Post and Rail Fencing: How Many Rails Do You Actually Need?

Start with the job the fence has to do

Rail count is really a question about gaps. Every extra rail closes the space between the top rail and the ground, so the more you need to keep in or out, the more rails you want. A fence marking a field edge along a lane near Kings Worthy has a very different job from one holding a pony in a paddock at Twyford.

As a rough rule, we use two rails for purely visual boundaries, three for most stock and family gardens, and four where you have small animals, escapologist dogs or foals. A wire mesh fixed to the inside of the posts can also do the work of extra rails at lower cost, which is worth knowing before you commit to four.

• Two rails: decorative boundaries, driveways, front gardens, splitting a large garden into zones
• Three rails: horses, cattle, larger dogs, most rural garden boundaries
• Four rails: sheep, goats, foals, small dogs, or anywhere you want a solid visual barrier without going to close board

Height sets the rail count more than anything else

Post and rail is normally built at three standard finished heights: roughly 0.9 m (three feet), 1.2 m (four feet) and 1.35 m (four and a half feet). Once the height is fixed, the rail count follows from how big a gap you can tolerate.

A 0.9 m fence with two rails leaves gaps of around 400 mm, which is fine for a boundary but useless for a terrier. At 1.2 m, three rails give gaps of roughly 300 mm, which is the standard for horses and cattle. At 1.35 m with four rails the gaps drop to about 250 mm. Going taller with only two rails looks odd and leaves a gap you could climb through, so height and rail count really have to be decided together.

• 0.9 m high: two rails, sometimes three
• 1.2 m high: three rails is the norm
• 1.35 m high: three rails for horses, four for smaller stock

Working out quantities for your run

Once you know the rail count, the arithmetic is straightforward. Rails are usually supplied in 3.6 m lengths and posts go in at 1.8 m centres, so each rail spans two bays and joins on a post. For a 50 m run with three rails you would need about 28 posts (allowing for the end post) and 42 rails, plus a few spares for cuts and any rails that arrive twisted.

Sloping ground, corners and gateways all add posts and reduce how neatly the rails divide, so on a garden in Winchester with a change of level we tend to add 10 to 15 per cent to the rail count. Half round or square posts, and whether the rails are morticed into the post or nailed to the face, also change the length you can buy. Morticed fencing looks neater and lasts longer at the joints but takes more labour to fit.

On cost, the rails are the cheapest part of the fence. Going from three rails to four on a typical run adds a fairly modest amount to materials and a bit of extra fixing time, whereas adding height means longer posts and deeper holes. If you are undecided, it is usually cheaper to add a rail than to add a foot of height later.

Timber, treatment and how long it will last

Around Winchester the common choice is pressure treated softwood, which will typically give 15 to 20 years on the rails and rather less on the posts if they are not set well. Chalk soil drains freely which helps, but the low lying clay along the Itchen holds water and shortens post life. Sweet chestnut cleft rails cost more but need no treatment and can last 25 years or more.

Whatever you choose, the number of rails does not change the posts, and the posts are what fail first. Set them at least 600 mm into the ground, and use concrete or well rammed stone in soft ground. A four rail fence on posts that rock after five winters is money wasted.