The 18' can be used, but you may need to have a larger eave overhang than ordering them built to 17.5. I generally cut the overhang to the length I desire, but there is a limit on how much you can shorten them or the soffit won't be low enough.

The roof trusses are for a room addition of 17.5 ft x 17 ft (as the code allows), with 4/12 pitch roof.

Can 18 ft span roof trusses be used for 17.5 ft span, or it should be custom ordered to exact span dimension? Or if there are other options? Thanks.