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Measuring Slope and Grade Across a Gate Opening
To measure slope across a gate opening, stretch a string tight and level between the two posts or ends, then measure down from the string to the ground at the left end, the middle and the right end. The difference between those three readings is the rise or fall across the opening, and it decides whether the gate bottom should follow the ground, step with it or stay level with a gap. Ten minutes and a few cheap tools give a fabricator numbers that photographs cannot.
Why slope changes the design
A gate is a rigid frame, and ground is not. If the ground rises four inches from one side of the opening to the other, a level gate will sit close to the ground on the low side and clear it by four extra inches on the high side. That gap may let a dog through, trap leaves or look odd. A gate whose bottom follows the slope hides the gap but may drag when it swings, because the ground rises under the arc.
The same slope also affects the swing. A gate hinged on the low side swinging toward the high side will scrape the ground as it opens unless the hinges are set to lift it or the ground is cut back. A gate swinging toward the lower side is easier. Knowing the slope in advance lets the fabricator choose the hinge style and the direction of swing.
Tools
You need a tape measure, a ball of string or mason's line, a four foot level, two stakes or clamps if the ends are not already fixed, and a notebook. A line level, a small bubble that hangs on a string, is a good addition and costs little. A helper makes the work faster, but one person can do it with a clamp.
Measuring across the opening
Tie the string to one post or stake at a height of about a foot above the highest point of ground along the line of the gate. Pull it tight across to the other side and tie it. Hang the line level on the middle of the string and raise or lower one end until the bubble centers. If you do not have a line level, hold the four foot level against the string at several points and adjust.
When the string is level, measure straight down from the string to the ground at the left end, at the middle and at the right end. Write down the three numbers. Subtract the smallest from the largest to get the total fall across the opening. If the middle reading does not sit between the two ends, the ground dips or humps, and you should note that. A crown in the middle behaves differently from a steady slope.
Take more than one line
A gate swings through an arc, so the ground along that arc matters as well. Repeat the measurement along the line the free edge of the gate will travel. Place a stake where the latch end will sit and measure how the ground changes as you walk toward the hinge post. If the gate swings inward, repeat on the inside. If it swings outward, repeat on the outside.
It also helps to take a reading at the center of the opening along the direction of travel. Standing in the opening and looking toward the house, does the drive rise or fall? A rise toward the house can force an inward swinging gate to climb, so it may need a limit on how far it opens.
Reading the numbers
As a rough guide, a difference of an inch or two across a walk gate is easy to handle and is often hidden by a small gap or a slightly adjusted bottom rail. A difference of three to four inches across a driveway opening begins to affect the design. More than that usually means the fabricator will suggest a stepped bottom, a racked frame that follows the slope, or some regrading of the ground. These are general ideas, and the shop will judge each case.
Notice also whether the slope is toward or away from the gate. Water runs downhill, and if the low side is also where the posts sit, water can collect in the footing. Mention it to the fabricator, since drainage affects the footing design.
The options for dealing with slope
There are four common approaches. The first is a level gate with a variable gap, where the top and bottom rails stay straight and the gap under the gate changes with the ground. It is simple and strong but leaves an uneven gap. The second is a racked gate, in which the frame is built as a parallelogram so the bottom follows the slope while the pickets stay vertical. It looks tidy but takes more careful building.
The third is a stepped gate, whose bottom rail drops in steps to follow the ground. This suits steeper ground but makes a distinct pattern. The fourth is to change the ground, by regrading, adding a concrete threshold or a small retaining edge, so a standard level gate fits. This is often done with driveways, where concrete is already involved. Each approach has trade-offs in cost, look and function, and the fabricator can explain which suits your opening.
What else to record
While you are out with the string, record anything else that affects grade. Is there a curb, a drain, a step or a change in surface? Does a driveway apron meet the road at an angle? Does a sidewalk cross the opening? Note where each begins and ends. These features can interfere with the swing, and they can also affect where concrete footings sit.
Take photos at each measuring point with the tape visible, so the shop can match numbers to pictures. A quick sketch showing the string line, the three readings and the direction north helps too.
Measurement mistakes to avoid
The most common error is a string that sags. A loose string produces false readings, so pull it as tight as you can, and check that the level reading is steady. Another error is measuring from the wrong reference, such as a post top that is not level with the string. Always measure straight down from the string. A third is forgetting to account for a step or a curb. If the ground drops sharply at one end, write it down separately instead of folding it into the same number.
If your numbers look odd, repeat the measurement. A second try takes five minutes and often catches a slip. The aim is not to give the fabricator perfect data but to give rough, honest numbers that make the first conversation useful.
Passing the numbers on
When you contact a fabricator, include the three readings, the direction of swing you prefer and a note about anything unusual in the ground. The shop will then know early whether the opening is easy or demanding, and whether a site visit is needed straight away. It also lets you ask a helpful question: "The ground drops four inches left to right, what would you recommend?" The way a shop answers tells you a lot about its experience. More on preparing before a call is in the guide to measuring a gate opening before you call a fabricator.