Wood  Decay

Since decay can be an important issue in the inspection of log homes, inspectors should be familiar with techniques used to locate both accessible and hidden decay and to determine the limits of its extent.

VISUAL EVALUATION

The first method for finding decay is to look for conditions which would encourage the growth of decay fungi.

Areas of the roof design and drainage system which might route water to areas in which it could be trapped are a good place to start.

Decks, balconies and elevated walkways attached to a home can also route rain and snowmelt to areas which may trap moisture. Especially vulnerable are exterior corners of older homes, since this is where log end-grain is exposed and end-grain absorbs water much more quickly than the outer surfaces of logs.

PROBING

Probing with an awl or screwdriver is a simple way to identify advanced wood decay, when it's accessible. Wood with  advanced decay is soft and easily recognized. Wood which is infected with decay fungi and has undergone less severe deterioration may be more difficult  to identify.

At the other end of the spectrum, incipient (early) decay may be extremely difficult to identify, even though wood may have lost up to 50% of its strength. At the time of an inspection of course, wood may be at any stage of decay between advanced and incipient. 

Inspectors will fail to find decay either because it is hidden, inaccessible or ar a stage in which discovery requires specialized techniques which lie beyond the scope of a General or Log Home Inspection. Inspectors may wish to include in the inspection report language specifically disclaiming decay which is hidden or located in inaccessible areas.

The PICK TEST

In using the pick test, the soundness of wood is judged from the manner in which wood splinters when an awl, ice pick or knife blade is inserted under wood fibers, then pried up.

Sound wood will break with a snap. The broken edges will appear splintery and sharp, often breaking to one side.

Decayed wood will break quietly, usually directly over the prying tool.

LIABILITY and HIDDEN DECAY

Inspectors may want to attempt to identify areas of hidden decay when conditions indicate a high probability that it may exist. By undertaking this task, they may open themselves to additional liability. Locating hidden decay lies well beyond the scope of the General Home Inspection and is an area each inspector should discuss with their attorney, as the amount of liability inherent in performing any inspection for hidden decay may vary by state and by the quality of the contract and inspection language used.

Liability is linked to educating the client on the limits of what an inspector can do.

It’s important that clients understand that methods for locating hidden decay are not exact and are still being developed. So many variables and conditions exist that developing protocols which will address all situations and conditions is impossible. Using the methods discussed below involves using what few tools are at our disposal VS using no tools, making no effort and waiting until failure occurs at which point the inspector’s client gets to foot the bill for repairs.

SHELL of SOUND WOOD

Decay seen at a log surface often extends into parts of the log which appear sound at the exterior, but which the inspector may suspect of containing internal decay. From a structural perspective, the inspector is interested in gaining at least a rough idea whether the shell of sound wood is thick enough for the log to retain sufficient structural integrity to support the structure. The shell of sound wood is the layer of solid wood remaining in a log when the core of the log has lost all strength from wood decay.

Of course inspectors won’t perform engineering calculations to determine whether the shell of sound wood is sufficient, but determining the rough extent of any problems found can offer some guidelines on whether to recommend a specialist inspection, the specialist being a qualified log home restoration contractor or structural or forensic engineer. The fact is… not many qualified specialists exist. One more reason why a knowledgeable inspector willing to make the effort will develop a business advantage over those who are not.

Log Extension Decay

When looking at the end grain of a log, the inspector may see a section of decayed wood which he can pretty safely assume extends back into the log for a certain distance. Determining the severity of the problem requires determining the extent of decay. This is called determining the profile of internal decay.

In log extensions, this profile is typically roughly cone-shaped, as if the cone were lying on its side with the base at the end of the log.

In testing the 35-year old lodgepole pine logs in this photograph, the cone of decay varied from 1:4 to 1:5.  This means that for every 1 inch of decay area height visible at the log end, decay extended back into the log for 4 to 5 inches.

This means that an area of decay visible at a log end which is about 4 inches high has hidden decay extending back into the log 16 to 20 inches.

The cone of decay profile may vary with…

• log wood species
• original log moisture content
• Decay present in the original log
• homesite conditions
• local average annual humidity
• home design
• type of finish
• condition of finish
• long-term maintenance
• any structural changes

LOCATING HIDDEN DECAY

RESONANCE TESTING

One method for determining the presence of hidden decay is resonance testing or malleting, two different terms for tapping logs suspected of containing hidden decay with a rubber mallet.

The deteriorated fibers of decayed wood transmit sound less efficiently than the intact fibers of solid wood. For this reason it’s sometimes possible to locate areas of hidden wood decay by tapping logs at intervals and listening for a change in tone.
Although changes in tone may be affected by other properties of the log such as thickness, length, growth characteristics and contact with other logs, resonance testing can still aid in indicating areas which are candidates for more accurate testing techniques.

RESISTANCE DRILLING

Resistance drilling is an accurate method for locating and documenting hidden wood decay. It involves drilling into wood and using the resistance met by the drill bit as it moves forward to gain information about the condition of the wood. This requires the homeowner’s permission!

DRILLING by HAND

Resistance drilling can be done by hand using a conventional power drill and paddle bit.

The ability to accurately and consistently locate interior wood decay in logs by hand-drilling will vary from person to person. Practice will help improve a person's ability to differentiate between decayed and sound wood.

By using a tapered paddle bit, the resistance felt will be from the tip of the bit as it moves forward through the wood. If a twist or auger bit is used, friction from the sides of the bit rubbing against the sides of the hole will reduce the effectiveness of this method.

The driller should make an effort to use constant, even pressure while pushing the bit into the wood. Decayed wood fibers offer less resistance than sound fibers, so the driller should concentrate on feeling changes in resistance as the bit encounters sound and decayed wood.

Using a smaller bit will be less invasive, but changes in resistance may not be as obvious. Using a larger bit also allows the wood shavings to be examined. Shavings from sound and decayed wood will have a different feel when rubbed between fingers and a different look, especially if examined under a magnifying glass or hand lens.

Drilling up at an angle from underneath will avoid creating a hole which will catch and trap water.  Inspectors who drill completely through a log will make the seller unhappy.

RESISTANCE-MEASURING INSTRUMENTS

Resistance-measuring instruments work by inserting a needle-like drill bit into wood. As the instrument inserts the needle, it measures and records variations in torque created as the needle meets areas of greater and lesser resistance, sound wood (peaks) offering greater resistance than decayed wood (valleys). The results are recorded by a stylus on waterproof graph paper. 

Resistance-measuring instruments are the most accurate and dependable method for locating and documenting hidden wood decay. This is the IML Resistograph.

Test results are recorded in actual 1:1 size to give as accurate a reading as possible.                 

 Resistance-measuring instruments are sensitive enough to record annual tree rings. The path of the needle in testing this 15-inch log lies just below the edge of the paper. Dips in the line show softer wood and decay. Because the results are in actual size it’s easy to see where decayed wood starts and ends.           

This type of testing is low-invasive. Holes are easy to plug and cause no structural and very minimal cosmetic damage.

INCREMENT BORING

Increment borers are tools designed to remove a wood core from a tree or log. Their usual purpose is to examine the annual ring pattern to gather climatic data or to date logs or timbers through dendrochronology. This should be kept in mind by those inspecting historic structures and wanting to accurately date wood members. The International Tree Ring Data Base offers inspectors the chance to give clients the option of accurately dating a historic structure.

Increment borers can also be used to find decay hidden in logs. At about $350.00 in cost they are a compromise between hand resistance drilling (cost of a drill and bit) and resistance recording instruments (about $4,000).

STRESS WAVE TIMING

Stress wave timing involves electronically timing the speed with which sound waves travel through wood. Typically an electronic receiver is placed at one end or on one side of a log and an electronic sound transmitting decice is placed at the other end or on the other side. Both devices are connected to an electronic timing device.

The objective is to locate areas of internal decay by comparing changes in the time taken for the sound to travel from the point of impact to the receiver, when the log or timber is tested at various locations.

This type of testing works much better on individual timbers than on logs in a wall and is not generally used for locating hidden decay in log homes. It's mentioned here because the subject of stress wave timing will appear in researching wood decay.

Sources for More Information 

Wood and Timber Condition Assessment Manual Forest Products Society, 2004 ISBN 1-892529-39-4

One of the few publications to provide detailed coverage of decay and fire assessment methods relevant to  log and timber homes.

Increment Borers

Forestry Supplies Inc.    (800) 647-5368

Ben Meadows Co.        (608) 743-8007

Resistance Drilling Instruments

IML    (888) 514-8851

Sibtech Scientific    +44 1483 440 727 (England)