I plan on making a G.Loomis GLX 9' rod for mostly lake fishing. I will build it on a 6 Wt rod for use primarily with a 7 Wt, weight forward sinking line and occasionaly a 5 or 6 Wt double taper floating line. Will the line one size larger make a good combination, or would it be better to use a 7 Wt rod for a 7 Wt line?

Depends on the distance you are going to be fishing. Line weight designation corresponds to a certain amount of line length. If you are building on a 6 wt blank then a 7 weight line will work, but at shorter distances. If you want to fish average or better distances and want to use a 7 weight line, you'd be better off with the 7 weight blank.

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Taking one step farther, if you go to a 6wt on a 7wt rod, you will get better recovery (making it feel like a stiffer faster action rod) and it will allow you to really stretch the distance out there, but we are talking serious double hauling distance here. Manufacturers (especially the notable ones) do alot of testing and science to determine the correct wt for each blank. There are certainly going to be variables, but my suggestion is if you are primarily fishing 7wt, do a 7wt. rod. Then go to a local fly shop and ask to test different size lines if you don't have em. You may like a 6wt. feel better, or you may not. The drawback to downsizing the line is you will not get the rod to load properly at shorter distances. Kinda like casting with a stiff pole. To really understand this, try taking a 2-3wt line and put it on a 5-6wt rod. You will understand what I mean. Doing goes a lot farther than listening.
Elrod

Rex,

Airflo's "Tackle Advisor" at www.flylines.com explains this perfectly.
Alan

Keeping in mind what Elrod has said. A friend of mine, who owns a fly shop, has talked to Sage about new rods in the same line weight, that don't cast the same. The small differences in the blanks, that we talk about when static testing, account for this. A 6 wt. rod built to Sage's own spec.s, might really be a 5 wt. or a 7 wt. Sage does not test every rod for line weight accuracy. As custom rod builders, the time we spend setting up guide placement and test casting is well worth the effort. Sage factory rods are fine rods, millions own them and love them, but generic guide charts are not accurate.

Bill Doherty