With a 12â deck span, the beam may be either two 2x8s or two 2x10s, depending on wood used. CASE II SOLUTION: Use âaâ to determine joist size and âaâ+âbâ to determine beam a b and footing sizes.The length of âbâ is restricted by both the length of âaâ and the size of the joists.Example: a=8â, b=2â, Posting Spacing=10â Refer to the Joist Span table. For an 8â joist span, either 2x8s at 24â O.C. or 2x6s at 16â O.C. are acceptable.For sizing the beam, use a joist length of 10â (8â+2â) and a posting spacing of 10â.xample: a=8â, b=2â, Posting Spacing=10â Refer to the Joist Span table. For an 8â joist span, either 2x8s at 24â O.C. or 2x6s at 16â O.C. are acceptable. For sizing the beam, use a joist length of 10â (8â+2â) and a posting spacing of 10â.