Siding Trim: J-Channel, Starter, Corner Posts
How much J-channel, starter strip and corner post a house needs, measured line by line and turned into stock-length pieces, next to the quick per-square factors.
How much J-channel you need depends on the windows, doors and gables, not on the wall area. A window takes J-channel on all
4 sides; a door on 3, because the bottom length of door openings will most likely not include J channel
(BYU-Idaho). The gable rakes take it too. Starter strip follows the base of the house, and every corner needs posts as tall as the
wall. Measure those lines, add the waste and divide by the stock length: that is the take-off below.
What each trim piece follows
- Starter strip:
To estimate the amount of starter strip required, measure the linear feet around the entire base of the house.
(VSI manual) - J-channel:
J-Channel: For siding installations, measure around doors and windows, under eaves, at rake edges of gables where dormer meets roofline, and anywhere else required to provide a finished appearance.
(CertainTeed). CladCount counts openings and rakes, not the eaves. - Corner posts:
Outside cornerpost: Measure length of outside corners.
(CertainTeed). Each corner gets whole posts. - Stock lengths: J-channel
J-Channel – 1-1/2" 12' 6" length
andInside Corner Post 10' 0" length
(Alside).
Two houses, measured
Both with 10 % waste on starter strip and J-channel and none on corner posts, as the calculator counts them.
| Trim | Measured | Linear ft | Pieces |
|---|---|---|---|
| Starter strip | base of the house | 140.0 | 13 × 12' 6" |
| J-channel | openings 162.0 + rakes 67.1 | 229.1 | 21 × 12' 6" |
| Outside corner posts | 4 × 8' | 32.0 | 4 × 10' |
| Inside corner posts | 0 × 8' | 0.0 | 0 × 10' |
| Trim | Measured | Linear ft | Pieces |
|---|---|---|---|
| Starter strip | base of the house | 120.0 | 11 × 12' 6" |
| J-channel | openings 239.0 + rakes 50.6 | 289.6 | 26 × 12' 6" |
| Outside corner posts | 6 × 18' | 108.0 | 12 × 10' |
| Inside corner posts | 2 × 18' | 36.0 | 4 × 10' |
On the two-story house the outside corners are 18' tall, so each takes 2 posts of 10 ft, 12 in all for its 6 outside corners. Whole posts per corner: a cut-off from one corner is too short to finish another.
The per-square factor method on the same houses
Ply Gem's estimating guide skips the measuring: multiply the squares of siding by a factor per accessory. Its example: Starter Strip:
25 squares × 0.8 = 20 pieces of starter strip.
, “J” Channel: 25 squares × 2.0 = 50 pieces of “J” channel.
, Outside Corner
Post: 25 squares × 0.2 = 5 pieces of outside corner posts.
and Inside Corner Post: 25 squares × 0.1 = 2.5 pieces.
(Ply Gem). Applied to the two houses, rounded up:
| House | Squares | Starter: measured / factor | J-channel: measured / factor | Outside posts: measured / factor | Inside posts: measured / factor |
|---|---|---|---|---|---|
| One-story house | 13.01 | 13 / 11 | 21 / 27 | 4 / 3 | 0 / 2 |
| Two-story house with an attached wall | 20.19 | 11 / 17 | 26 / 41 | 12 / 5 | 4 / 3 |
The factors assume an average house. They come out high on J-channel for both houses here, and low on corner posts for the tall one,
because corner posts grow with the wall height and J-channel with the number of openings, not with the squares. The factor method is
quick for a rough supply-yard quote; the take-off is what the walls actually have. Ply Gem itself notes that many homes may not
require inside corner posts
(Ply Gem).
Open in calculator and change the piece lengths under Corners & trim lengths
to what your supplier stocks.