5. Using the wrong paint. One of the biggest DIY projects around, painting can make a place look great. Manfredini says flat paint should only be used for ceilings because it's usually not as washable as paints with an eggshell or satin finish. On outdoor decks, "sun and rain tear the heck out of the wood," he says. Clear sealers don't block the UV rays, and they peel. Use a linseed-oil-based stain -- it drives the pigment into the wood and preserves it.
6. Improper preparation of walls for painting. A good, quality paint job is 90-percent preparation, Manfredini says. Clean the walls, sand them and patch any holes before you paint. A coat of primer or stain blocker is advisable if you're trying to cover over oil-based paint, stains or peeling paint, or if you're painting a lighter color over a darker color.
7. Unsafe job conditions. Nothing diminishes your return on investment like a trip to the emergency room. Wear safety goggles when using power tools or working with drywall or wood; wear hard hats when you're working under other people on scaffolding; and open some windows when you're painting or staining, or stripping old finishes off of floors or walls, Del Grande says. And don't wear loose-hanging clothing, especially when using power tools. Wear gloves when carrying wood, metal and rock, or when hammering, and wear a nail or tool pouch to prevent damage to your floors and more importantly, the feet of people and pets.
8. Inaccuracy. Successful DIYers live by this rule: Measure twice, cut once. It's so important for things like building walls, hanging drywall or cutting baseboards, countertops or pipe. If you're going to err, err on the side of too long. You can always make something shorter; you can't make it longer. Spackle can cover only up to a 1/8-inch seam.
9. Working beyond your limits. Everybody has them. Del Grande won't work on a roof; yours might be plumbing or electrical work. Don't stand on the top steps of ladders, and don't try to work beyond your reach. Ladder accidents send more than 164,000 people to the emergency room every year, according to the U.S. Consumer Product Safety Commission.
10. Failure to get a clue. You don't want to start to learn how to do a project on your own house. If you have a friend who is a contractor or an experienced DIYer, offer your assistance on one of his projects so you can learn. No one will turn away free labor. If you need to remove a supporting wall, have an engineer look at it to see what kind of beam you need to replace it. "If you have a saw in your hand and have a question about what you're doing," Del Grande says, "stop. Follow that little voice in your head."