Easy Tips for Measuring Fence Height Using a Plumbline

Getting a fence height right the first time saves you from neighbor complaints, permit fines, and lopsided aesthetics. A plumbline—nothing more than a weighted string—turns this tricky task into a one-person job, even on sloped ground.

Below you’ll learn how to pick the right cord, weight, and anchors, then turn that simple tool into a precision instrument that beats laser levels for cost and reliability.

Why a Plumbline Outperforms Other DIY Methods

Laser beams refract in bright sun and cheap models drift 3 mm every 10 m; a plumbline never needs calibration. A spirit level forces you to balance on a ladder while squinting at a bubble; the string stays put while you step back and sight.

Water-filled vinyl tubes are accurate only when both ends are visible, but a plumbline drops straight through shrubs or over masonry without pause. Even a $300 rotary laser can’t tell you the exact drop from post top to grade unless you also own a grade rod; the string already is the rod.

Professional masons still carry plumb bobs in their pouches because gravity doesn’t run out of batteries.

Choosing the Right String and Weight

Braided nylon mason’s line is cheap, but it stretches 1 cm under a 200 g load on a 3 m drop; switch to 0.9 mm Kevlar kite cord for zero stretch in summer heat. The bob itself should weigh 225–340 g; lighter bobs wander in a 5 mph breeze, while heavier ones can snap thin twine.

Brass bobs are worth the extra dollar because their slim profile slips past chain-link mesh without snagging. A 4 cm stainless clip on the line lets you detach the bob quickly when you move to the next bay, keeping the cord tangle-free in your pocket.

Marking a Reference Datum Before You Drop the Line

Drive a 50 mm galvanized nail flush into the post at the permitted height—say 1.8 m—then hook the string loop over it. This nail becomes your immutable zero; even if you later trim the post, the string still references the original permit line.

On panel fences, mark the datum on the top rail instead of every picket; you’ll transfer the mark downward in one smooth motion later.

Single-Person Technique for High Posts

Clamp a spring-loaded clothes peg to the line 300 mm above the bob, then wedge the peg jaw onto the nail head; the tension holds the string vertical while you step back to sight. When you need to slide the line to the next post, pinch the peg and walk; no second set of hands required.

This trick also keeps the bob from pendulum-swinging in light wind because the clamp adds lateral damping.

Reading the Plumbline on Sloping Ground

Drop the bob until it barely kisses the soil, then pinch the string against the post at that exact spot; the distance between your datum nail and pinch point equals the height shortfall. Transfer this measurement uphill by holding a short spirit level flat against the string, then marking the level’s endpoint on the next post.

If the slope exceeds 15°, switch to a 2 m ruler held parallel to the string; the ruler’s graduations cancel trigonometry errors that creep in when you eyeball an angle.

Accounting for Wind and Temperature Drift

A 10°C swing stretches nylon by 0.3 %; on a 2 m drop that is 6 mm—enough to trigger a by-law breach. Kevlar and Spectra cords have thermal coefficients below 0.05 %, so spend the extra 80 cents if you’re working in desert sun.

When gusts hit 15 mph, damp the bob by lowering it into a coffee can half-filled with sand; the rim stops sideways swing without dulling the point. Clip a second clothespin 100 mm above the bob to act as a wind vane; the added drag steadies the line within seconds.

Converting Plumbline Readings to Permit Paperwork

Photograph each post with the string in place; include a tape measure aligned to the datum nail and a close-up of the bob touching grade. Upload these images to the city portal so inspectors can pre-approve heights before you pour concrete.

Record heights in millimetres on a simple sketch; rounding to centimetres invites 5 mm rounding errors that accumulate across 20 bays. Save the sketch as a PDF named with the date; inspectors love time-stamped proof when neighbors dispute the final sightline.

Using the Plumbline to Spot Bowed or Twisted Rails

Stretch a second string horizontally between posts at the datum height, then drop perpendicular plumbines every 600 mm along the rail; any gap between rail and string reveals a sag or crown. A 3 mm hollow means the rail will read low when you later attach pickets, so shim now with a cedar wedge.

This dual-string grid also exposes twist: if the horizontal string touches the rail on the north edge but gaps on the south, the rail has rotated; loosen brackets and re-square before pickets hide the defect.

Quick Fixes When the Fence Is Already Too High

Mark the excess height with the plumbline, then slip a Japanese pull saw between string and post; saw until the blade just kisses the cord for a perfectly flat top. On vinyl, heat a sharp utility knife with a heat gun and slice along the string line; the warmed vinyl cuts like butter and leaves a melt-sealed edge.

For chain-link, count the weave diamonds above the string, then unbolt the top rail and slide out the excess diamonds; reattach tension wire to keep fabric taut.

Maintenance Checks That Take 30 Seconds per Post

Once a year, hook the plumbline to the original nail and verify the bob still hovers at your grade mark; soil heave can raise a post 10 mm without visible cracking. If the gap has changed, loosen the concrete collar with a 12 mm masonry bit, then tap the post down with a block and mallet until the string kisses the mark again.

Log the measurement on the same sketch you used for permits; consistent records prove stability if a future buyer questions fence compliance.

Advanced Tricks for Curved or Stepped Panels

On concave scalloped tops, drop the plumbline from the lowest point of each arc, then record the vertical distance to a taut mason’s line strung between end posts; this gives you uniform scallop depth despite varying ground height. For stepped wood fences, transfer the datum downhill by stretching a diagonal string from the top of the upper post to the base of the lower; where the diagonal intersects each intermediate post, mark a fresh datum that compensates for the grade break.

Use a different colored Sharpie for each step so you never confuse intermediate marks during final trimming.

Turning the Plumbline Into a Budget Transit

Clamp a cheap smartphone angle meter to the string with a rubber band; zero the app while the bob hangs free, then tilt the phone until the screen reads 90.0° to confirm perfect vertical before you mark. The combo costs under $5 yet gives digital reassurance that rivals a $200 contractor level.

Film the phone screen in slow-mo while you nudge the post; stop when the digit holds steady for three seconds—you’ve eliminated parallax error entirely.

Common Mistakes That Void Your Measurements

Wrapping the string around a round post distorts the line by 5 mm; always anchor to a nail driven to full depth so the cord exits at the exact post face. Knotting the line at the bob eyelet creates a bump that can catch on foliage; instead, use a swaged sleeve or tiny figure-eight follow-through.

Never trust a bob that isn’t symmetrical; a dropped dent in lead shifts the center of gravity 2 mm, enough to show a false lean on critical boundary fences.