The maximum span of 2×8 ceiling joists without support will depend on various factors, including the type of wood, the spacing between the joists, and the load they need to support. In general, for residential construction with common framing lumber such as SPF (Spruce-Pine-Fir), 2×8 ceiling joists spaced 16 inches on center (oc) can typically span up to about 20 to 23 feet.
Span is determined by species, grade, dimensions, spacing, use, and load. 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 ceiling joist is 10.5”, 14.5”, 17.7”, and 22.5”.
Standard industry ceiling joist spacing in inches are 12”, 16”, 19.2”, and 24” on-center, which means the actual space between the joist 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 joist often results in joist of greater depth too.
In the construction world, span is a word used to mean the linear distance a board, joist, rafter, or roof must bridge or cover without bending or requiring support. The distance is determined by many factors and conditions.
A rule of thumb is 1.5 times a joist’s depth but in feet when spaced at 16” centers. In general, a 2×8 will span 1.5 x 8, so 12-feet. Based on all factors though, a 2×8 joist span is 7′-1” to 16′-6”
There are different factors that affect how far a 2×8 can span, including use or location, species of wood, grade, the spacing between joists or rafters, and load.
There are different depth of ceiling joist are used in floor construction such as 2×4, 2×6, 2×8 and 2×10 made of different species of wood (southern yellow pine & Douglas fir) and different grade such as grade – 1, grade – 2, grade – 3. Southern yellow pine is strongest species of wood that can span more distance compared to other species of wood. Grade 1 lumber have higher strength than other grade Lumber due to fewer knots.
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How far can a 2×8 ceiling joist span without support
A #1 Douglas fir-larch ceiling joist at 12” on-center (oc) may span 26’ at 12”. OC, 23’-10” at 16” OC, 22’-5” at 19.2” OC, and 20’-1” at 24” OC.
The live and dead load rates combined are also important. A 2×8 will support a dead load of 20psf and a live load of 40psf resulting in a total load of 60psf. Depending on the species, grade, and spacing, a 2×8 may span between 7’-1” and 15′ 0″.
A 2×8 spaced 16 inches apart can span a maximum distance of 18 feet 2 inches when used as a joist.
As per general thumb rules and guideline, a 2×8 ceiling joists can span up to 18 feet and 2 inches from beam to beam when spaced the standard 16 inches apart with a maximum ceiling loads are assumed to be 20 PSF and roof pitch is not steeper than 3 in 12.
A 2×8, #2 grade, Douglas fir-larch ceiling joist can span 25’ 8″ at 12” OC, 23’-0” at 16” OC, 22’-5” at 19.2” OC, and 18’-9” at 24” OC without support at live load of 10 Psf and dead load of 5 Psf and deflection of L/240.
A 2×8, #2 grade, Hem fir ceiling joist can span 24’ 0″ at 12” OC, 21’-9” at 16” OC, and 18’-6” at 24” OC without support at live load of 10 Psf and dead load of 5 Psf and deflection of L/240.
A 2×8, #2 grade, Spruce/pine fir (SPF) ceiling joist can span 24’ 7″ at 12” OC, 22’-4” at 16” OC, and 18’-9” at 24” OC without support at live load of 10 Psf and dead load of 5 Psf and deflection of L/240.
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Conclusion:
A 2×8, #2 grade, Douglas fir-larch ceiling joist can span 25’ 8″ at 12” OC, 23’-0” at 16” OC, 22’-5” at 19.2” OC, and 18’-9” at 24” OC without support.