafters are ideal for closely-spaced rafters. Generally, you need thicker dimensional lumber (2×8 or higher) for wider-spaced rafters, especially at 24 inches on-center. However, most roofers use 2×6 and 2×8 rafters. Though important, we’ll leave out the length for now.Ĭommon rafter sizes are 2×4, 2×6, 2×8, 2×10, and 2×12. Size here primarily refers to the dimensions that impact the thickness, thus the width and depth. Rafter Sizeīesides material and grade, consider each rafter’s size. Similarly, you need more rafters when using lower-grade lumber. For instance, weaker softwood lumber, such as spruce, necessitates more rafters per square than a hardwood like oak. The type and grade of the timber matter a lot. Meanwhile, steel offers better strength properties for industrial applications. Wood is the material of choice for residential roof construction. Rafters are typically made from timber or steel. We’ll only consider the six most important ones. Rafter spacing depends on multiple factors. This means measuring from the first rafter’s center to the next rafter’s center. Rafter spacing is measured “Off Center” (OC). Alternatively, it’s a near-horizontal member that doubles up as a roof joist on flat roofs. A rafter is a diagonal framing member on pitched roofs running from the apex of the roof to the roof plate or the eaves. Rafter spacing refers to the horizontal distance between two parallel rafters on a roof. But 24-inch on-center spacing requires 50% fewer rafters, thus significant cost savings. Also, the rafters must be deeper (thicker), of a higher grade/quality, and from a stronger wood species. However, 24-inch on-center spacing exerts about 50% more weight on each rafter than 16-inch on-center spacing. Which is better? This guide explains everything you need to know to make an informed decision.īoth options are practical. But the big dilemma is choosing one between the two. But, more importantly, they’re the sole framing members responsible for distributing roof weight to the walls.įortunately, many roofers and homeowners already understand that 16 and 24-inch on-center are the standard rafter spacing measurements. Roof rafters, the roof framing members running from the wall plate to the roof ridge, must be properly spaced.įor one, they directly impact joist and ceiling spacing and the integrity of the roof deck.

Pre fab metal roof rafter spacing