Pressure Preservative Treatment of Structural Glued Laminated Timber

Maurice J. Rhude
P.E., President
Sentinel Structures, Inc.
Peshtigo, Wisconsin


Tim bur-r-r-r-r. Look out below.

It you look below you are not apt to see that tree tailing on you from above. Loggers work with overhead dangers from above.

Historic wood ship maintenance, repair and preservation should be directed at what takes place below the deck. Look out below. With rainy days, with water coming inside almost as fast as the rain is falling outside, the problem is below deck.

U.S.S. Constitution, in 1949, exhibited extensive damage from marine borers, insects and decay. Replacements were made for the first time with pressure preservative treated wood, on a large scale, and certain other procedures were initiated:

1. a roof was removed
2. aerated the lower decks
3. discontinued swabbing the lower decks
4. dry cleaned or vacuumed and broomed to permit faster drying of wood

In the 1970's much hull planking was replaced from the top of the bulwark to a few feet below the waterline. Commander Thomas Coyne, 58th commanding officer, said, "It stays great in the water but above the waterline the ship rots out like mad." The underwater hull proved to be in good condition due to the salinity and low temperature of the water; all of the 360,000 board feet of white oak ordered would not be used.

The USS Constellation, in Baltimore since 1955, with hogged keel, open deck seams, topside planking seams open to the weather, and knees and frames decaying, has serious structural problems.

Does the owner let her sit and decay, tow her out to sea and let her sink, or spend money for needed repairs. With the only logical solution being repair, full advantage must be taken of the wood laminating process and pressure preservative treatment to make her highly resistant to decay and with a minimum of annual maintenance.

Wood is a renewable and most versatile construction material. There will always be enough wood for boats. Wood is economical and structurally efficient. It will decay if used in certain atmospheres when not pressure preservatively treated to define retentions and penetrations. Without preservative treatment, wood ships will surely be on a slow, downhill slide.

Preservative treatments protect wood if outdoors fully exposed to the weather without roof cover, when in ground contact, when indoors in high humidity atmospheres, and for marine applications.

The American Institute of Timber Construction (AITC) Standard 109 "Pressure Preservative Treatment of Structural Glued Laminated Timber" covers preservative treatment for laminations prior to gluing and for treatment of finished members after gluing. Refer to American Wood Preservers Association (AWPA) Standards C28 and C14 for additional information.

Someone recently told of the effect of wood on humidity, with the wood acting as a quiet humidifier, working automatically to exchange moisture in sufficient amounts to hold humidity at comfortable room levels. For ranch-type homes it was believed that there would be no need for air conditioning or humidifiers since the wood is absorbing moisture at times of rising humidities and giving oft moisture when the wood is becoming drier. I cannot imagine wet wood being placed in a building to add humidity to that environment.

It is much more appropriate to note the effect of humidity on wood.

The words and pictures which follow illustrate when-

1. Wood need not be preservatively treated.
2. It must be pressure-preservatively treated.
3. Treatment is needed or is not required dependent upon construction details.

Preservative treatment is not required where the wood is kept below 20% moisture content or where oxygen is excluded from the wood.

Click here to see a Relative Humidity of Air vs. Moisture Content in Wood Graph.

When enclosed buildings have proper ventilation, and the wood is protected from the weather, fully covered by a roof.

If the wood is permanently and totally submerged in water, there is no need for treatment.

Preservative treatment is required where the wood, in service, will be at or above 20% moisture content.

In portions of members extending outside the building walls and fully exposed to the weather without roof cover. For wet-process operations in enclosed buildings, with a high humidity environment, where despite ventilation, the wood moisture content is above 20%.

Wood in direct contact with ground or water where the wood is not permanently and totally submerged, it must be treated.

Marine organisms dictate that fully submerged untreated wood in salt water is not protected.

Preservative treatment may or may not be required for certain installations where wood can be protected from decay with various construction details and connections maintaining an in-service moisture content of less than 20%. The wood with a moisture content of 20% or greater requires preservative treatment.

Ventilation, insulation, vapor barriers, and separation between wood and concrete or masonry, influence the need for treatment.

The amount of resistance offered to insect attack must be determined.

Protection from attack by marine organisms by pressure preservative treatment is required.

The preservative treatment to be selected could be one of the following:

Creosote or creosote/coal tar solutions.
While appropriate for the most severe exposure conditions, it is dark and oily and generally cannot be stained or painted. There is an odor and may stain most anything it contacts. It is used only on finished glued members.

Oil-borne treatments.
Pentachlorophenol is the most common of the oil-borne preservatives. The Type A is used only after gluing and can stain other materials and does not readily stain or paint. Type C (light hydrocarbon solvent) can leave the wood close to a natural appearance and it can be stained or painted after the light solvent volatiles have evaporated.

Water-borne treatments.
These preservatives, water soluble, are generally applied prior to gluing, then the wood is kiln dried and the members are glued to the required shape and size. The preservatives leave the member a light green-gray or brown color but wood can readily be stained or painted immediately since the wood is dry.

Penetration of preservative into the sapwood is necessary to provide good protection. Some species are highly resistant to penetration and are thus unsuitable for preservative treating. Treatments will not penetrate all species. Sometimes the phrase "treated to refusal" is used on such resistant species, usually a sign that the wood cannot be properly treated even with a longer pressure period. Incising is required for some species; it is not required for Southern Pine.

Pre-treatment (drying or seasoning) is important since green wood will not allow proper penetration of the treating solution. Kiln drying after treatment with water-borne preservatives is usually required by building codes for treated wood used inside buildings.

It would not make sense to build or repair wood ships with green lumber so as to control the humidity within the ship.

Glued laminated members to be treated by pressure processes or pre-treated lumber to be made into laminated members must be glued with wet-use adhesive.

In general, longer curing times or higher temperatures are required for gluing pretreated laminations. Treatment may affect adhesive spread and assembly times. Different combinations of species-treatments-adhesives may not produce the same quality of glue bond so each combination must be investigated prior to use.

Whenever possible, all fabrication should be completed prior to treating the members. When treatment is on the lumber prior to gluing, the same attention to fabrication and machining is not required.

A replica of the Viking Golstad ship made the voyage from Norway to the 1893 Chicago Columbian Exposition.

Recently it was moved from under roof cover in Lincoln Park to the suburb West Chicago. A Viking Ship Restoration Committee would search for a climate and humidity-controlled enclosure for preservation.

The wood was dry for over 100 years, the ship was doing great and only needed a home with similar dry conditions.

The Mystery Ship was raised from 110 feet of water in Green Bay in the summer of 1969. Built in 1846, this two-masted schooner, Alvin Clark, had been in the cold waters off Chambers Island for 105 years. It was in excellent shape to have a roof over it but it went back into the water, with no root cover, and hopelessly decayed in 25 years by 1994. It was bulldozed to make room for a parking lot.

Click here to see a wood kiln.

Need Not Be Pressure-Preservatively Treated

Our Lady of Queen of Peace, Madison, WI
Korean Central Covenant, Northbrook, IL
Hope Chapel, Granger, IN
Hannahville High School Gym, Wilson, MI

Must Be Pressure Preservatively Treated

Two Hearted River Bridge, Luce Co., MI
Senator Road Bridge, Montcalm Co., MI
Richard Center Bridge, Richland Center, WI
Eagle River Bridge, Keweenaw Co., MI
St. Joseph Bell Tower, Madison, WI
Nebraska Public Power, Nebraska
Cookson, Manistique River, Schoolcraft Co., MI
Chicago Botanical Garden, North Brook, IL

May Be Treated or Untreated Dependant on Construction Details

Lewis University Pool, Romeoville, IL
University of Wisconsin Center, Marinette, WI
EMP Simulator Tower, Patuxent River, MD - Tower Members
Smith Rapids Covered Bridge, Chequamegon National Forsest, WI
MCM (Mine Countermeasure Ship), U.S. Navy
U.S.S. Constitution, Boston, MA
Replica of Captain Cook's Endeavour, Freemantle, Australia
Mast for Endeavour, Freemantle, Australia
U.S.S. Bittern (MHC 43) Minehunter Coastal
Aqualab III, SMTC (formerly MCH43), Portland, ME - another view.

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Version 1.02, 9 July 1997