Hardboard (Composite Wood) Siding

Hardboard (Composite Wood) Siding


Hardboard siding is most commonly seen as horizontal lap siding. It is also seen as vertical panels and as horizontal strips that simulate wood shingles. Finishes include smooth and textured patterns that simulate wood. Thickness is about 7/16 inch. It should be installed by fastening siding to walls covered by a water-resistive barrier. This material was first available in the early 1900s and became widely available in the 1930s. It was most popular from 1940s through 1990s, and is experiencing a comeback in current construction. Do not describe this material as plywood, oriented strand board, or wood siding. Newer versions of material are sometimes called composite wood siding and engineered wood siding.

Hardboard siding should be installed according to manufacturer’s instructions. These instructions will vary based on the manufacturer, the model of siding being installed, and the location of the house.

Some hardboard siding is manufactured using wood byproducts and binders. Older versions of hardboard siding are a known problematic wall covering. Many of the problems experienced with this siding were caused by improper installation, and by improper maintenance (painting and caulking). Class action lawsuits alleging product defects were filed against many manufacturers of this wall covering in the 1990s. These lawsuits were settled, and the recovery funds were exhausted many years ago.

Typical Defects Typical defects that home inspectors should report include:

  1. inadequate clearance above grade, hard surfaces, flashing, and roof coverings (1 or 2 inches above hard surfaces and above roof coverings, 1 to 6 inches above grade, all clearnaces per manufacturer’s instructions),
  2. absent and deteriorated flashing and sealant around doors, windows, and other penetrations,
  3. absent and deteriorated sealant at corner boards,
  4. absent movement gap (about ΒΌ inch) at vertical surfaces (e.g., corner boards, penetrations) and at horizontal flashing,
  5. absent and improperly installed flashing and kick out flashing at wall intersections,
  6. damaged and deteriorated siding,
  7. improperly installed fasteners (e.g., wrong fastener type, improper location, over and under driven),
  8. excessively wavy or buckled siding,
  9. absent or deteriorated paint at the bottom edge.

Standards (1) IRC 2018 Section R703; (2) manufacturer’s instructions.


Three images of deteriorated harboard siding with the caption "Hardboard siding is often deteriorated at joints and at bottom edges."
Image is captioned "Example of why kick out flashing is important."