How trails are built

1. Reading the ground

A contour line joins up every point at the same height. Walk along one and you stay level. Cross them and you are going up or down.

The only thing you need to read off a contour map is spacing. Lines squeezed together mean steep ground. Lines spread apart mean gentle ground. That is it. Everything else in this guide is about choosing where to put a line through them.

Same drop in height between every pair of lines. Where they bunch up, that drop happens over a much shorter distance.

In the Lab: contours are 2 m apart, and every fifth one is drawn heavier so you can count 10 m at a glance.

2. The fall line

The fall line is the path a dropped ball would take: straight down the hill, at right angles to the contours. It is the most obvious way to get to the bottom, and it is almost always the wrong one.

Water takes the fall line too. Put a trail on it and you have built the water a channel — smooth, bare, and pointing downhill. Every storm moves a bit more dirt, the channel deepens, and within a few years the trail is a rock-lined trench nobody wants to ride.

A trail that traverses — cuts across the slope at a shallow angle — sheds water sideways instead of collecting it.

Blue arrows are water. On the left it has nowhere to go but down the tread. On the right it crosses the trail and carries on down the hill.

3. The half rule

This is the one number worth memorising.

A trail should never be steeper than half the slope it crosses.

If the hillside falls at 20%, the trail across it stays under 10%. On a gentle 10% hillside, the trail stays under 5%.

The reason is the same as the fall line, just more precise. Water coming down the hill arrives at the trail and has a choice: carry on straight down, or turn and follow the tread. It follows whichever is steeper. Keep the trail under half the sideslope and the hill always wins, so the water crosses and leaves.

Break the half rule and the trail becomes the easiest way down for the water as well as for you.

The other limit: even on very steep ground, keep the trail itself under about 10% on average and 15% at its steepest. Half of a 40% hillside is 20%, but a 20% trail is miserable to climb and skids out on the way down.

4. Grade reversals

A grade reversal is a short climb in the middle of a descent — the trail dips down, then rises again over a few metres.

It looks like a small thing. It is the single most important piece of drainage on a trail. Water running along the tread reaches the bottom of the dip and cannot climb the far side, so it leaves at the low point and heads off down the hill. Every reversal is a full stop for the water.

Aim for one every 20 to 50 metres on anything that descends. On a bike they barely register — if anything they add a pump you can push against for speed.

Side view. The trail loses height overall, but not in one uninterrupted run — and that is what keeps it there.

In the Lab: reversals show as blue ticks across the trail and blue dots on the profile. If you go more than 60 m of descent without one, the scorecard says so.

5. Outslope

Outslope is the sideways tilt of the tread. A finished trail should fall away from the hill by about 5% — roughly a 5 cm drop across a metre-wide tread.

It is the difference between water sheeting straight off the edge and water sitting in a puddle, softening the dirt until tyres churn it into a bog. New trails are built with outslope, and it is the first thing that disappears as tyres and boots push dirt to the outside edge and build a berm of it.

Looking along the trail from behind. The hill is on the left of each pair, the drop-off on the right.

6. Corners and berms

Corners are where riders brake, and braking is what digs holes. How you build a corner decides whether it holds up.

Flat corner. Tread stays level across. Slow, and it wants a grippy surface. Fine on gentle ground.
Bermed corner. The outside edge is banked up so the wall pushes you round. You keep your speed, you brake less, and the corner lasts far longer.
Switchback. A tight reversal of direction used to gain height on steep ground. Build it on the flattest bit you can find and make the turn itself as close to level as possible.

Radius is what makes a corner easy or hard. Anything under about 3 m and most riders have to stop and shuffle around. Wide, open corners ride fast; tight ones scrub speed, which is sometimes exactly what you want before something steep.

A berm is just dirt piled and packed on the outside of the turn, high enough that the bike leans into it rather than sliding off.

7. Features

Features are the fun. They are also what decides who can ride your trail, so they need thinking about rather than just building.

Two rules cover most of it. Jumps need speed, so they go on a downhill run-in with a straight approach — you should never be pedalling at the lip. And anything a rider cannot see the landing of should have a way around it, because committing blind to something you have not looked at is how people get hurt.

Build progression in. A trail with a small tabletop, then a bigger one, then a gap teaches riders to jump. A trail that opens with a gap just filters them out.

Roller — a smooth bump. Push down the back and you gain speed for nothing.
Table top — a jump with a flat top. Come up short and you land on the table, not in a hole. The safe way to learn.
Step-up — a jump onto a landing higher than the lip.
Drop — ride off an edge. The landing wants to slope away, never be flat.
Gap jump — nothing between the lip and the landing. Always build a ride-around.
Rock garden and rock roll — chunky rock to pick through, or a smooth slab to roll down.
Skinny — a narrow plank or log. All balance, so keep it flat and straight.

In the Lab: drop a feature onto the line and it checks the spot. Put a gap jump on flat ground and it will tell you there is not enough speed there to clear it.

8. Difficulty ratings

Trails are graded by the hardest thing on them that you cannot ride around. One committing feature makes the whole trail that grade, no matter how mellow the rest of it is.

The symbols came from ski fields and mean the same thing here. They are relative to the area, not absolute — a black in one place can be a blue somewhere gnarlier.

Trailbaker works the grade out from three things: the steepest sustained grade, how tight the corners get, and the hardest feature you have placed. Whichever is worst sets the rating.

9. Dirt and tools

The dirt you have decides what you can build.

Loam — dark, soft forest soil. Wonderful to ride and hopeless as a base; it holds water and never really packs down. Dig through it to what is underneath.
Clay — packs hard and holds a shape, which makes it the good stuff for berms and lips. Turns to grease when wet.
Sand and gravel — drains beautifully, holds nothing together. Needs finer material mixed through it to bind.

Whatever you build with, the rule is the same: compact it in layers. Loose dirt piled up and left is gone by the end of the first month. Water it, pack it, then pack it again.

The tools that matter are a mattock or hoe for cutting the bench, a McLeod for shaping and levelling, a rake for clearing, and a tamper for packing. A pair of loppers saves more time than any of them.

10. Permission

Designing trails is free. Building them is not, and it is not yours to decide.

Trails cut without permission get bulldozed, and they cost riders access to whole areas — land managers who find unsanctioned digging tend to close the lot rather than work out which bits were fine. Every rider who comes after you pays for it.

If you want to build something real, the route is straightforward: find your local mountain bike club and turn up to a trail day. They already have the land agreements, the tools and the insurance, and people who have been reading dirt for twenty years and will happily show you how. Bring your design. Good ideas get built.

11. Glossary

Bench cut: A flat tread cut into a hillside, with the dug material used to widen the outside edge.
Berm: A banked corner. The bank does the turning so you keep your speed.
B-line: An easier alternative route around a hard feature.
Fall line: The steepest way down a slope, at right angles to the contours. Where water goes.
Grade: Steepness along the trail, as a percentage. 10% means dropping 1 m every 10 m travelled.
Grade reversal: A short climb inside a descent, giving water a low point to leave at.
Half rule: Trail grade should stay under half the sideslope it crosses.
Outslope: The sideways tilt of the tread, about 5% away from the hill, so water sheets off.
Sideslope: How steep the hillside is across the trail, ignoring the trail itself.
Switchback: A tight turn reversing direction to gain or lose height on steep ground.
Tread: The surface you actually ride on.