Installation Instructions

CertainTeed Vinyl Siding Installation Guide

Inside Cornerposts

There are three options for trimming inside corners: Standard

3/4" inside cornerpost, single J-channel, and two J-channels.

To ﬂash the inside corner, bend a 20"-wide piece of aluminum

coil stock 90° so you have two 10" legs. Insert the ﬂashing into

the corner. If you use more than one piece of ﬂashing, overlap

the upper pieces of the ﬂashing over the lower pieces.

To install inside cornerpost, hang the post from the top of the eave.

The bottom should extend 3/4" below the starter strip. Remove

the bottom 3/4" of the nailing ﬂange so it does not show below

the siding. Set the post straight and true. Position the top nail in

the top of the nailing slot. All other nails should be in the center

of the nail slots

If you have to splice the inside cornerpost, cut 1" off all but the

outer face of the lower post. Lap 3/4" of the upper post over the

lower post, leaving 1/4" for expansion.

If you are using two pieces of J-channel instead of inside

cornerpost, ﬂash the corner with a 10" x 10" “L” corner fabricated

from aluminum coil stock or any weather-resistant barrier. Hang

the J-channel from the top of the eave. The bottom should extend

3/4” below the starter strip. Remove the bottom 3/4" of the nailing

ﬂange so that it will not show below the siding. Use the same

positioning and nailing guidelines as inside cornerpost.

To create a narrower corner, you can also use a single length

of J-channel and ﬂashing. First, install the siding on one wall.

Then place the J-channel lightly against the siding and nail it to

the substrate on the adjacent wall. Follow the same positioning

and nailing guidelines as inside cornerposts.

inside cornerpost splice

hang from top

nail, center

remaining nails

8" to 10"

1/4" to 3/8"

3/4"

solid nailable

substrate

corner flashing

(scrap aluminum coil, for example)

standard 3/4"

inside corner post

single J-channel

two J-channels