Seal or stain all exposed beams in the home. This reduces the chance that airborne grease and dirt will be absorbed into the wood. Rid these surfaces of loose dust and cobwebs with a vacuum brush mounted on extension tubes and attached to a backpack, shop or canister vacuum cleaner. Extension tubes are available at home centers and janitorial supply shops. While a dust mop may also be used, a vacuum cleaner will reduce dust “fallout” on surfaces below the beam.
After vacuuming, heavy accumulations of dirt and grease on beams with rough surfaces will mean employing methods that aren’t for the faint of heart, or for those who might tend to rush the work. The job sometimes must be done while standing on a ladder, and that means thinking safety.
Wear rubber gloves and eye protection and prepare a solution of foaming degreaser and water. Apply with a scrub brush and dry using a moistened terry-cloth towel. Protect floors and furniture with drop cloths or plastic, but be careful when using plastic sheets, which will get very slippery when wet. Stained, rough wooden beams have a little bit of protection against moisture; even so, a foaming degreaser will help keep absorption to a minimum. Use a scrub brush to gently clean a small section of the beam at a time, being careful to maintain balance and footing. Immediately wipe each cleaned section with a moist terry towel to rinse. You'll have to get off the ladder and rinse out the towel after using it several times.
When working on any ladder, don’t extend beyond arm’s reach on either side — move the ladder instead. Be sure to place an extension ladder far enough from the adjacent wall to reduce chances of falling backward while working. Federal safety guidelines call for a minimum 1:4 ratio when placing extension ladders — one foot or more away from the wall for every four feet up the wall. In other words, if the top of the ladder is to be 12 feet off the floor, it should be placed at least 3 feet from the wall.
Reference:
Don Aslett in The Cleaning Encyclopedia: Your A to Z Illustrated Guide to Cleaning Like the Pros.