Check: let's look at the AITC Table DF-26 (Roof Beams subject to Snow Loads, Douglas Fir) ... for 5-1/8 x 13.5 for span of 18 ft it shows 884 plf. NICE! ... I have that same number! Note also the `B'. That means that `B'ending controls the value. If we did a similar calc ... we would find that for Shear, Snow load and Total load deflections, the capacity (in this case the allowable spacing) values would all be greater (than the 9.1 ft). This kind of contradicts what I said in an earlier post - about glulam being governed by deflection for long spans. But - 18 ft is not a very long span for glulam. Plus, if you look at the AITC table ... you only have to go out to 20 ft ... to get `D'eflection governing (the `D').
The AITC DF-26 table is ... here.