Corrosion of Roof Truss Gusset Plates in Farm Buildings

Agdex#:	714
Publication Date:	02/94
Order#:	94-035
Last Reviewed:	02/94
History:	Original Factsheet
Written by:	J. Johnson - Engineer (Structural Design)/OMAFRA

Table of Contents

1. Introduction
2. Causes of Corrosion
3. Prevention of Corrosion
4. Repair Recommendations
5. Summary

Introduction

Deterioration of metal truss gusset plates is a major concern in buildings that contain high humidity and corrosive environments. Normal galvanized steel plates exposed to moisture, condensation and ventilation air containing manure gases, will corrode rapidly. Many of these buildings show severe corrosion within five to ten years. This corrosion can weaken the building and could potentially lead to structural failure.

Truss plates are light gauge metal plates used to connect prefabricated wood trusses. Truss plates are produced by punching light gauge galvanized steel (normally 16, 18 or 20 gauge) so teeth protrude from one side as shown in Figure 1. The truss plates are galvanized prior to punching, leaving numerous unprotected metal edges. During truss fabrication, these truss plates are pressed into the lumber with either a hydraulic press or a roller to fully embed the teeth.

Figure 1. A Typical Truss Plate

The buildings most affected by this corrosion are cold, naturally ventilated beef and dairy barns with slatted floors and deep manure storages. Also affected are warm, naturally ventilated, swine barns. In all cases, the roof trusses have been included in the building’s environmental air space. This exposes the entire truss assembly to a potentially wet service condition. In most cases, farm trusses are designed for a dry service condition.

The truss plates show the greatest deterioration near the building air exchange openings - typically at the heel (Figure 2) and peak joints of the truss. These are the areas of greatest air mixing and temperature change, producing high humidity and condensation problems. Unfortunately, these are also very critical joints in the structural integrity of the truss.

Figure 2. Moderate corrosion has occurred at the heel connection of the trusses in this naturally ventilated swine barn.

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Causes of Corrosion

There are many potential causes of corrosion in animal buildings. Animals exhale large quantities of moisture into the air. This creates high relative humidity in the building if the moisture is not properly vented. High humidity increases the potential for condensation which wets the entire truss assembly.

Ammonia gas, typically found in animal environments combines readily with this moisture and becomes ammonium hydroxide. This chemical attacks most metal surfaces. Free moisture on poorly protected steel will also initiate the rusting process. The moist wood accelerates the corrosion of the metal fasteners since wood itself is slightly acidic. In addition, long term wetness can raise the wood moisture content above 30% and accelerate wood decay.

Dust, commonly found in animal environments, provides a surface on which acids and gases can react, significantly accelerating the rate of corrosion. Common bacterial colonies found in barns tend to form biofilms on building and equipment surfaces, allowing bacterial growth and the production of other corrosive acids.

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Prevention of Corrosion

A) Properly Ventilate the Building

A good ventilation system should move enough fresh air through the building to reduce the levels of moisture, gas and dust to acceptable levels. A well designed system will minimize corrosion problems. Proper ventilation requires good building design and good ventilation management. A ventilation specialist, equipment supplier or building contractor can help to ensure that ventilation is not contributing to the corrosion problem.

Often, an owner tightens up a barn to raise the building temperature or to save on supplemental heat. Unfortunately, this reduces the ventilation rate and allows the humidity level to increase. For fan ventilated barns, it is important to keep at least one exhaust fan operating to constantly remove the respired moisture. Similarly, naturally ventilated barns require a constant exchange of air to control moisture. Additionally, barns with continuous ridge ventilation provide a condensing surface at every peak truss connection along the barn. Intermittent chimneys constructed between the trusses keeps this ventilation air away from the truss plates and reduces the potential for corrosion.

B) Apply a Protective Coating to the Metal Truss Plates

A protective coating can be applied to the plates by brush either before or after truss installation. This coating must cover each metal plate including its edges (Figure 3). One of the recommended epoxy coatings which is lead and chromate free is the following:

Epoxy-Polyamide Primer and Topcoat. (SSPC¹ Paint No. 22 or CGSB² Paint No. 1-GP-146)

This epoxy paint is a two component product that requires specific mixing and application expertise. It is recommended that the plates be prepared with a cleaning solvent or by sandblasting prior to coating. This task could be undertaken by a commercial painter, building contractor, or the owner. It will be a labour intensive job, that will only be as effective as the quality of workmanship employed. Your truss manufacturer or industrial paint supplier can help you find this coating material or an equivalent product.

Figure 3. These metal truss plates were coated by brush at the job site.

C) Use Pre-coated or Stainless Steel Truss Plates

Various pre-coated and stainless steel truss plates are available, however, they are expensive. Coated truss plates cost almost five times the price of standard G90 galvanized plates. They must be larger than standard plates because of their slippery surface. Stainless steel plates can cost almost ten times the price of standard plates. This makes a significant difference in the cost of the truss.

D) Install Ceiling with Insulation and a Vapour Barrier

If the truss assembly is completely partitioned out of the animal environment, it will not be influenced by the high humidity condition and related problems. A 4 or 6 mil polyethylene vapour barrier is necessary to prevent moisture migration into the attic space. Steel or plywood ceilings by themselves do not provide an adequate vapour barrier. The sheet seams, fasteners and material porosity will allow moisture to pass through. A minimum of ceiling insulation is also required to prevent condensation from occurring on the ceiling surface itself. It is still possible to accommodate natural ventilation by installing exhaust chimneys through the attic space between the trusses as shown in Figure 4. Consult a ventilation designer for the correct number and size of chimneys required.

Note: ¹SSPC refers to the Steel Structures Painting Council (U.S.)
²CGSB refers to the Canadian Government Specifications Board

Figure 4. A naturally ventilated barn with chimneys through the attic space.

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Repair Recommendations

• If the truss deterioration is minor, employ one or more of the prevention procedures outlined above to halt further corrosion.
• If the deterioration is more severe, a structural repair should be considered. A consulting engineer should be approached for an assessment and repair specification. The Ontario Building Code requires roof systems to be engineered.
  

Summary

While the extent of the truss deterioration problem is not known, it is a very serious situation for those buildings affected. Owners should inspect their buildings periodically for signs of wetness and corrosion on the truss plates. A repair should be initiated if necessary. A building contractor, truss manufacturer or engineering consultant can assist you. It is anticipated that the building repair or retrofit expenditure undertaken now will be cheaper than replacing the structure prematurely.

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