Hi, the plans for the 20'x30' 1 1/2 Story Cottage calls for 2"x12" joists 16" o.c. for the loft but all the span tables I have checked don't allow a 2"x12" to span anything near that distance. I have seen several pictures posted with clear span joists across this cottage. What am I missing. I am planning to widen the cottage to 24'. Do I need a centre beam to support the joists midspan? Thanks for any advice you could give. :D
Joist selection depends a lot on species and grade of lumber as well as the projected or design load.
John's 20x30 plans call for select structural Doug Fir or I beams. Loft floors are also usually rated for less live load than a main or full headroom floor.
Your real problem will be coming from widening the structure by 4 feet. This online calculator http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Douglas+Fir-Larch&size=2x12&grade=Select+Structural&member=Floor+Joists&deflectionlimit=L%2F360&spacing=12&wet=No&incised=No&liveload=30&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span
indicates that 2x12 select structural Doug Fir on 12" centers would be okay. But before go ahead and order let's see what John says.
Also see this thread...scroll way down for John's response
http://www.countryplans.com/cgi-bin/yabb2/YaBB.pl?num=1192031145
20' (19' actual span) is about the limit for 2x12's.
If you are going 24' wide do what I do in the 24' wide Solar Saltbox. Run a center beam and go with 2x8 or 2x10 joists. Alternatively you can get floor trusses but for most builders it is not worth the expense.