Overlining and Underlining a Fly Rod

Generally, when you put line on a fly reel, you match the weight of the line to the rod and reel. This means a 5-weight rod will get 5-weight line, an 8-weight will get 8-weight line, and so on. This makes sense, right?

While this is the most common way to rig up, there are two other options for choosing a line weight — overlining and underlining.

In a nutshell, overlining a rod means you’re using a heavier-weight line than your rod calls for. Usually, you don’t want to go more than a weight or two over the weight of the rod. Similarly, underlining a rod means you go one or two weights lower than your rod’s weight when choosing a fly line.

Here are some pros and cons of both overlining and underlining a rod. Also, I address some considerations to keep in mind before trying either. These will help you determine when to use each on the water.

Overlining

Overlining a rod definitely has its perks. At the same time, it’s not a cure-all for every casting issue out there.

The problem with a lot of articles I see is that they’re labeled something along the lines of, “why you should overline your rod.” I don’t have a problem with overlining rods, but there are also times not to overline. Here are some pros and cons of using a heavier line. Take into account your goals and your style of fishing to decide if overlining might be right for you.

A man in silhouette casts a fly rod in a lake.
Overlining can make fast action rods load more easily.

Pros

More loading for fast action rods – Many rods these days are fast action, with a stiff backbone that doesn’t like to bend much. These fast action rods are great for launching line, but they can be hard to “feel,” especially for beginners. In order to cast properly, it’s important to know when the rod has loaded, and unless you’re keeping an eye on it the whole time, this is best done by feeling when the rod has fully bent as it throws the line back. For experienced casters, this probably isn’t an issue. For beginners, though, it can be hard to detect. Having a heavier line will put more bend in the rod, making it easier to feel the loading. This, in turn, will make it easier to throw accurate casts.

Loading with less line or long leaders – Another big benefit of overlining, for experts and beginners alike, is being able to load a rod with less fly line out. When there’s less fly line out the end of a fly rod (either because you’re making very short casts or using a very long leader), there won’t be much line weight to carry the weightless fly. By using a heavier fly line, you can get away with having less line out, since the weight will be greater. So, overlining may be a great option for fishing very narrow streams that require short casts.

Wind (light weights) – This applies to lower-weight rods like 3 and 4-weights. Having a heavier line on these rods may allow you to cast more easily into wind. The added weight can help to punch through wind that would normally throw a lightweight fly line off course. It can be argued that the opposite is true of higher-weight lines like 10 or 11-weights, which I’ll address in the cons.

A man's hand holding a fly rod and reel

Cons

Drag – If you’ve got a lot of fly line sitting on the water during a drift, overlining will cause excess drag. With more weight and a larger profile, the line will easily get caught up in currents. This can cause the fly to move at the wrong pace, which is often considered one of the easiest ways to lose a fish’s interest. If you’re trying to fish with a lot of line on the water, avoid overlining.

Stealth – If you’re fishing around particularly spooky fish, overlining may not be the best choice. Naturally, a larger and heavier line is going to be more noticeable in the water. This is especially true when the line first hits the water after a cast. This probably isn’t even a consideration when fishing for species like bass or bluegills. Trout, however, may not be fans of overlining.

Distance – One big drawback of overlining is that you’ll probably sacrifice some distance. If you’re taking short casts, as mentioned before, overlining is great. If you’re trying to cast way out into a lake or across a big river, though, your distance will suffer. This is understandable, since it’s obviously easier to throw something farther when it’s lightweight. Keep your target distance in mind when deciding whether to overline.

Slow action rods – Slow action rods, like glass rods, will suffer under the added weight of a heavy line. For the same reason that fast action rods thrive with overlining, slow action rods struggle. These rods bend very easily even with standard line weights. By adding more weight, the rod will probably start to sag before you even start to cast. Once you start casting, the rod won’t have enough backbone to support the heavier line.

Wind (heavy weights) – Unlike the lower-weight rods, powerful rods like 10 or 11-weights may not be ideal for overlining in wind. That’s because their lines are already heavy enough to punch through gusts, but adding extra unnecessary weight can make the line’s profile so big that it’ll get caught in the wind like a sail. To be honest, I’m not sure where the line would be drawn between overlining being a pro or con in the wind, but in general the higher weights probably aren’t ideal.

Underlining

Underlining a rod doesn’t seem to come up in conversation nearly as often as overlining does. The pros and cons of underlining are, naturally, pretty much just the opposite of overlining. For that reason, I’ll give a much more abbreviated discussion of the pros and cons of underlining. In general, the qualities of an overlined reel will be reversed for an underlined reel.

Pros and Cons

Unlike overlining, underlining will make it harder to load a fast action rod and cast the rod at a short distance. Without the added weight, the rod will have less bend and you’ll need more line out to fully load the rod. This’ll make it harder, especially for beginners, to get accurate casts. Underlined rods may also be hard to cast in the wind, since the line will easily get caught in gusts.

Underlining, however, does have some huge benefits for certain anglers. If you fish glass or bamboo rods with lots of bend, underlining will bring out the playfulness of the rod without overwhelming it. In fact, some fly lines are even made lighter, specifically to be used with slow action rods. While overlining a glass rod is essentially a death sentence for your casting, underlining it can improve your cast.

Additionally, looking at the other cons of overlining shows that underlining can help with drag and stealth when fishing for picky fish. If you need to sneak up on a spooky brook trout, consider underlining by one weight.

Considerations

From the pros and cons listed, hopefully you can determine whether overlining, underlining, or matching the rod’s weight is best for you. However, there’s more to the story.

In an ideal world, fly lines would be made to exact specifications every time. Then you could choose to match the rod’s weight (for well-rounded performance), overline the rod (to cater to fast rods and short casts), or underline the rod (for glass or bamboo).

The problem is that fly lines aren’t always as they appear.

The chart below shows the upper and lower limits (in grains and grams) of each line weight. It also shows the sweet spot, highlighted in yellow, for each weight.

A fly line weight chart.

Now, if you look up some actual line specifications, you’ll notice an issue. Here are the grain weights of two Rio lines. The first is Rio’s Trout LT WF line, a standard trout line. The second is their LightLine WF, a lighter line made for slow action rods.

2wt5wt
Trout LT WF90 grains150 grains
LightLine WF80 grains140 grains

If you compare these grain weights to the American Fly Fishing Trade Association (AFFTA) standards above, you’ll see that the regular trout line runs heavier than even the highest limit of a given line weight. The LightLine WF, on the other hand, matches the sweet spot of a given weight exactly.

Now, to be fair, I assume this is for a good reason. Since more anglers will be using fast action rods, it makes sense to have heavier-than-average lines (since heavy lines load fast rods better).

The problem, though, is that unless you’re aware of that when you buy the line, you might try to overline a rod that is already effectively overlined. If you know overlining is good for rod loading and you go a weight or two above the standard rod weight, now you’ve actually gone two to three weights over.

Additionally, if you decide that for your glass rod you’ll stick to the standard weight to avoid overlining, you’ll inadvertently overline anyway. Unless you went with the LightLine (which beginners may not even think to buy), you’d have to buy a weight down for glass to put it in the sweet spot.

The AFFTA chart, to be fair, is inherently flawed, since it doesn’t differentiate between fast and slow action rods. But, at the same time, if line manufacturers kept their products in line with the chart, at least there would be a standard. Then, you could adjust up or down for your rod type without having to double check that the specifications of the line match the standard.

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This is a list I made and use for my own trips, and I think any backcountry angler will find it handy.