Gutters need a drop of at least 1/4 inch for every 10 feet of run. A drop of 1/2 inch per 10 feet is better if you get a lot of rain or have trees nearby. That is 1 inch of fall over 20 feet, and it keeps water moving even when the gutter picks up a little debris.
A 30 foot gutter, for example, should sit 3/4 inch to 1 1/2 inches lower at the downspout end than at the far end. In this guide you will get a drop chart for common lengths, a simple formula for any run, and clear steps to mark and hang the slope. You will also see what to do with long runs and how to fix gutters that already hold water.
How much drop should a gutter have?
The minimum is 1/4 inch of drop for every 10 feet of gutter. The better target for most homes is 1/2 inch for every 10 feet. Both numbers drain water. The difference is how much room you leave for things to go wrong.
Gutters do not stay perfect for life. Hangers loosen, fascia boards shift, and wet leaves add weight. A gutter set at the bare minimum can lose its slope after a few seasons and start holding water. A gutter set at 1/2 inch per 10 feet has pitch to spare.
In metric, 1/4 inch per 10 feet is about 6 mm for every 3 metres, or roughly 2 mm per metre. The 1/2 inch version is about 13 mm for every 3 metres, or roughly 4 mm per metre. As a ratio, that is close to 1 in 480 for the minimum and 1 in 240 for the stronger slope.
So which one should you pick? Use these as a guide:
- Pick 1/4 inch per 10 feet for short runs, light rain, and roofs with no trees nearby.
- Pick 1/2 inch per 10 feet for heavy rain, tree-lined yards, steep roofs, and any run that tends to collect leaves.
Quick rule: divide the length of the run in feet by 10, then multiply by 1/4 inch for the minimum or 1/2 inch for the safer slope.
A steeper pitch is not always better, though. You will see why in the section on mistakes. For now, the safe range is between 1/4 and 1/2 inch per 10 feet.
Gutter drop chart by run length
This chart shows the total drop from the high end to the downspout for common run lengths. Measure the run in a straight line along the fascia board, from the high point to the downspout.
| Gutter run | Minimum drop (1/4 in per 10 ft) | Safer drop (1/2 in per 10 ft) |
| 10 feet | 1/4 inch | 1/2 inch |
| 20 feet | 1/2 inch | 1 inch |
| 30 feet | 3/4 inch | 1 1/2 inches |
| 40 feet | 1 inch | 2 inches |
| 50 feet | 1 1/4 inches | 2 1/2 inches |
The 50 foot row assumes the gutter slopes one way. Runs that long are better split at the middle, which changes the math. The next two sections cover how to find your own number and how to handle long runs.
If your gutter turns a corner, treat each side as its own run. Work out the drop for each leg based on the distance to the downspout it feeds.
How do you calculate gutter drop for your own house?
Use one simple formula. Take the length of the run in feet, divide it by 10, then multiply the answer by the drop per 10 feet that you picked.
Here is an example for a 32 foot run. Thirty-two divided by 10 is 3.2. Multiply 3.2 by 1/4 inch and you get 0.8 inch, which is about 13/16 inch. Multiply 3.2 by 1/2 inch and you get 1.6 inches, which is about 1 5/8 inches. So the downspout end should sit between 13/16 inch and 1 5/8 inches lower than the high end.
Here is a shortcut for the stronger pitch. A drop of 1/2 inch per 10 feet is the same as 1 inch per 20 feet. For a 40 foot run, that is 2 inches. Round to the nearest 1/16 inch. A chalk line cannot show anything finer than that.
Two details help you get the number right. First, measure from the high point to the downspout, not the length of the whole house. Second, if a downspout sits in the middle of a run, work out each side on its own.
What if your gutter run is longer than 40 feet?
For runs longer than 40 feet, put the high point in the middle and slope the gutter down to a downspout at each end. This keeps the drop on each side small and the water load on each downspout lower.
Take a 50 foot run as an example. Each half is 25 feet. At 1/4 inch per 10 feet, each side drops 5/8 inch. At 1/2 inch per 10 feet, each side drops 1 1/4 inches. If you sloped the whole 50 feet one way instead, the stronger pitch would add up to 2 1/2 inches, and a tilt like that is easy to spot from the street.
You can also flip the layout. Slope the gutter down from each end toward a single downspout in the middle. Pick whichever puts the downspouts where water can drain away from the house best.
A good rule is about one downspout for every 20 to 30 feet of gutter. The Home Depot rain gutter planning guide covers downspout spots, hanger spacing, and ground drainage in more detail.
Set each downspout at a low point of the run. Place it a few inches in from the end of the gutter so it lines up with the corner of the house. Then send the water at least 4 feet away from the foundation.
How to set the right slope, step by step
Here is the full process for hanging a new gutter with the correct drop. A step-by-step gutter install guide shows the whole job with photos if you want to see each stage.
- Check the fascia. Look for rot and fix it before anything else. Brackets need solid wood to hold.
- Find the high point. It is the end farthest from the downspout. On a run with a downspout at each end, it is the middle.
- Mark the high point. Rest a level or straightedge on the roof so it hangs over the edge. Mark the fascia 1 to 1 1/2 inches below it. Do not push the gutter up against the underside of the shingles.
- Mark the low point. Use the formula to find the drop, then measure down from the high mark at the downspout location and make a second mark.
- Snap a chalk line. Stretch the line tight between the two marks and snap it. Use a level to check that the line falls toward the downspouts.
- Fix the brackets. Place one about every 18 to 24 inches along the line. Screw them into the rafter tails (the ends of the roof rafters behind the fascia) and not only into the fascia board.
- Hang the gutter. Line up the top of the gutter with the chalk line. Join sections so each overlap sends water from the higher piece to the lower piece, then seal the joints.
- Test it. Run a garden hose at the high end and watch the water travel. It should reach the downspout without stopping anywhere.
The gutter follows your chalk line, not the fascia board. If the fascia is not level, that is fine. The line is what sets the pitch.
What happens if the slope is wrong?
Too little slope is the more common problem. Water sits in low spots instead of running out. Leaves and grit settle into that water and build up into a dam. Soon the gutter spills over the front edge, runs down the siding, and soaks the soil next to the foundation.
Standing water causes other trouble too. It speeds up rust in steel gutters, wears out seams and sealant, and gives mosquitoes a place to breed. Water that sits against the fascia can also rot the wood behind the gutter.
Too much slope has its own problems. A gutter that tilts hard is easy to spot from the street, and it can run out of fascia room on long runs. In a downpour, fast water can also overshoot the outlet at the low end. That is why 1/2 inch per 10 feet is a good ceiling for most homes.
How do you check and fix gutters that are already up?
The easiest check is to look after rain. Set a ladder on firm, level ground and look for water still sitting in the gutter once the rain has stopped. Standing water means a flat or low spot.
You can also test on a dry day. Run a hose at the high end and watch the flow. Water should move steadily to the downspout. If it stalls, mark that spot.
To measure the slope, hold a 2 to 4 foot level against the bottom of the gutter, or pull a string line along the brackets and measure down from it at both ends. Compare the result to your target drop.
To fix a low spot, loosen the hangers nearby. Raise the low section or lower the high one, then tighten the hangers again. Fractions of an inch are usually enough. If the gutter sags between brackets, add hangers so they sit every 18 to 24 inches.
Clean the gutter before you measure anything. Wet leaves add weight, pull the gutter down, and create dams that look like slope problems. Clearing them is sometimes the whole fix.
Common mistakes that ruin a good slope
- Measuring from the wrong end, so the gutter slopes away from the downspout.
- Letting the gutter dip or wave between brackets. The slope should be one steady line.
- Hanging the gutter against the underside of the shingles instead of 1 to 1 1/2 inches below the roof line.
- Putting the downspout outlet at the high end instead of the low end.
- Overlapping joints the wrong way. Sealant will not stop a leak if the overlap points water in the wrong direction.
- Skipping the hose test before you pack up the ladder.
Every one of these is cheap to avoid while the gutter is still on the ground or the line is still chalk. They get expensive once the brackets are in the wood.
Conclusion
Gutters should drop at least 1/4 inch for every 10 feet, and 1/2 inch per 10 feet gives you a safer margin. Divide the run by 10, multiply by your chosen drop, and mark the low end at the downspout. Split runs over 40 feet at the middle, snap a chalk line, and test with a hose. A good result is a gutter that drains fully soon after the rain stops, with no standing water left.
Frequently asked questions
What is the minimum slope for a gutter?
The minimum slope is 1/4 inch of drop for every 10 feet, which is about 2 mm per metre. Anything flatter leaves water standing in low spots, especially once a little debris builds up.
Should gutters be level?
No. A level gutter holds water because nothing pushes it toward the downspout. Give it at least 1/4 inch of drop per 10 feet, or 1/2 inch per 10 feet for a safer margin, toward the downspout.
How much drop does a 20 foot gutter need?
A 20 foot gutter needs at least 1/2 inch of drop from the high end to the downspout. For the safer pitch of 1/2 inch per 10 feet, use a full 1 inch of drop.
Can a gutter slope too much?
Yes. Too much pitch makes the gutter look crooked and can use up your fascia room on long runs. Fast water can also overshoot the outlet in a downpour. Stay between 1/4 and 1/2 inch per 10 feet.
How many downspouts does a gutter run need?
A good rule is one downspout for every 20 to 30 feet of gutter. For runs over 40 feet, put the high point in the middle and a downspout at each end. Heavy rain areas may need larger downspouts.
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