Anyone who lives on a tropical island knows that salt air and high humidity can make metals corrode faster than they would in a more temperate climate.
But since January 2004, there has been another potential source of corrosion: the chemicals used in pressure-treated lumber and plywood.
Metals that could be affected include the screws and nails, hurricane straps and other connectors that hold pieces of wood together.
More than 90 people in the construction industry attended a lunch presentation titled ‘Connector Issues for Pressure Treated Wood’ last Friday at the Wharf Restaurant.
The lunch was hosted by A. L. Thompson Home Depot, with invitations sent out by the Building Control Unit of government’s Planning Department.
Guests heard from Brad Lambert, who described corrosion tests recently carried out by the company he works for – Simpson Strong-Tie.
‘We’re not trying to sell you anything,’ Mr. Lambert told the audience.
Instead, he said, he was offering serious information that was easy to understand. The relevance of the address was reflected by the number of people who remained to ask questions or exchange opinions.
Developments in the construction industry are of interest to building professionals any time, while Cayman’s ongoing post-Ivan reconstruction has heightened awareness of connector products and techniques.
Chemicals in wood
Mr. Lambert explained that, for years, pressure-treated wood had been produced through use of chromated copper arsenate, which protects wood against insects and fungal decay.
US concerns arose that the arsenic in the formula was dangerous. Alternative chemical treatments were created for residential use.
Some contain different ingredients or greater quantities, making them more corrosive to metal than the old arsenate formula.
One wood-pressure treatment, for example, contains ammonia, which reacts with aluminium. Another formula has four times the amount of copper.
Still another problem arises from wood treated with a chemical that is less corrosive to metal, but which leaches into water, making it unsuitable if exposed to rain.
Stainless steel connectors and fasteners or those that are hot-dip galvanised or mechanically galvanised offer resistance to corrosion.
Mr. Lambert warned that metals should not be mixed. Stainless steel is more expensive than other products, creating a temptation to use galvanised fasteners, for example, with a stainless-steel connector. The reaction between the two metals, however, causes faster corrosion.
Advice to the lay person
While professionals discussed barrier membranes and retrofitting, Mr. Lambert did not ignore ordinary homeowners and do-it-yourselfers.
‘You should be asking the person selling the wood what kind of fasteners to use,’ he said.
Pressure-treated lumber will usually have a tag with the manufacturer’s specifications, he said.
Persons who have engaged a builder should discuss fasteners and connectors with the contractor.
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