How to Install Z-Flashing on T1-11 Siding

Written by paul massey
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T1-11 exterior wood siding has been in wide use for over 30 years and is distinguished by vertical grooves that simulate a spaced board appearance, available in 4-, 6-, 8- and 12-inch groove spacing. As part of the T1-11 installation, wherever a wall height requires more than a full sheet of siding, a Z-shaped metal flashing is added at all horizontal seams to ensure a proper seal against rain or moisture.

Skill level:
Moderately Easy

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Things you need

  • Tape measure
  • Hammer
  • T1-11 wood siding
  • Z-flashing
  • Metal snips or hacksaw
  • Nailset (optional)
  • 8d galvanised nails
  • Exterior-grade silicone caulking (optional)

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  1. 1

    Complete the installation of the first horizontal row of T1-11 siding following standard construction practices. Typical installation aligns the bottom edge of the first row of siding 2 to 4 inches below the wall's bottom sill plate and over a continuous layer of waterproof kraft paper or polythene plastic sheeting.

  2. 2

    Install the first length of Z-flashing. Begin at either end of the wall and set the level middle section of the Z-flashing onto the top edge of the siding panel, with the wider flange upward and against the wall framing. Flush the end of the flashing length with the wall corner and secure the flashing to the framing with an 8d nail at each wall stud. Keep the nails toward the top edge of the flashing and use a nailset to punch a hole for the nail, if necessary.

  3. 3

    Continue installing the Z-flashing at the top edge of the siding panels across the full length of wall. Position the flashing lengths as defined in Step 2, overlapping the ends a minimum of 2 inches onto the previous flashing piece and securing the flashing with 8d nails at each wall stud. Cut the Z-flashing, as needed, using metal snips or a hacksaw. When the full length of the siding's top edge has been flashed, continue installing the T1-11 siding with the bottom edge resting on top of the installed Z-flashing.

Tips and warnings

  • To add an extra layer of weather seal to the Z-flashing, run a 1/4-inch continuous horizontal bead of exterior silicone caulking to the inner vertical section of the flashing before installing the next section of T1-11 siding over it.

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