Bridge - Probing

Target of investigations

Probing is beneficial in investigating the near-surface decay of wood members and estimating the depth of splits and checks—inherent defects of timber and sawn lumber.


Probing involves using a moderately sharp object, such as an awl, pocketknife, or pick hammer, to locate the near-surface decay of wood members. Deteriorated wood may show excessive softness, less resistance against the probe penetration, and a different breaking pattern compared to sound wood.(1) When lifting a small sliver of wood with a pick or pocketknife, decayed wood breaks abruptly, while sound wood splinters.(2) In addition, probing with flat-bladed probes such as feeler gauges, which are typically graduated in centimeters and/or inches, can be used to estimate the depth of splits or checks in wood members.(1)

Source: U.S. Department of Agriculture, Forest Service.
A. Decay (discoloration at the end of member) in a split of a timber member.(1)
Source: U.S. Department of Agriculture, Forest Service.
B. Side checking in vertical substructure members (timber pile and timber column).(1)
Source: FHWA.
C. Breaking pattern of sound wood.(2)
Source: FHWA.
D. Breaking pattern of decayed wood.(2)
Source: U.S. Department of Agriculture, Forest Service.
E. Probing the end-grain surface of a wood deck member.(1)
Figure 1. Photos. Methods of probing.

Physical Principle

Because wood tends to exhibit excessive softness as it deteriorates, the resistance of wood against probing with a moderately sharp object can provide information about the decay of a member. Probing can also provide information about the depth of splits and checks.

Data Acquisition

Depending on the purpose of the inspection, an awl, pocketknife, or pick hammer can be used for probing.

Data Processing

No data processing is required

Data Interpretation

Care must be taken to differentiate between water-softened wood and deteriorated wood. Probing may not be effective for assessing deterioration in low-hardness wood members such as the ones made from western red cedar.(1)


Probing advantages include the following:

  • Simplicity.
  • Low cost.
  • Sensitivity to the moderate and advanced level of deterioration.


Some of the limitations of probing include the following:

  • Probing is mainly sensitive to near-surface decay.
  • Probing may not be an effective tool in low-hardness wood species.
  • Probing does not provide quantitative information about the physical and mechanical properties of wood members.


  1. White, R. H., and R. J. Ross, eds. 2014. Wood and Timber Condition Assessment Manual2nd ed. General Technical Report No. FPL-GTR-234. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.
  2. Ryan, T. W., J. E. Mann, Z. M. Chill, and B. T. Ott. 2012. FHWA Bridge Inspector’s Reference Manual (BIRM). Publication No. FHWA NHI 12-049. Washington, DC: Federal Highway Administration., last accessed February 1, 2022.