How to Set a Straight Fence Line — Getting It Right the First Time

A straight fence line is the foundation of the whole job. If your line is off by even 10 or 20 millimetres at the start, that error compounds along the run — every post shifts a bit further out, every panel sits slightly wrong, and by the time you reach the other end you can see it. You'll see it every time you look at the fence, and so will your neighbour.

The good news is that setting a straight line isn't difficult. It just takes a bit of care at the start. This guide covers the full process — from marking your corners through to dealing with slopes — so you can get it right the first time and not have to pull anything out and redo it.

What You Need

You don't need much gear for this part of the job, but getting the right string line matters more than you'd think.

• Bricklayer's line (nylon) — this is a braided nylon string that holds tension well and doesn't stretch or sag. You can get it from Bunnings for a few dollars. Don't use cotton string or twine — cotton stretches when it gets damp and sags in the middle, which defeats the whole purpose.
• Star pickets or timber stakes — you need something solid to tie the line to at each end. Star pickets are ideal because they're rigid, they won't move once they're driven in, and most people have a couple lying around. Timber stakes work fine too.
• A hammer or star picket driver — to drive your stakes in.
• Tape measure — a long one. If your fence run is over 8 metres, a 5-metre tape means you're measuring in sections, which introduces error. A 30-metre tape is worth borrowing or buying if you don't have one.
• Spirit level — a 1200mm spirit level is ideal. You'll use this with a straight edge to check the string line is level along the run.
• Spray paint — marking paint for the ground. This is optional but makes life much easier when you're marking post positions. You can pick up a can of survey marking paint at Bunnings.

Start With the Corners and Ends

Every fence run starts and finishes somewhere — a corner of the property, a wall, a gate post, or a boundary peg. These are your fixed points, and everything else lines up between them.

Before you do anything with a string line, you need to know exactly where your corner posts and end posts will go. Walk the fence line and mark those positions first. If you're building along a boundary, the corner positions are usually set by the surveyor's pegs (if they're still in the ground) or by existing boundary markers.

If you're building a new fence alongside an existing one — say, replacing an old timber fence with PVC — don't assume the old fence is on the boundary. It might be 50 or 100 millimetres off, or it could be well inside or outside the property line. If there's any doubt about where the boundary is, get a surveyor to place pegs. It costs a few hundred dollars, but it's significantly cheaper than having to move a fence after it's built.

Once you've identified your corner and end positions, drive a star picket or timber stake into the ground at each one. Push them in far enough that they're solid — they need to hold a tight string line without moving.

Setting the String Line

This is the part that determines whether your fence ends up straight or not. Take your time here.

Step 1 — Tie the line at the first stake

Tie your bricklayer's line firmly to the first stake. Use a clove hitch or just wrap it around a few times and tie it off — the knot doesn't matter much as long as it's secure and won't slip.

Step 2 — Run it to the other end and pull it tight

Walk the line out to the stake at the other end of the run and pull it tight before you tie it off. The key word here is tight. Any sag in the middle of the line means the posts in the middle of the run will be offline. Over a 10 or 15-metre run, even a small sag can push your middle posts out by 20 or 30 millimetres — which is enough to notice.

Nylon bricklayer's line is designed for this. It holds tension well and doesn't stretch the way cotton does. Pull it firm, tie it off, and then check it from the side. If you can see it drooping in the middle, it's not tight enough.

Step 3 — Position the line on the face of the fence

This is the detail that a lot of first-timers get wrong. The string line should sit where the face of the fence will be — that's the finished front surface of the panels — not the centre of the post.

Why does that matter? Because the face of the fence is what you see. If your string line runs down the centre of the posts and your posts are 127 millimetres wide, the face of the fence is sitting 63 millimetres off the line. That's fine if you're consistent, but most people aren't, and the result is a fence face that wanders in and out.

Run the line on the side of the posts that faces the direction you care about — usually the front of the fence or the side that faces the street or your neighbour.

Step 4 — Set the height

Run the string line at the height where the tops of your posts will be. For a standard 1.8-metre PVC fence with posts set 600 millimetres into the ground, the top of the post is 1.8 metres above finished ground level. Running the line at this height lets you check that all your posts will be the same height once they're installed.

On flat ground, this also doubles as a level reference. If the line is level and all the posts are cut or set to touch the line, the top of the fence will be even.

Step 5 — Check for level

Place a straight edge (a length of straight timber or a long spirit level) underneath the string line at a few points along the run and check it with a spirit level. If the ground is flat, the line should be level from end to end. If it's not, adjust the height at one end until it is.

On sloping ground, the process is different — we'll cover that in the fencing on a slope section below.

Marking Post Positions Along the Line

Once your string line is up and tight, you can mark where each post will go.

For Probuild PVC fencing, the standard panel width is 2.4 metres (2400mm). That means your posts will be spaced at 2.4-metre centres along the run, plus the width of the post itself. Your specific spacing will depend on the product — check the installation details for the panel you're using, because it varies slightly between styles.

How to measure accurately — use running dimensions

This is important, and it's where a lot of DIY fences go wrong. When you're marking post positions along a run, always measure from the same starting point — not from the last post you marked.

Here's what that means in practice. Say your post spacing is 2.4 metres:

• Post 1 is at 0 metres (your start point)
• Post 2 is at 2.4 metres from post 1
• Post 3 is at 4.8 metres from post 1
• Post 4 is at 7.2 metres from post 1

You always measure from post 1, using the full distance along the tape. This is called "running dimensions" and it's how builders and surveyors set things out. If you measure 2.4 metres from post 1, then move the tape and measure 2.4 metres from post 2, then 2.4 metres from post 3, every tiny measurement error stacks up. Over a 20-metre run, you could end up 30 or 40 millimetres off at the far end.

Hook your tape at the first post position, run it along the string line, and mark each post position from the one tape pull. Use spray paint on the ground directly below the string line, or push a small stake in at each point.

What if the run doesn't divide evenly?

It usually won't. If your total fence run is 11 metres, that's four panels at 2.4 metres (9.6 metres) with 1.4 metres left over. You've got two options:

• One shorter panel at the end. Cut the last panel down to fit the remaining gap. For PVC fencing, this is straightforward — PVC cuts cleanly and you can trim a panel to the size you need.
• Adjust the spacing slightly. Instead of four bays at 2.4 metres plus a short one, you could do five bays at 2.2 metres. This gives you even spacing across the whole run, which looks tidier. It depends on whether your panel system allows for that flexibility — for Probuild panels, give us a call and we'll talk you through what works for your run length.

Dealing With Corners and Angles

If your fence changes direction — around a corner of the property, for example — you'll need to set up a separate string line for each straight section. Each run of fence gets its own line, tied off at the shared corner post.

Checking for square — the 3-4-5 method

If two fence runs meet at what should be a 90-degree angle, you can check whether the corner is actually square using the 3-4-5 method. It's based on the Pythagorean theorem, but you don't need to remember the maths — just the numbers.

Here's how it works:

1. From the corner post, measure 3 metres along one string line and make a mark.
2. From the same corner post, measure 4 metres along the other string line and make a mark.
3. Now measure the diagonal distance between those two marks. If the corner is square (exactly 90 degrees), that diagonal will be exactly 5 metres.

If the diagonal is more or less than 5 metres, the corner isn't square. Adjust the position of one of your string lines until the diagonal measures exactly 5 metres, and you'll have a true right angle.

You can scale the numbers up for more accuracy — 6-8-10 works the same way, as does 9-12-15. Bigger numbers mean more precision because a millimetre of error matters less over a longer distance.

Angles other than 90 degrees

Not every corner is a right angle. If your boundary runs at an odd angle, the easiest approach is to set your string lines to follow the boundary and let the fence follow the property shape. Drive a stake at the corner where the direction changes, tie off both string lines at that point, and check that each straight section is properly aligned on its own.

If you're working from surveyor's pegs, the boundary line will already define the angle for you — just follow it.

Fencing on a Slope — Stepped vs Raked

Flat ground is easy. But a lot of Perth blocks have some slope to them — sometimes gradual, sometimes steep — and the way you handle the slope affects how the fence looks and how you set your string line.

There are two main approaches: stepped and raked.

Stepped fencing

With a stepped fence, each panel sits level (horizontal), and where the ground drops between one post and the next, the panel "steps" down. The result is a staircase effect — each panel is at a slightly lower level than the one before it as the fence follows the slope downhill.

Stepped fencing is the more common approach for PVC panel fencing because the panels stay square and don't need to be cut at an angle. It works well on gentle to moderate slopes. On steep slopes, the steps can get quite large, which can leave noticeable gaps under the higher side of each panel — something to think about if keeping animals in or out matters.

Setting the string line for a stepped fence: Instead of running one continuous line along the full slope, set a level string line for each individual bay. The line should be level across each panel span, stepping down at each post to match the slope. You'll end up with posts that are all different heights above ground level — shorter at the top of the slope, taller at the bottom.

Raked fencing

With a raked fence, the panels follow the angle of the slope. The top of the fence runs parallel to the ground, giving a smooth, flowing appearance rather than a staircase. It looks cleaner on moderate slopes and eliminates the gaps that stepping can create at the bottom of each panel.

Raked panels need to be cut at an angle to follow the slope, which means more work during installation. Not all panel styles rake easily — it depends on the design. Some PVC panel styles are well suited to raking and others aren't. If you're thinking about raking your panels, give us a call and we can let you know what works for your chosen style.

Setting the string line for a raked fence: Run the string line along the top of where the posts will be, following the natural slope of the ground. The line will angle downhill, and all your posts should be the same height above ground level (assuming the slope is consistent). Mark the ground level at each post position so you know how deep to dig each hole — the deeper you go into the slope, the taller the post needs to be above ground on the downhill side.

How to decide

There's no universal rule. On a gentle slope, either approach works and it comes down to what you prefer the look of. On a steeper slope, stepping is usually the easier and more practical option because the panels don't need to be modified. On very gentle slopes, raking looks seamless and most people wouldn't even notice the fence is on a slope.

If you're not sure which approach suits your block, take a photo of the slope and give us a call. We've seen most situations and can suggest what tends to work best.

Common Mistakes

These are the ones that come up regularly, and they're all avoidable if you know to watch for them.

• String line not tight enough. A saggy string line gives you a curved fence. If the line dips in the middle, your middle posts will be offline. Pull it tight, check it from the side, and retighten if you need to. On longer runs, you may need to add an intermediate stake to keep the line from sagging under its own weight.
• Measuring from the wrong reference point. Measure from the property boundary — not from your neighbour's existing fence. Their fence might be 50 or 100 millimetres off the boundary, and if you line up to their fence, yours will be off by the same amount (or more, once your own measurement errors stack up).
• Measuring post to post instead of from a single start point. This is the "incremental measurement" trap described above. Always use running dimensions from your first post to avoid cumulative creep.
• Not checking for square on corners. If you eyeball a 90-degree corner instead of checking it with the 3-4-5 method, it will almost certainly be off. Even two or three degrees out of square is visible on a finished fence.
• Forgetting to account for post width. If your string line is on the face of the fence and you mark the post positions at the line, remember that the post extends back behind the line by its full width. Make sure the post hole is centred on where the post actually needs to sit, not centred on the string line.
• Not allowing for gates. If there's a gate in the run, the gate opening needs to be accounted for when you're spacing posts. Mark the gate post positions first, then divide the remaining run into panel bays. Don't discover you need a gate opening after you've already set your posts at regular intervals.

Tips From the Field

• Wind is your enemy. If you're in Perth, you already know about the Fremantle Doctor and the afternoon sea breeze. A lightweight string line will blow around in the wind and give you an unreliable reference. Use a heavier nylon bricklayer's line — it cuts through the wind better than thin string. Some people use fishing braid for the same reason, though it's harder to see against the ground.
• Double-check your boundary before you start. If you're building on the boundary line, both you and your neighbour should agree on where it is. Walk the line together, find the surveyor's pegs if they're still there, and confirm you're both happy before a single stake goes in. Under the Dividing Fences Act in WA, boundary fences involve both property owners, so it's worth getting this sorted early.
• If the boundary is unclear, get a surveyor. A boundary peg-out survey in Perth typically costs a few hundred dollars. That sounds like a lot for what amounts to a couple of pegs in the ground, but it's a fraction of the cost of pulling out a fence and rebuilding it on the correct line. It also protects you if a neighbour dispute comes up later — you've got a surveyor's report to back up your position.
• Use bright-coloured string. Fluorescent pink or orange bricklayer's line is easier to see than white, especially against sandy Perth soil or in afternoon light. It's the same product, just easier to work with.
• Check with Dial Before You Dig before you put anything in the ground. Even driving a star picket into the ground to hold your string line can hit a shallow gas or comms line. We've covered this in detail in our Dial Before You Dig guide — it's free, it's quick, and it's a legal requirement in WA before you excavate.
• Take photos of your string lines before you start digging. Once the holes are dug and the posts are in, you can't easily check whether the original layout was right. A few photos from each end of the run give you a reference to come back to if something doesn't look right during the build.

This guide is general advice for setting out a fence line on residential properties. Site conditions vary — ground type, slope, access, and boundary situations are all different. If you're unsure about any part of the process, give us a call and we'll help you work through it.