TIMBER PRODUCTS July 2025 HT Newsletter Volume: 07/08/2025

ALSC Requirements for Surrogate Block Use

On July 26, 2023, ALSC distributed the information below regarding surrogate blocks.  These points are already part of TP's inspection review but it is important facilities see that ALSC's use of "must", "shall", and "requirements" below show that these are not recommendations but requisite to conformance when employing surrogate blocks in the heat-treating process.

"During the last couple of years, surrogate block use for verifying heat treatment has become very common. Some foreign intercepts of banned pests have been linked to facilities using surrogate blocks. (Requirements for heat treatment are outlined in ISPM 15 and the ISPM 15 Explanatory Document.) ALSC must ensure that surrogate block use provides for the proper heat treating of HT or IPPC marked material. ALSC offers the following requirements for using surrogate blocks during the heat treatment cycle:

• Surrogate blocks must be larger than the thickest effective components being heat treated. This means if three 2x4’s (1-1/2" thick) are nailed together, this would be equivalent to a 4-1/2" piece of wood, so the surrogate must be at least 4-1/2" thick.

• The surrogate block must be free of large checks, splits or other defects that allow air to penetrate into the block. The block must be a similar species and moisture content of the material being heat treated.

• The thermocouple must be drilled into the block at least ½ of the depth of the effective thickness. For instance, if a 3-1/2x3-1/2-inch block is used, the hole and the tip of the thermocouple must be at least 1-3/4" deep in the block. The tip of the thermocouple in this case must be no closer that 1-3/4" from any edge of the block. Care must be taken that the hole for the thermocouple does not extend through the block.***

• Thermocouples must be sealed.

• Surrogate blocks must be placed in the wood being treated, not on top of or to the side of wood unless a verification study has been completed showing an acceptable placement of the blocks compared to the lumber being treated.

The use of surrogate blocks during the heat treatment cycle:

• Surrogate blocks must be equal to or cooler in temperature than the coolest lumber being heat treated at the start of the HT cycle.

• When a facility runs more than one charge in a day, surrogate blocks can retain heat from previous charges. Care must be taken to ensure surrogate blocks still represent the internal temperature of the wood being treated. The facility shall be required to change surrogate blocks between charges when multiple charges are within 24 hours of each other.

• Surrogate blocks must be adequately cooled to equivalent or cooler than the wood being treated. Cooled blocks should not be stored in the sun, or in areas where artificial heat may affect the blocks such as on a heated floor or near a heat source or in a heat chamber that could be warmer than the area lumber being treated is stored prior to heat treatment.

• All personnel involved in the heat-treating process should be well versed in all of these practices.

• A surrogate block process would make review of these situations clearer to inspectors and personnel at facilities. This process should be unique to each facility.

• Failure of a facility to adequately follow all of the above situations will result in the previously heat-treated material on site having to be held and re-heat treated."

***Note that the depth of wood from the back of the hole to the end of the block is at least equal to the depth of the hole drilled. For example, if a 1-3/4" hole is drilled then there should be at least 1-3/4" wood from where the hole stops in the block, to the end of the block.

If these requirements are not followed, a non-conformance will be issued to the facility so please make certain to have a solid understanding of the above requirements and properly adhere to them when utilizing surrogate blocks.

If there are any questions regarding these requirements, please contact your Inspector or the Director of Wood Packaging.

WPM Class Registration Open

September 9, 2025 at the Timber Products - Conyers, GA facility, 1641 Sigman Road, Conyers GA 30012

Registration Cost Per Person:   $250 for TP Clients, $750 for non-clients

Lunch will be provided.

Click on the link below to register.

Training Services: Timber Products Inspection

Space is limited to 30 participants.