A 10 ft (120") deck railing between posts takes 23 wood balusters (1-1/2") at 3-9/16" gaps or 26 metal balusters (3/4") at 3-3/4", both under the IRC 4" sphere. Hook the tape on the post once: the first wood edge goes at 3-9/16", the next at 8-5/8", then every 5-1/16".
120" between posts
Hook the tape on the post face once and mark every baluster edge from there; each mark is rounded once, so a 1/16" never compounds down the rail.
| Done | # | Mark | Tape | Centre | Decimal |
|---|---|---|---|---|---|
| 1 | Leading edge | 3-9/16" | 4-5/16" | 3.563" | |
| 2 | Leading edge | 8-5/8" | 9-3/8" | 8.625" | |
| 3 | Leading edge | 13-11/16" | 14-7/16" | 13.688" | |
| 4 | Leading edge | 18-3/4" | 19-1/2" | 18.750" | |
| 5 | Leading edge | 23-13/16" | 24-9/16" | 23.813" | |
| 6 | Leading edge | 28-7/8" | 29-5/8" | 28.875" | |
| 7 | Leading edge | 33-15/16" | 34-11/16" | 33.938" | |
| 8 | Leading edge | 39" | 39-3/4" | 39.000" | |
| 9 | Leading edge | 44-1/16" | 44-13/16" | 44.063" | |
| 10 | Leading edge | 49-1/8" | 49-7/8" | 49.125" | |
| 11 | Leading edge | 54-3/16" | 54-15/16" | 54.188" | |
| 12 | Leading edge | 59-1/4" | 60" | 59.250" | |
| 13 | Leading edge | 64-5/16" | 65-1/16" | 64.313" | |
| 14 | Leading edge | 69-3/8" | 70-1/8" | 69.375" | |
| 15 | Leading edge | 74-7/16" | 75-3/16" | 74.438" | |
| 16 | Leading edge | 79-1/2" | 80-1/4" | 79.500" | |
| 17 | Leading edge | 84-9/16" | 85-5/16" | 84.563" | |
| 18 | Leading edge | 89-5/8" | 90-3/8" | 89.625" | |
| 19 | Leading edge | 94-11/16" | 95-7/16" | 94.688" | |
| 20 | Leading edge | 99-3/4" | 100-1/2" | 99.750" | |
| 21 | Leading edge | 104-13/16" | 105-9/16" | 104.813" | |
| 22 | Leading edge | 109-7/8" | 110-5/8" | 109.875" | |
| 23 | Leading edge | 114-15/16" | 115-11/16" | 114.938" |
4x4 posts at 123-1/2" on centre exceed the 6'-0" AWC DCA 6 guard-post limit: add a middle post or use a railing kit rated for this length.
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| 23 | 3-9/16" | 5-1/16" | 3-9/16" | 26 | ||
| 26 | 3-3/4"− | 4-1/2" | 3-3/4" | 29 | ||
| 25 | 3-5/8"+ | 4-5/8" | 3-5/8" | 28 | ||
| 17 | 3-3/8" | 6-7/8" | 3-3/8" | 19 |
| 1 | 3-9/16" | 4-5/16" | 3.563" |
|---|---|---|---|
| 2 | 8-5/8" | 9-3/8" | 8.625" |
| 3 | 13-11/16" | 14-7/16" | 13.688" |
| 4 | 18-3/4" | 19-1/2" | 18.750" |
| 5 | 23-13/16" | 24-9/16" | 23.813" |
| 6 | 28-7/8" | 29-5/8" | 28.875" |
| 7 | 33-15/16" | 34-11/16" | 33.938" |
| 8 | 39" | 39-3/4" | 39.000" |
| 9 | 44-1/16" | 44-13/16" | 44.063" |
| 10 | 49-1/8" | 49-7/8" | 49.125" |
| 11 | 54-3/16" | 54-15/16" | 54.188" |
| 12 | 59-1/4" | 60" | 59.250" |
| 13 | 64-5/16" | 65-1/16" | 64.313" |
| 14 | 69-3/8" | 70-1/8" | 69.375" |
| 15 | 74-7/16" | 75-3/16" | 74.438" |
| 16 | 79-1/2" | 80-1/4" | 79.500" |
| 17 | 84-9/16" | 85-5/16" | 84.563" |
| 18 | 89-5/8" | 90-3/8" | 89.625" |
| 19 | 94-11/16" | 95-7/16" | 94.688" |
| 20 | 99-3/4" | 100-1/2" | 99.750" |
| 21 | 104-13/16" | 105-9/16" | 104.813" |
| 22 | 109-7/8" | 110-5/8" | 109.875" |
| 23 | 114-15/16" | 115-11/16" | 114.938" |
Every baluster is 5-1/16" on centre after the first.
| 1 | 3-3/4"− | 4-1/8"− | 3.722" |
|---|---|---|---|
| 2 | 8-3/16" | 8-9/16" | 8.194" |
| 3 | 12-11/16"− | 13-1/16"− | 12.667" |
| 4 | 17-1/8" | 17-1/2" | 17.139" |
| 5 | 21-5/8" | 22" | 21.611" |
| 6 | 26-1/16"+ | 26-7/16"+ | 26.083" |
| 7 | 30-9/16" | 30-15/16" | 30.556" |
| 8 | 35"+ | 35-3/8"+ | 35.028" |
| 9 | 39-1/2" | 39-7/8" | 39.500" |
| 10 | 44"− | 44-3/8"− | 43.972" |
| 11 | 48-7/16" | 48-13/16" | 48.444" |
| 12 | 52-15/16"− | 53-5/16"− | 52.917" |
| 13 | 57-3/8" | 57-3/4" | 57.389" |
| 14 | 61-7/8" | 62-1/4" | 61.861" |
| 15 | 66-5/16"+ | 66-11/16"+ | 66.333" |
| 16 | 70-13/16" | 71-3/16" | 70.806" |
| 17 | 75-1/4"+ | 75-5/8"+ | 75.278" |
| 18 | 79-3/4" | 80-1/8" | 79.750" |
| 19 | 84-1/4"− | 84-5/8"− | 84.222" |
| 20 | 88-11/16" | 89-1/16" | 88.694" |
| 21 | 93-3/16"− | 93-9/16"− | 93.167" |
| 22 | 97-5/8" | 98" | 97.639" |
| 23 | 102-1/8" | 102-1/2" | 102.111" |
| 24 | 106-9/16"+ | 106-15/16"+ | 106.583" |
| 25 | 111-1/16" | 111-7/16" | 111.056" |
| 26 | 115-1/2"+ | 115-7/8"+ | 115.528" |
Every baluster is 4-1/2" on centre after the first.
05 / Field guide
This sheet lays out a 10 ft (120") deck railing between post faces. It compares 1-1/2" wood with 3/4" metal, gives every mark from one hook of the tape, and checks the 4" sphere rule and post spacing.
120" ÷ (1-1/2" + gap) fits 23 balusters with 24 equal gaps of 3-9/16" (3.563"), 5-1/16" on centre.
From the post face the leading edges fall at 3-9/16", 8-5/8", 13-11/16", 18-3/4" and on in 5-1/16" steps. Buy 26 to allow 10% for splits and miscuts.
Thinner balusters need more of them: 26 at 3-3/4"− gaps, 4-1/2" on centre. The first four marks: 3-3/4", 8-3/16", 12-11/16", 17-1/8".
1" metal or composite takes 25 at 3-5/8"; 3-1/2" flat pickets take 17 at 3-3/8".
With 4x4 posts the centres would be 10' 3-1/2" apart, past the 6'-0" DCA 6 limit for prescriptive guard posts. Add 1 intermediate post (2 bays of about 58-1/4" clear), or use a railing kit rated for this section length.
The guard top sits at least 36" above the deck and the bottom rail less than 4" above it.
Questions
23 of 1-1/2" wood or 26 of 3/4" metal for 120" between posts, centred with equal gaps. Buy about 10% more.
3-9/16" clear between 1-1/2" balusters (5-1/16" on centre) or 3-3/4" between 3/4" balusters (4-1/2" on centre).
Its leading edge sits one gap from the post face: 3-9/16" for 1-1/2" wood or 3-3/4" for 3/4" metal.
Under AWC DCA 6, yes: guard posts may be at most 6'-0" apart, and 120" clear on 4x4 posts is 10' 3-1/2" on centre. Railing kits rated for longer sections are the exception.
25 at 3-5/8" gaps; 3-1/2" flat pickets take 17 at 3-3/8".
Raised decks, basements and storeys