Roof rafter spacing, span and sizes | Rafter spacing | Rafter sizes | Rafter span
A rafter is a structural component build of wood or steel that is used as part of a roof construction. Rafter runs from the ridge or hip of the roof to the wall plate of the external wall. Rafters are generally provide in series, side by side, providing a plateform to support roof decks and roof coverings.
In many countries such as in US, Canada United Kingdom roof rafter made of wood/ timber are much common to build a house. Many species of wood are used to provide timber which have much strength.
Rafters are usually made of pine or cedar tree, for longer span rafters, building materials several manufacturers have created Laminated veneer lumber (LVL) rafters that can be 2–5 times longer than typical wood rafter. In US, most wood rafters have maximum length of 20 feet.
The area which has heavy rainfall and snowfall require good drainage for roof, roof drainage depending on slope of the roof provided, based on slope of roof it is categorised into two types flat roof and pitched roof or sloping roof.
The spacing of roof rafter is just one of the component used to determine the minimum size of roof rafter. There is no any standard rafter spacing measurements. But generally rafter spaced typically either 12, 16 or 24 inches apart in one of several industry -standard increments.
What Is Rafter Spacing
Rafter spacing is the distance between parallel rafters or roof joists. It is measured from the center of one rafter to the center of the next, also known as ‘on-center’, which makes it easier to determine the number of rafters based on the building’s length.
The typical on-center spacing, according to the International Residential Building Code (IRC), is 12”, 16”, 19.2”, and 24”, which means the actual space between the rafters is 10.5”, 14.5”, 17.7”, and 22.5”.
Standard industry roof rafter spacing in inches are 12”, 16”, 19.2”, and 24” on-center, which means the actual space between the rafters is 10.5”, 14.5”, 17.7”, and 22.5”. Depending on all contributing factors though, 16” and 24” are commonly used for most buildings. The wider the space between rafters often results in rafters of greater depth too.
Rafter spacing in mm for different sizes are kept at 300mm, 400mm and 600mm apart. Standard spacing between rafter is 400 mm centre to center. The rafter which are kept at lower spacing can allow longer distance to span as compared to joist spaced further apart.
Most common size of dimensional lumber as roof rafter are 2″×6″, 2″×8″, 2″×10″, 2″×12″ and 2″×14″ used in roof framing. Thicker dimensional lumber, such as 2x10s and 2x12s can span further. Thickness of rafter should be not less than 2 inches and their depth should be range from 4″ to 12″. Length of Rafter depending on climatic condition, style, slope and type of structure. It is used over brick external wall, wall plate, over two support and over top of beam.
A 2×4 size of roof Rafter can allow to maximum span upto 9 feet when spaced 12″ O.C, 8′ 3″ on 16″ O.C, and 7′ 3″ on 24″ O.C.
A 2×6 size of roof Rafter can allow to maximum span upto 14′ 6″ when spaced 12″ O.C, 13′ 0″ on 16″ O.C, and 11′ 6″ on 24″ O.C.
A 2×8 size of roof Rafter can allow to maximum span upto 19′ 0″ when spaced 12″ O.C, 17′ 0″ on 16″ O.C, and 15′ 0″ on 24″ O.C.
A 2 ×10 size of roof Rafter can allow to maximum span upto 23′ 0″ when spaced 12″ O.C, 19′ 11″ on 16″ O.C, and 16′ 3″ on 24″ O.C.
A 2×12 size of roof Rafter can allow to maximum span upto 26′ 9″ when spaced 12″ O.C, 23′ 2″ on 16″ O.C, and 18′ 11″ on 24″ O.C.
Roof rafter spacing are 12” (300mm), 16” (400mm), 19.2” (500mm), and 24” (600mm) on-center. Most common size of dimensional lumber as roof rafter are 2×6, 2×8, 2×10, 2×12 and 2×14 used in roof framing.