A 7 ft (84") deck railing between posts takes 16 wood balusters (1-1/2") at 3-1/2" gaps or 18 metal balusters (3/4") at 3-11/16", both under the IRC 4" sphere. Hook the tape on the post once: the first wood edge goes at 3-1/2", the next at 8-9/16", then every 5".
84" 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 | 3-1/2"+ | 4-1/4"+ | 3.529" | |
| 2 | Leading edge | 8-9/16" | 9-5/16" | 8.559" | |
| 3 | Leading edge | 13-9/16"+ | 14-5/16"+ | 13.588" | |
| 4 | Leading edge | 18-5/8" | 19-3/8" | 18.618" | |
| 5 | Leading edge | 23-5/8"+ | 24-3/8"+ | 23.647" | |
| 6 | Leading edge | 28-11/16" | 29-7/16" | 28.676" | |
| 7 | Leading edge | 33-11/16"+ | 34-7/16"+ | 33.706" | |
| 8 | Leading edge | 38-3/4" | 39-1/2" | 38.735" | |
| 9 | Leading edge | 43-3/4" | 44-1/2" | 43.765" | |
| 10 | Leading edge | 48-13/16"− | 49-9/16"− | 48.794" | |
| 11 | Leading edge | 53-13/16" | 54-9/16" | 53.824" | |
| 12 | Leading edge | 58-7/8"− | 59-5/8"− | 58.853" | |
| 13 | Leading edge | 63-7/8" | 64-5/8" | 63.882" | |
| 14 | Leading edge | 68-15/16"− | 69-11/16"− | 68.912" | |
| 15 | Leading edge | 73-15/16" | 74-11/16" | 73.941" | |
| 16 | Leading edge | 79"− | 79-3/4"− | 78.971" |
4x4 posts at 87-1/2" 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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| 16 | 3-1/2"+ | 5" | 3-1/2" | 18 | ||
| 18 | 3-11/16"+ | 4-7/16" | 3-11/16" | 20 | ||
| 17 | 3-3/4"− | 4-3/4" | 3-3/4" | 19 | ||
| 12 | 3-1/4"− | 6-3/4" | 3-1/4" | 14 |
| 1 | 3-1/2"+ | 4-1/4"+ | 3.529" |
|---|---|---|---|
| 2 | 8-9/16" | 9-5/16" | 8.559" |
| 3 | 13-9/16"+ | 14-5/16"+ | 13.588" |
| 4 | 18-5/8" | 19-3/8" | 18.618" |
| 5 | 23-5/8"+ | 24-3/8"+ | 23.647" |
| 6 | 28-11/16" | 29-7/16" | 28.676" |
| 7 | 33-11/16"+ | 34-7/16"+ | 33.706" |
| 8 | 38-3/4" | 39-1/2" | 38.735" |
| 9 | 43-3/4" | 44-1/2" | 43.765" |
| 10 | 48-13/16"− | 49-9/16"− | 48.794" |
| 11 | 53-13/16" | 54-9/16" | 53.824" |
| 12 | 58-7/8"− | 59-5/8"− | 58.853" |
| 13 | 63-7/8" | 64-5/8" | 63.882" |
| 14 | 68-15/16"− | 69-11/16"− | 68.912" |
| 15 | 73-15/16" | 74-11/16" | 73.941" |
| 16 | 79"− | 79-3/4"− | 78.971" |
Every baluster is 5" on centre after the first.
| 1 | 3-11/16"+ | 4-1/16"+ | 3.711" |
|---|---|---|---|
| 2 | 8-3/16"− | 8-9/16"− | 8.171" |
| 3 | 12-5/8" | 13" | 12.632" |
| 4 | 17-1/16"+ | 17-7/16"+ | 17.092" |
| 5 | 21-9/16" | 21-15/16" | 21.553" |
| 6 | 26" | 26-3/8" | 26.013" |
| 7 | 30-1/2"− | 30-7/8"− | 30.474" |
| 8 | 34-15/16" | 35-5/16" | 34.934" |
| 9 | 39-3/8"+ | 39-3/4"+ | 39.395" |
| 10 | 43-7/8"− | 44-1/4"− | 43.855" |
| 11 | 48-5/16" | 48-11/16" | 48.316" |
| 12 | 52-3/4"+ | 53-1/8"+ | 52.776" |
| 13 | 57-1/4" | 57-5/8" | 57.237" |
| 14 | 61-11/16" | 62-1/16" | 61.697" |
| 15 | 66-3/16"− | 66-9/16"− | 66.158" |
| 16 | 70-5/8" | 71" | 70.618" |
| 17 | 75-1/16"+ | 75-7/16"+ | 75.079" |
| 18 | 79-9/16"− | 79-15/16"− | 79.539" |
Every baluster is 4-7/16" on centre after the first.
05 / Field guide
This sheet lays out a 7 ft (84") 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.
84" ÷ (1-1/2" + gap) fits 16 balusters with 17 equal gaps of 3-1/2"+ (3.529"), 5" on centre.
From the post face the leading edges fall at 3-1/2", 8-9/16", 13-9/16", 18-5/8" and on in 5" steps. Buy 18 to allow 10% for splits and miscuts.
Thinner balusters need more of them: 18 at 3-11/16"+ gaps, 4-7/16" on centre. The first four marks: 3-11/16", 8-3/16", 12-5/8", 17-1/16".
1" metal or composite takes 17 at 3-3/4"; 3-1/2" flat pickets take 12 at 3-1/4".
With 4x4 posts the centres would be 7' 3-1/2" apart, past the 6'-0" DCA 6 limit for prescriptive guard posts. Add 1 intermediate post (2 bays of about 40-1/4" 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
16 of 1-1/2" wood or 18 of 3/4" metal for 84" between posts, centred with equal gaps. Buy about 10% more.
3-1/2" clear between 1-1/2" balusters (5" on centre) or 3-11/16" between 3/4" balusters (4-7/16" on centre).
Its leading edge sits one gap from the post face: 3-1/2" for 1-1/2" wood or 3-11/16" for 3/4" metal.
Under AWC DCA 6, yes: guard posts may be at most 6'-0" apart, and 84" clear on 4x4 posts is 7' 3-1/2" on centre. Railing kits rated for longer sections are the exception.
17 at 3-3/4" gaps; 3-1/2" flat pickets take 12 at 3-1/4".
Raised decks, basements and storeys