Cutting, Ripping and Milling

The Underwriters Laboratories (UL) mark on each piece of Pyro-Guard, Exterior Fire-X and other brands of fire retardant treated lumber or plywood allows basic on-the-job cutting of lumber and plywood.

On the other hand, the UL mark carries restrictions on ripping and milling since these operations alter the surface of the wood which could reduce the effectiveness of treatment. Below is a description of what is and is not permitted by UL.

• Cutting of lumber to length (cross-cutting) is permitted by UL without restriction.
• Ripping (cutting) of plywood is permitted by UL without restriction.
• Light sanding of lumber or plywood is permitted by UL to remove raised grain or to prepare for finishing.
• Resurfacing, shaping or planing should be done before treatment.

Ripping of lumber is not allowed by UL, as in ripping a 2x4 into 2x2's, or as in ripping a wider piece of lumber into a narrower piece. Similarly, cutting of stair stringers after treatment should not be done because the effect is similar to ripping.

In some circumstances, the building inspector, at his discretion, may approve the use of ripped FRTW (example: 2x2's ripped from 2x4's) if the installation allows complete masking of the ripped edge by installing it flush against an intact piece of FRTW or against a non-combustible surface.

Milling (resurfacing) of lumber or plywood is not allowed by UL. If special shapes or thickness are required, milling should be done prior to treatment.