Do the things you nail together sometimes fall apart. If they do, it may be because you don't understand all the variables in the holding power of nails. Let's take a quick look at how nails work.

You probably already know that the longer a nail is, the better it will hold. Increasing diameter will also increase holding power.

But nails will also hold much better when driven into sidegrain than when driven into end grain. In low-density woods such as pine, a nail driven into sidegrain may have twice as much holding power as a nail driven into end grain.

Try it yourself. Drive a nail into the end of a piece of pine, then pull it out. Repeat with the same size nail driven into the side of a pine board, and see how much harder it is to pull out.

There's a reason for this difference. When a nail is driven into sidegrain, it tears wood fibers, compressing them and bending them in the direction the nail is driven. These fibers try to recover, and this presses their ends against the nail.

None of this happens when a nail is driven into end grain, because the shaft of the nail runs parallel to the wood fibers.

Of course you can't always avoid driving nails into end grain. Many jobs require it. But whenever you must, you can increase holding power somewhat by driving the nails at an angle, so at least they intersect and compress the ends of some fibers. If possible, use a pair of nails at converging angles. This will help lock the joint together.

Another trick that will increase holding power is to prebore pilot holes for your nails. Use a bit equal to about 60 percent of the nail diameter if you are working with soft woods, or about 85 percent working hard woods.

Preboring increases strength in a number of ways. First, it allows the proper compression of the wood fibers without tearing them and destroying some of their elasticity. Second, it lets you use large nails without splitting. Larger nails hold better than small ones. Third, it can help assure better contact between the two pieces being joined.

Very often, when you drive a nail through one piece into another, the nail chips away splinters from the top piece of wood as it passes through. These splinters then prevents a good fit between the two pieces of wood, and the strength of the joint suffers. This is especially true if the joint is also being glued. Without good contact between mating parts, glue joints lose most of their strength.

The type of nail you choose can also affect holding power. If you want maximum strength, use nails with ringed or treated shafts. Remember, however, that nails of this type are difficult to remove later without damaging the surrounding wood.

A nail with a slim, tapered spear point will have more holding power than one with a blunter point. But it also will be more likely to split the wood. You'd think the opposite would be true, but a spear point forces fibers aside, and that type of wedging action is what caused splits.

That's why the old trick of tapping the tip of a nail with a hammer to blunt it will help prevent splitting. A blunt nail punches it own hole. If you use this trick, remember that it may help prevent splitting, but it decreased holding power. Preboring is a better way to avoid splitting. Squeaks & Leaks Q: I'm having trouble with carpeted stairs made of pine boards in a high-traffic area. They squeak. I've tried nailing small pieces of shingles under the steps, but now the squeaking seems to be worse. What can I do? A: As you probably know, the squeaking is caused by loose boards rubbing together. If you can determine which boards are loose and rubbing, curing the problem shouldn't be difficult. Have someone walk on the stairs while you check for moving boards from below. The movement is usually too slight to spot by eye, but you can feel it if you place your hands against the steps. Try to determined where the squeaks are coming from. The movement may come near the middle of a tread, or at the end.

If the squeak is near the middle of a tread, glue blocks will offer a quick and a simple fix. Cut some hardwood blocks about 1 1/2 inches square by three or four inches long. Take one and coat two adjacent edges with white or yellow glue. Press the block firmly into the joint between the squeaking tread and the riser beneath it. Fasten it first to the tread with a two-inch No. 10 screw. Then, while your helper stands on the tread to press it down against the riser, drive a second screw through the block and into the riser. Add another block in the same manner.

Glue blocks also will work if the movement is at the end of the tread.

You can increase the effectiveness of the repair by working some glue into the crack between the tread and the riser or stringer it rests upon. Spread the glue along the crack. Then force it into the joint with a thin putty knife or an artist's pallette knife. Have your helper step on and off the loose tread a few times, too. This will work the glue deeper into the joint. Then add your glue blocks. Q: My basement wall leaks at the top of the foundation on the east side of my home when the rain comes from that direction. The exterior of the house is brick and the mortar seems to be intact. Still, water gets in between the bricks and the top of the foundation. Since I am planning to finish the basement, is there anyway to stop this? A: There must be a crack between the foundation and the first course of bricks. Try running a bead of silicone masonry caulk along the joint. This stuff is gray and will blend in fairly well with the existing mortar. Let it set and then spray a hose against the problem wall and check for leaks. I'm betting you won't find any.