How Far Apart Should Deck Railing Posts Be?

Plan Your Railing To Meet Post Spacing Code

If you're planning to renovate your deck railing or add a new one, we're sure you've asked the question: "How far apart can I space my deck railing posts?"

The answer depends heavily on the size and material of the posts and rails you're using. But in general, most posts (including 4x4 wood posts) can only be 6 feet apart, while some stronger systems (or 6x6 wood posts) can span up to 8 feet between posts.

We'll run through the different post spacing requirements below, plus show you how to space your posts evenly and meet building codes so you can plan your deck railing upgrade down to the last detail!

A rectangular deck with 90 degree corners and evenly spaced deck railing posts

Table of Contents

Deck Railing Post Spacing Tips

A long stretch of deck railing with posts spaced evenly across it

#1: Different Materials, Sizes & Systems Require Different Post Spacing

The first step to figuring out your deck railing post spacing is to decide on the type of railing you'll be installing.

Complete Railing Systems:

If you're working with a complete railing system, that system will have its own post spacing requirements. For example, the Trex Transcend composite railing system shown above can have posts up to 8 feet apart on-center - that means from the center of one post to the center of the next.

Some railing systems can only span 6 feet between posts. Meanwhile, an especially strong metal deck railing system, like Fortress FE26 Steel Railing, can span up to ten feet if you use it with a fortifying accent top panel.

An extra-long section of Fortress FE26 railing using an accent top panel to span ten feet between posts
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Wood Posts:

If you're using lumber to build your own railing, the following rule of them generally applies:

4x4 posts can be up to 6 feet apart, on-center.

6x6 posts can be up to 8 feet apart, on-center.

Make sure you check your local building codes, though, to see if any special restrictions apply.

#2: How To Space Your Posts Evenly

So your posts can sit 6 feet apart… but should they?

In general, it will look better to space your posts evenly across a railing run. So instead of splitting a 15-foot run into spans of 6 feet, 6 feet and 3 feet, you'd be better off planning for three 5-foot runs that will look better and still require the same amount of materials.

Here's the easiest way to space your posts evenly across a span:

A diagram showing a deck with dimensions measured and labeled

Step 1: Measure Your Space

Make a rough drawing of the outline of your deck. Measure each distance and mark it.

A diagram showing a deck with evenly-spaced deck railing posts added

Step 2: Divide The Distance & Space Evenly

Divide each distance by 6 (or by the longest span you can have between posts, if your system allows for 8- or 10-foot spans).

Round that number up. That's how many sections of railing you'll need. Add one and you'll have the number of posts you need for that span..

You can determine the exact distance between posts when you start installing. For now, just make sure that none of your spans are longer than 6 feet.

#3: Know The Difference Between On-Center and True Measurements

Typically, railings use "on-center" measurements. That technically means the distance from the middle of one post to the middle of the next post.

In planning your railing, you may sometimes see materials listed with "true" dimensions. If you buy a top rail that's a "true" 6-foot length, that means it'll actually be able to create a railing section slightly longer than 6 feet, when you include the posts on either side.

A diagram showing the difference between on-center deck railing measurement and true deck railing measurement

Read more: What's The Difference Between "On-Center" and "True" Deck Railing Dimensions?

This isn't really a major distinction, because almost any railing system you buy will need to be cut down to a shorter size to fit the exact spans between posts on your deck.

The important thing is to make sure that your rails are long enough to complete the span. If you're measuring six feet of actual distance from the near side of one post to the near side of the next post, an "on-center" 6-foot rail kit won't be long enough to reach.

If you make sure to measure "on-center," and compare that to the lengths of your rails, you'll avoid the headache of having to move posts or order longer rail sections.

#4: Leave 1-1/2 Inches Between Your House And First Post

If your deck is attached to a house (as most decks are), you'll need to build in a little space between the house and the first post of your railing system.

We typically recommend leaving at least 1-1/2 inches between your house and post. This makes sure that you can mount your post without drilling through your ledger board flashing.

Make sure the gap is smaller than 4 inches to continue to meet building codes

A diagram showing how far to space your first deck railing post from the side of your house

Sometimes, railings can mount directly to the house, but this brings up more complications like accounting for your home's siding and not compromising any flashing or waterproofing behind the siding. In general, we'd mount a post near your house and start your railing from there.

#5: Factor In Your Deck Stairs

The last tip is to factor in stairs if your deck has them - or if you plan to add them down the road. While one side of your deck might be an even 12 feet, two 6-foot sections wouldn't leave room for stairs.

A deck railing planned around a short set of stairs that also have railing

Even if you don't have stairs yet, you may want to add them in the future, so consider your post placement to leave an avenue for entering or exiting your deck.

Pro Tip: if you have an especially long set of stairs, the post spacing rules still generally apply - don't plan your stair posts to be more than 6 feet apart unless your railing system specifically allows for longer stair spans.

Deck Railing: Next Steps

If you need help planning your deck layout or determining how many posts you need, don't hesitate to give us a call at 1-888-824-5316. We've got a team of experienced project planners who will make it easy to plan for the deck you want.

If you haven't decided what kind of deck railing you want, you can browse our huge selection of deck railing systems by category, or learn more about what type of railing will best suit your home and deck:

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Click to read our article: How To Match Deck Railing To Your Home's Style

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