How much slope should gutters have?
The working number is ¼ to ½ inch of drop per 10 feet of run — about 1/40 inch per foot. Enough to move water, little enough that the gutter still reads level from the yard.
Gutters are not hung level. They fall toward the downspout so water keeps moving, and the amount of that fall — the slope, also called pitch or drop (or eavestrough slope, if you are reading Canadian guides) — is one of the few numbers in guttering with a genuinely standard answer: ¼ inch of drop for every 10 feet of run, up to ½ inch per 10 feet where you want faster drainage. Per foot, that is roughly 1/40 to 1/20 of an inch — invisible from the ground, decisive inside the gutter.
The drop chart
Total drop = run length × slope. At the two ends of the standard range:
| Run length | Drop at ¼ in / 10 ft | Drop at ½ in / 10 ft |
|---|---|---|
| 10 ft | ¼ in | ½ in |
| 20 ft | ½ in | 1 in |
| 30 ft | ¾ in | 1½ in |
| 40 ft | 1 in | 2 in |
| 50 ft | 1¼ in | 2½ in |
The gutter slope calculator does this arithmetic for any run and slope, and shows the result as the height difference between the high end and the downspout end.
Why not more slope?
If a little fall is good, a lot is not better. Three things go wrong with steep gutters:
- It shows. Past about ½ inch per 10 feet, the gutter visibly diverges from the fascia line and the trim starts to look crooked.
- You lose capacity at the low end. The gutter cross-section is sized for water depth; a steep run stacks all its water at the downspout end and can overflow there in hard rain while the high end runs dry.
- You run out of fascia. On a long run, 2–3 inches of total drop can push the low end below the fascia board or under the drip edge, leaving nothing to fasten to.
Long runs: slope from the middle
Runs much over 40 feet should not drain to one end. Set a high point mid-run and slope both ways to a downspout at each end: each half is a 20–25 foot run with its own modest drop, the fascia reveal stays even, and each outlet carries half the roof. This is the same split the downspout count & sizing calculator assumes when it spaces outlets every 30–40 feet.
Setting the slope with a string line
The method is the same for new gutters and for re-hanging saggy ones:
- Mark the high end just below the drip edge, at the top of the fascia.
- Compute the total drop for the run (chart above) and mark the downspout end that much lower.
- Snap a chalk line between the marks and hang the gutter to the line, hanger by hanger.
To check an existing gutter, pour a bucket of water in at the high end after a dry day: it should sheet toward the downspout and be gone in a minute or two. Water that sits in the middle means a sag or a back-pitched section; re-set those hangers rather than living with the pond.
Is a little standing water a problem?
A thin film that evaporates in a day is normal — few gutters are perfect. A standing pool is not: it adds weight the hangers were not sized for, breeds mosquitoes, and in cold climates freezes into a bar of ice that pries the gutter off the fascia. If a level check shows the run is flat or back-pitched, the fix is re-pitching the hangers, not another cleaning. Persistent overflow at the downspout end with a correct slope usually means the run needs another outlet, not more fall — check it against the downspout guide.
Bottom line: ¼ inch per 10 feet is the floor, ½ inch per 10 feet the practical ceiling; split anything over 40 feet from a mid-run high point. These are labeled planning conventions — follow the gutter manufacturer’s hanging instructions where they differ, and let a pro handle multi-story work.