A 12 ft (144") deck railing between posts takes 27 wood balusters (1-1/2") at 3-11/16" gaps or 32 metal balusters (3/4") at 3-5/8", both under the IRC 4" sphere. Hook the tape on the post once: the first wood edge goes at 3-11/16", the next at 8-7/8", then every 5-3/16".
144" 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 | 3-11/16" | 4-7/16" | 3.696" | |
| 2 | Leading edge | 8-7/8"+ | 9-5/8"+ | 8.893" | |
| 3 | Leading edge | 14-1/16"+ | 14-13/16"+ | 14.089" | |
| 4 | Leading edge | 19-5/16"− | 20-1/16"− | 19.286" | |
| 5 | Leading edge | 24-1/2"− | 25-1/4"− | 24.482" | |
| 6 | Leading edge | 29-11/16" | 30-7/16" | 29.679" | |
| 7 | Leading edge | 34-7/8" | 35-5/8" | 34.875" | |
| 8 | Leading edge | 40-1/16" | 40-13/16" | 40.071" | |
| 9 | Leading edge | 45-1/4"+ | 46"+ | 45.268" | |
| 10 | Leading edge | 50-7/16"+ | 51-3/16"+ | 50.464" | |
| 11 | Leading edge | 55-11/16"− | 56-7/16"− | 55.661" | |
| 12 | Leading edge | 60-7/8"− | 61-5/8"− | 60.857" | |
| 13 | Leading edge | 66-1/16" | 66-13/16" | 66.054" | |
| 14 | Leading edge | 71-1/4" | 72" | 71.250" | |
| 15 | Leading edge | 76-7/16" | 77-3/16" | 76.446" | |
| 16 | Leading edge | 81-5/8"+ | 82-3/8"+ | 81.643" | |
| 17 | Leading edge | 86-13/16"+ | 87-9/16"+ | 86.839" | |
| 18 | Leading edge | 92-1/16"− | 92-13/16"− | 92.036" | |
| 19 | Leading edge | 97-1/4"− | 98"− | 97.232" | |
| 20 | Leading edge | 102-7/16" | 103-3/16" | 102.429" | |
| 21 | Leading edge | 107-5/8" | 108-3/8" | 107.625" | |
| 22 | Leading edge | 112-13/16" | 113-9/16" | 112.821" | |
| 23 | Leading edge | 118"+ | 118-3/4"+ | 118.018" | |
| 24 | Leading edge | 123-3/16"+ | 123-15/16"+ | 123.214" | |
| 25 | Leading edge | 128-7/16"− | 129-3/16"− | 128.411" | |
| 26 | Leading edge | 133-5/8"− | 134-3/8"− | 133.607" | |
| 27 | Leading edge | 138-13/16" | 139-9/16" | 138.804" |
4x4 posts at 147-1/2" 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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| 27 | 3-11/16" | 5-3/16" | 3-11/16" | 30 | ||
| 32 | 3-5/8" | 4-3/8" | 3-5/8" | 36 | ||
| 30 | 3-11/16" | 4-11/16" | 3-11/16" | 33 | ||
| 20 | 3-1/2"+ | 7" | 3-1/2" | 22 |
| 1 | 3-11/16" | 4-7/16" | 3.696" |
|---|---|---|---|
| 2 | 8-7/8"+ | 9-5/8"+ | 8.893" |
| 3 | 14-1/16"+ | 14-13/16"+ | 14.089" |
| 4 | 19-5/16"− | 20-1/16"− | 19.286" |
| 5 | 24-1/2"− | 25-1/4"− | 24.482" |
| 6 | 29-11/16" | 30-7/16" | 29.679" |
| 7 | 34-7/8" | 35-5/8" | 34.875" |
| 8 | 40-1/16" | 40-13/16" | 40.071" |
| 9 | 45-1/4"+ | 46"+ | 45.268" |
| 10 | 50-7/16"+ | 51-3/16"+ | 50.464" |
| 11 | 55-11/16"− | 56-7/16"− | 55.661" |
| 12 | 60-7/8"− | 61-5/8"− | 60.857" |
| 13 | 66-1/16" | 66-13/16" | 66.054" |
| 14 | 71-1/4" | 72" | 71.250" |
| 15 | 76-7/16" | 77-3/16" | 76.446" |
| 16 | 81-5/8"+ | 82-3/8"+ | 81.643" |
| 17 | 86-13/16"+ | 87-9/16"+ | 86.839" |
| 18 | 92-1/16"− | 92-13/16"− | 92.036" |
| 19 | 97-1/4"− | 98"− | 97.232" |
| 20 | 102-7/16" | 103-3/16" | 102.429" |
| 21 | 107-5/8" | 108-3/8" | 107.625" |
| 22 | 112-13/16" | 113-9/16" | 112.821" |
| 23 | 118"+ | 118-3/4"+ | 118.018" |
| 24 | 123-3/16"+ | 123-15/16"+ | 123.214" |
| 25 | 128-7/16"− | 129-3/16"− | 128.411" |
| 26 | 133-5/8"− | 134-3/8"− | 133.607" |
| 27 | 138-13/16" | 139-9/16" | 138.804" |
Every baluster is 5-3/16" on centre after the first.
| 1 | 3-5/8" | 4" | 3.636" |
|---|---|---|---|
| 2 | 8"+ | 8-3/8"+ | 8.023" |
| 3 | 12-7/16"− | 12-13/16"− | 12.409" |
| 4 | 16-13/16"− | 17-3/16"− | 16.795" |
| 5 | 21-3/16" | 21-9/16" | 21.182" |
| 6 | 25-9/16" | 25-15/16" | 25.568" |
| 7 | 29-15/16"+ | 30-5/16"+ | 29.955" |
| 8 | 34-5/16"+ | 34-11/16"+ | 34.341" |
| 9 | 38-3/4"− | 39-1/8"− | 38.727" |
| 10 | 43-1/8" | 43-1/2" | 43.114" |
| 11 | 47-1/2" | 47-7/8" | 47.500" |
| 12 | 51-7/8" | 52-1/4" | 51.886" |
| 13 | 56-1/4"+ | 56-5/8"+ | 56.273" |
| 14 | 60-11/16"− | 61-1/16"− | 60.659" |
| 15 | 65-1/16"− | 65-7/16"− | 65.045" |
| 16 | 69-7/16" | 69-13/16" | 69.432" |
| 17 | 73-13/16" | 74-3/16" | 73.818" |
| 18 | 78-3/16"+ | 78-9/16"+ | 78.205" |
| 19 | 82-9/16"+ | 82-15/16"+ | 82.591" |
| 20 | 87"− | 87-3/8"− | 86.977" |
| 21 | 91-3/8" | 91-3/4" | 91.364" |
| 22 | 95-3/4" | 96-1/8" | 95.750" |
| 23 | 100-1/8" | 100-1/2" | 100.136" |
| 24 | 104-1/2"+ | 104-7/8"+ | 104.523" |
| 25 | 108-15/16"− | 109-5/16"− | 108.909" |
| 26 | 113-5/16"− | 113-11/16"− | 113.295" |
| 27 | 117-11/16" | 118-1/16" | 117.682" |
| 28 | 122-1/16" | 122-7/16" | 122.068" |
| 29 | 126-7/16"+ | 126-13/16"+ | 126.455" |
| 30 | 130-13/16"+ | 131-3/16"+ | 130.841" |
| 31 | 135-1/4"− | 135-5/8"− | 135.227" |
| 32 | 139-5/8" | 140" | 139.614" |
Every baluster is 4-3/8" on centre after the first.
05 / Field guide
This sheet lays out a 12 ft (144") 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.
144" ÷ (1-1/2" + gap) fits 27 balusters with 28 equal gaps of 3-11/16" (3.696"), 5-3/16" on centre.
From the post face the leading edges fall at 3-11/16", 8-7/8", 14-1/16", 19-5/16" and on in 5-3/16" steps. Buy 30 to allow 10% for splits and miscuts.
Thinner balusters need more of them: 32 at 3-5/8" gaps, 4-3/8" on centre. The first four marks: 3-5/8", 8", 12-7/16", 16-13/16".
1" metal or composite takes 30 at 3-11/16"; 3-1/2" flat pickets take 20 at 3-1/2".
With 4x4 posts the centres would be 12' 3-1/2" apart, past the 6'-0" DCA 6 limit for prescriptive guard posts. Add 1 intermediate post (2 bays of about 70-1/4" 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
27 of 1-1/2" wood or 32 of 3/4" metal for 144" between posts, centred with equal gaps. Buy about 10% more.
3-11/16" clear between 1-1/2" balusters (5-3/16" on centre) or 3-5/8" between 3/4" balusters (4-3/8" on centre).
Its leading edge sits one gap from the post face: 3-11/16" for 1-1/2" wood or 3-5/8" for 3/4" metal.
Under AWC DCA 6, yes: guard posts may be at most 6'-0" apart, and 144" clear on 4x4 posts is 12' 3-1/2" on centre. Railing kits rated for longer sections are the exception.
30 at 3-11/16" gaps; 3-1/2" flat pickets take 20 at 3-1/2".
Raised decks, basements and storeys