Can I add a rail later if the boards start to bow?

You can, and it is a common repair, but it means unfixing enough of the pickets to get the rail in behind them. Doing it at build time costs a length of timber; doing it later costs an afternoon.

Should the rails go on the neighbour side or mine?

Traditionally the smooth side faces out and the rails face in, and some local rules require it. Where the fence is shared, a board-on-board or good-neighbour design puts pickets on both faces so neither side gets the rails, at the cost of more boards.

Do the rails have to be one piece per section?

It is much better if they are. Butting two short rails mid-span puts a joint where there is no support, and joints are where fences fail. Ordering rail stock in the same length as the post spacing avoids the problem entirely.

Fence Ledger

How many rails does a 6 ft privacy fence need?

Rails are the horizontal 2x4s the pickets are nailed to, and how many each section needs is decided by the height of the fence rather than by taste.

Rails per section

Derived from the fence height, not chosen. A taller fence needs another rail to stop the boards bowing in the middle.

What the usual answer misses

The rail count is not a style choice, it is what stops the boards bowing. A picket fixed only at top and bottom over six feet has an unsupported middle, and it will cup outward within a couple of seasons of sun and rain. That is why the count steps up with height rather than staying at two — and it is the first thing quietly dropped from a cheap quote, because it is invisible from the street on the day the fence is finished.

Work out your own

Every assumption above is editable. Change the length, the spacing, the hole size — the numbers and the working update as you go.

Units

Shape

Fence length

Height

Gates

Advanced options

These are typical values, not requirements. Post depth in particular is governed by your local frost line and building code — check before you dig.

Post spacing

Gate width

Picket gap

Hole diameter

Hole depth

Gravel base

Bag yield

Waste allowance

Posts

Sections

Pickets

Concrete

5 bags

Estimated cost

Materials only (DIY)

$264 – $760

Professionally installed

$400 – $1,040

National planning ranges compiled from published US construction cost guides, last checked September 2026 (confidence: medium-low). Prices vary sharply by region, season and contractor. These are estimates for planning, not quotes.

Sources: homeguide.com, calcsummit.com, angi.com, ergeon.com, homewyse.com

Shopping list

Bill of materials for this fence
Material	Size	Quantity
Posts	4x4	3
Rails incl. waste	2x4	4
Pickets incl. waste	1x6	10
Concrete mix		5bags
Gravel		2bags
Exterior screws		≈80
Walk gate kit		1

Why these numbers

How we got the section count

Run 1: 4 ft ÷ 8 ft = 0.5, rounded up to 1 section

How we got the post count

3 posts are fixed by the shape itself
2 at the open ends
1 carrying a gate
0 line posts spaced between them
3 posts in total

How we got the picket count

Each picket covers 5.5 in plus a 0.25 in gap, so they repeat every 5.75 in
4 ft of fence needs 9 pickets
Plus 10% for offcuts, rounded up to 10

How we got the concrete

Each 10 in hole is 30 in deep, less 4 in of gravel base, leaving 26 in of concrete
The 4x4 post itself takes up part of that, so each hole needs the remainder
Across 3 posts
Divided by the 0.6 cu ft a bag yields: 4.99, so buy 5 bags

Common questions

Can I add a rail later if the boards start to bow?: You can, and it is a common repair, but it means unfixing enough of the pickets to get the rail in behind them. Doing it at build time costs a length of timber; doing it later costs an afternoon.
Should the rails go on the neighbour side or mine?: Traditionally the smooth side faces out and the rails face in, and some local rules require it. Where the fence is shared, a board-on-board or good-neighbour design puts pickets on both faces so neither side gets the rails, at the cost of more boards.
Do the rails have to be one piece per section?: It is much better if they are. Butting two short rails mid-span puts a joint where there is no support, and joints are where fences fail. Ordering rail stock in the same length as the post spacing avoids the problem entirely.