How to plan a fence on sloping ground
On a slope, a panel fence is usually stepped: each panel sits level, and each one is set a little lower than the last. To plan it, measure the run and how far the ground falls along it, then work out the panels and posts as for a flat run. A 10 m run with 1.83 m panels needs 6 panels and 7 posts.
Measure the run and the fall
Measure the length of the fence line along the ground, in metres. Then measure how far the ground drops from one end to the other. The simplest way is a string line: tie it to a peg at the top end, pull it level with a line level or spirit level, and measure down from the string to the ground at the low end.
The gradient calculator turns the drop and the length into a slope. For a 600 mm drop over 10 m:
- 600 mm ÷ 10 m = 60 mm per metre
- that's 1 in 16.7, or 6%, or 3.43°
If the ground rises and falls along the run, measure the drop at each post position instead and note the highest and lowest points.
Stepped or sloping
There are two ways to follow a slope:
- Stepped panels. Each panel is level and set lower than the one before, so the top of the fence steps down. This is the usual way with standard panels. It leaves a triangular gap under the downhill end of each panel, which gravel boards fill.
- Sloping (raked). The fence follows the ground, so its top runs parallel to the slope. This suits fences built on site from rails and boards, such as closeboard or featheredge, rather than ready-made panels.
For a stepped fence, the step at each post is the total fall shared between the panels. With 6 panels and a 600 mm fall, that's about 100 mm a step if the slope is even. On uneven ground, set each step to suit the ground under that panel.
Panels, posts and gravel boards
Work out the panels and posts as you would for a flat run: the slope doesn't change the count. The fence calculator starts from 1.83 m panels and 100 mm posts, and you can change both to match yours. For a 10 m run:
- (10 − 0.1) ÷ (1.83 + 0.1) = 5.13, so 6 panels
- 6 panels + 1 = 7 posts
- 6 gravel boards
- the last panel needs trimming by 1.68 m, or use a shorter panel there
On a slope, the gravel boards fill the gap under each step, so check that their height covers it. If the steps are bigger than one board can cover, the ground may need levelling under each panel, or you may need a different fence type.
Post lengths
On a stepped fence, each post holds a panel at a different height above the ground. The post on the downhill side of each step stands taller out of the ground than the one uphill, so it needs to be longer. Work out each post's length from the panel height, the step, the gravel board and the depth in the ground, and order the longest length you need, or order two lengths.
The depth the posts go into the ground depends on the post-fix product, the fence height and how exposed the site is. Use the post-fix maker's instructions for the hole size and depth.
Post-fix concrete
The postcrete calculator works out the bags from the hole size and depth, the post size and the yield on the bag. With 7 posts, 300 mm round holes 600 mm deep and 100 mm posts, and a bag that yields 14 litres, it works out 36.4 litres a post, so 3 bags a post (21 in all), or 19 bags if you share part-bags between holes.
All the figures in that example are inputs to change: use the hole size, depth and bag yield from your post-fix maker's instructions.
Before you dig
- Check where the boundary runs, and who the fence belongs to, before you start.
- Check the planning rules for fence height where you live. The rules differ between England, Wales, Scotland and Northern Ireland, and a stepped fence is taller at some posts than others.
- Find out where buried pipes and cables run before you dig post holes.
- On a steep or exposed site, or where the fence also holds back soil, ask a fencing contractor or a structural engineer: a fence isn't a retaining wall.