A road crown raises the centre of the surface above its edges by 2-4% of the half-width, so water runs sideways to the ditches instead of lying on the road: on a 4 m road the centre is 4-8 cm higher than the edges. On a hillside road the whole surface slopes toward the downhill ditch and there is no crown. Re-establish the crown every time you grade or add material, because a road that loses its crown deteriorates fast.
Part of Road Construction Basics
The cross-section shape of a road surface that sheds rain toward its side ditches.
Water Leaves The Road Only If The Surface Has A Cross Slope
Rain that stays on a road softens the base, so every road surface needs a slope across it. The parent article calls it crown or camber: the surface is higher in the centre than at the edges, so water flows to the sides and does not pool. Its target is 2-4% from the centre to the edge. On hillside roads the parent says to slope the entire surface toward the downhill ditch and not to crown, because there you want all the water going one way.
Percent is rise over run: a 3% slope falls 3 cm over every 100 cm of width, or 3 cm per metre.
What You Need
- A crown template — a board as long as the road is wide, with the profile cut in it, or a straight board and a small shim.
- A level — a water level or bubble level. See Levels and Plumb Bobs.
- A tape measure and string — for the width and the crown height.
- Rakes and shovels — to move material.
Method: Shape And Check The Crown
Step 1: Compute the height Crown height = slope × half the road width. For 3% on a road 4 m wide: 0.03 × 2 m = 0.06 m, or 6 cm.
Step 2: Set string lines Stretch a string along the centreline at the finished height and strings along the edges lower by the crown height.
Step 3: Grade the surface Rake the surface to run from the centre string down to the edge strings in a gentle curve or two straight slopes. Fill low spots, shave high ones.
Step 4: Check with the template Hold the template across the road. A level on the board shows the cross-slope. Check at every few metres.
Step 5: Keep it Re-check after every grading and every addition of gravel.
Worked Example: Crown Height For Three Widths
Using the parent’s target of 2-4%, and the half-width times slope:
| Road width | Half-width | 2% | 3% | 4% |
|---|---|---|---|---|
| 3 m | 1.5 m | 3.0 cm | 4.5 cm | 6.0 cm |
| 4 m | 2.0 m | 4.0 cm | 6.0 cm | 8.0 cm |
| 5 m | 2.5 m | 5.0 cm | 7.5 cm | 10.0 cm |
The 4 m row is the parent’s 4-8 cm. Other figures in the corpus differ: Grading Basics gives a rise of 3-5% (60-100 mm on a 4 m road), Roads and Transport gives 2-3%, and the parent’s cobblestone method asks for 3-5 cm at the centre of a paved road, which on a 4 m road is 1.5-2.5%. Use the parent’s 2-4%. Rough and soft surfaces hold water on the road longer, so use the steeper end of the range on them and the flatter end on smooth, hard surfaces such as paving.
Common Mistakes
- No crown on a flat road — water lies on the surface and softens it.
- A crown on a hillside road — water runs both ways, one half onto the uphill side and across the road. Slope the whole surface to the downhill ditch.
- Forgetting it after grading — the parent says a road that loses its crown deteriorates fast.
- Too steep a crown — vehicles tilt toward the edges and loads shift. Keep within 4%.
Safety
Cross-slope and tipping loads
A cross-slope tilts a loaded cart toward its low side. Keep to the stated crown range, and keep tall loads low on side-sloping roads. See Cart & Wagon Loading for the tipping arithmetic.
What’s Next
- Learn next: Side Ditches — where the water goes next
- Combine with: Gravel Road — the surface you are shaping