Part 9 -The Pressure Treating Process

How Wood is Pressure Treated

Pressure treatment is a process that forces wood preservatives or fire-retardants into the wood. These processes are considered the best and most effective method to extend and preserve timber life. Preservatives protect the wood from attack by wood ingesting insects; like termites, and wood rot caused by fungal decay. Fire-retardant treatments help the wood to quickly char when exposed to flame, reducing the smoke and flame that occurs in a fire.

wood treating process video

Treated wood use applications include interior framing, exposed exterior wood for above ground or ground contact uses, fresh-water and salt-water exposures, and fire retardants for wall, roof and floor assemblies. Additionally, the pressure treatment process is used to protect utility poles, railroad ties, structural framing, fence pickets, deck boards, and posts.

A treatment facility obtains regionally specific wood such as Southern Yellow Pine, Douglas Fir, or Western Red Cedar for pressure treatments.

Pressure treating wood provides deep penetration and retention of preservatives for uniform protection for wood products, ensuring the treatment formulation meets the appropriate wood standards and building code requirements. With today’s advanced formulations, the treating process is very effective and computer controlled.

incising wood

For some western wood species, incising is a common process where the wood is resistant to preservative penetration, but the preservative will penetrate along the grain. These wood species are prepared by creating small incisions into the wood prior to the pressure treating process. It is common practice to incise all sawed Douglas fir three inches or more in thickness before treatment.

The Pressure Treating Process

  1. Wood is ordered and shipped to the treating plant from regionally sourced mills.
  2. Wood is checked for moisture content to ensure the moisture is not too high to accept the preservative treatment.applying end tags before treating process
  3. Once the wood is unloaded, end tags are applied to every piece of lumber to denote the plant name and location, application and its abbreviation, the preservative name or its abbreviation, the retention level of the preservative retained in the wood indicated in pounds per cubic foot, and the trademark of the approved third-party inspection agency.
  4. The pressure-treating process starts when forklifts place the wood on a tram that will move the wood into a large steel cylinder; called a vacuum pressure vessel.closed wood treating cyclinder
  5. Once the wood is loaded into the cylinder, the doors are closed and the cylinder is sealed.
  6. An industrial vacuum pump removes air from the cylinder, and that includes pulling air out of the wood too.
  7. The cylinder is then flooded with the preservative solution.
  8. Pressure is applied to the solution to force the preservatives deep into the wood cells.
  9. Cycle times and pressure settings are adjusted based on the retention levels needed and the species of the wood being treated.pressure treated wood coming out of cylinder
  10. Once the cycle has been completed, the cylinder is drained, then the industrial vacuum pump removes excess solution from the wood which is returned back to the work solution storage tank. 
  11. A final vacuum is run within the cylinder to extract excess preservative which is returned back to the work solution storage tank.
  12. The door is opened, and the wood is removed. The freshly treated wood is then placed on a drip-pad for 24 to 48 hours.core samples for penetration and retention testing
  13. Wood core samples are taken from each charge to test for penetration and retention to ensure wood is treated to minimum American Wood Protection Association (AWPA) standards.
  14. The wood is now ready for loading and shipping to building supply and distribution companies.

Other topics in this 10-part series

Part 1. 10/28/21
AWPA Category uses of pressure treated wood and end tags explained

Part 2. 11/04/21
Common treated wood sizes in nominal and actual dimensions plus popular project plans

Part 3. 11/11/21
Hardware and installation tips including field treatment for end cuts
Part 4. 11/18/21Staining treated lumbe

Part 4. 11/18/21
Staining treated lumber

Part 5. 11/25/21
Care and maintenance

Part 6. 12/02/21
Tips for removing snow and ice from wood decks

Part 7. 12/09/21
Safe handling

Part 8. 12/16/21
Research on treated wood used in raised garden beds

Part 9. 12/23/21
How wood is pressure treated

Last  Topic in this series

Part 10. 12/30/21
How to interview contractors and check references

If you have missed a topic or have suggestions for other treated wood related subjects, let us know.