A McLean firm is launching a nationwide computerized information service designed to facilitate the exchange of materials among companies in the construction and building industry.

According to Robert H. Walker, vice president of Exchange Technologies Inc., construction projects use many temporary items, are left with unneeded materials when plans change and produce significant quantities of scrap--all of which could be valuable to other builders. Leftover items can range from bolts and studs to trucks and cranes.

The information service, called Construction Exchange, will provide "an electronic marketplace" in which buyers and sellers of these materials can be matched, Walker said.

Construction Exchange will operate on a toll-free "800" telephone number. Companies with materials for sale can list them at a monthly rate of $30 per item per location. Potential buyers pay no charge.

A location is considered the first three digits of the company's ZIP code. For example, anyone seeking to buy scaffolding at a given location can determine by calling Construction Exchange whether it is available there.

This is how it would work: A firm that has dug a hole at a site might have tons of fill dirt. Another company may have a hole to fill. A match can be made quickly by computer.

Or take scaffolding: It may be cost less for a builder to sell the scaffolding used on a site to someone who will pick it up, than to ship it back to his yard to store for the next project. "It's a way of rotating useful materials that have outlived their purpose at a specific work site," Walker said.

Until now, the only way buyers and sellers in the construction trade could find each other has been by word of mouth or through advertising.

Walker contends that newspaper advertising is too local, too general and too limited, while trade journal ads cover too wide a geographic area, are too expensive and are too slow.

"It could be very quick," he said. "Someone could list in the morning, and someone could call in the afternoon. "More efficient use, re-use and conservation of resources, and timely matching of buyers and sellers of temporary-use equipment and materials . . . can be extremely cost-effective."

Construction projects typically operate on a profit margin of 5 to 15 percent, he said. The value of temporary-use items and leftover or unused materials also is about 5 to 15 percent of the project cost. An increase of 10 percent in the sale price of these items would mean a 10 percent increase in profits, he said. In addition, buying used items rather than buying or renting new ones also can lead to substantial savings, he added.

"We think that once we have established our identity and people understand the simplicity of the system, we will generate a lot of requests."

While access to Construction Exchange will be available only by telephone at first , Walker foresees a computer-to-computer information system. "There will come a time very soon when you will see a computer terminal in every construction trailer with access to data bases," he said.

Construction Exchange is the joint effort of Walker, a graphic artist and designer, and company president Farrokh Kanga, an engineer.