MARY DOWD, Arlington

A. PVC (vinyl) fencing is new, so it has no track record, although I am sure it has been adequately tested. I don't think it will crack, since it is more stable than wood and resists excessive expansion and contraction with the weather. It is put together loosely to allow for such movement, just as vinyl siding is loosely nailed to allow for it. But we won't know for sure until the fence goes through several seasons.

Good fences make good neighbors, saith the bard, although the motives for putting up a fence vary widely.

Some fences such as stockade, cyclone and other chain-link fences, sometimes topped with intimiading razor wire, say ``Keep out!'' and are designed to keep undesirables (dogs, cats and maybe even neighbors) out, and desirables in.

Such fences are put up for privacy, although they do reflect a fortress mentality and are not a nice thing to look at.

But most fences are indeed, neighborly, set on the line simply to divide the properties. Low (four feet) picket fences are ideal for this purpose. For privacy without intimidation, a four-foot picket topped by lattice will do nicely and still keep most undesirables out and desirables in.
They also provide a good background for growing plants and shrubs. And your neighbor can do the same.

Whenever you put up a border fence, build it a tad back from the line, and pay for all of it. Sharing a boundary fence with your neighbor can get sticky over time, esepcially if he moves away. There are an infinite number of styles out there. Cedar is the preferred wood, while pressure-treated lumber is often recommended for posts and rails.

Finishing? Why not leave it alone, as a fence should be. Both woods -- cedar and pressure-treated -- will weather nicely without decaying, and absolutey no maintenance is needed. Even spruce, often used in economy stockade fences, weathers well.

And, any wood that gets wet but is able to dry out between wettings, will last indefinitely.

Fences cost $10 and up per linear foot; about half that if you buy the materials (at any fence builder) and put it up yourself.

A big aid to putting up a fence these days are Post Ups, a steel stake 24, 30, and 36 inches long and topped by a steel box big enough to hold a 4 x 4 post. Such stakesd are cross-shaped and easy to drive into the ground. Secure the post and you're home free.

You can bury the posts instead, but if you do, make sure they are pressure-treated, and do not put any posts in concrete; the joint between concrete and post, no matter how tight and caulked, is a passageway for water to get in and stay in.

The Post Ups are fairly easy to drive if you don't run into any stones. Sometimes the driven stake will bypass a stone, although it might slope a little, even twist. Avoid this by relocating the stake.

If you hit a rock, pull up the Post Up and try to dig up the rock. Avoid driving the stake come hell or high water, trying to break through the stone or circumvent it; it almost never works, and you will end up with a stake that has a perfect U shape at its point. The handyman has one such bent stake in his trophy case.

Building your own fence not only saves money but allows you to make your own design and, and just as important, allows a random spacing of posts, which may be required, depending on how many stones you hit or the lay of the land. However random you make the posts, don't space them more than 10 feet apart; eight is better.

If you build a fence up or down a slope, do not slope the fence itself; it will look wrong. All posts should be plumb, and each section of rails level, so you will have to step the fence, setting one section level at one height, and the second also level at a higher or lower height, and so on. Fence designs are infinite. Examples:

Pickets spaced half an inch on rails for air circulation and some privacy. You can buy pickets up to six feet long, with tops that are square, fleur de lis, plain, curved, or Gothic. The curved tops are better than square ones because they shed more water. Or, cover the square tops with a rail. You can gussy up the fence by topping the posts with decorative tops: urns, balls, acorns, spikes and other interesting forms. Set 2 x 4 rails between posts with joist hangers. In fact, there are quite a variety of hardware sold in lumber stores, specifically designed for fencing.
A design variation is a two-foot-high strip of pressure-treated lattice on top of four-foot pickets, giving you six feet of good looks and privacy. Six feet is usually the maximum height for a fence. Check with your community for height limits and other restrictions.

Then there are board-on-board fences, with a vertical 1 x 6 or 1 x 8 on one side, spaced six or seven inches by similar boards covering the gap. This gives nearly total privacy but allows air circulation.

Getting any fancier increases the cost, except for a picket fence with a catenary curve or arc, a so-called natural curve. Catenary is from the Latin word ``katena,'' meaning ``chain.'' To achieve a catenary curve, suspend a chain from two posts; it will hang naturally. It will be a natural curve whether you make it long and shallow or short and steep. Once this curve is established, transfer it to the pickets all in a row and cut them to match.

One more point:

A long, straight fence looks nice, but may tend to lean over after a few years. To avoid this, and to give the fence a sculptured, architectural look, set the fence in a series of jigs and jogs: Set one section straight and level; set another to turn at right angles for one to two feet, then a straight line, and another jog back to the original plane of the fence.

If you have to paint it, don't. Instead, use a stain: a solid color latex stain will give you a good white color; two coats are neded, but no primer. All you have to worry about is cedar bleed.

Or, use a semitransparent stain. One coat is needed and you can recoat in five years without having to scrape and sand. Do fence me in!

Q. I plan to install a cedar fence, but there is evidence of termites in the area. The posts will be pressure-treated; can I use presssure-treated pickets as well?

MAUREEN FEVERMAN, Winchester

A. Yes, and let them weather. No need to paint or stain. Or, you can use the cedar pickets anyway, because termites are less likely to invade thin boards such as pickets that are above ground. Whatever you use for pickets, I suggest you use pressure-treated rails that the pickets will be nailed on. Cedar pickets set on rails in this fashion will last indefinitely, because they are not in contact with the ground, and will dry out after each wetting. What causes decay is contact with the ground and consistent wetness.

Q. There are two things I want to ask you: I put up 325 feet of a white cedar fence, and plan to stain it with a semitransparent stain to match the house and trim. How long should I wait before staining? The other thing is, I get a bee or some buzzing insect in my bathroom about once a week. I think they are coming in through the exhaust fan vent. How can I keep them out?

ANTHONY HURLEY, Georgetown

A. No need to wait; the wood is cured and will take the stain very well. In fact, semitransparent stain is the best way to treat a fence, other than not treating it at all and letting it weather. As for the winged intruder, when the insect comes in, close the door and turn on the fan; with luck the insect will leave the same way it came in. To keep insects out, put a screen either just in front of the fan or at the other end of the duct where it discharges outdoors.

Q. I am having a decorative fence made with red cedar, but the builder wants to make the balusters of ash. Although I am not getting what I ordered, would ash be OK to use. I plan to paint the fence.

EILEEN McEACHERN, Hampton, N.H.

A. If you can agree on the desire to use ash instead of red cedar, go ahead with it. I think the ash balusters will work well. Instead of painting the fence, I suggest you use two coats of a latex solid color stain.

Q. I'm building a fence, and wonder how to level each section. Would a string level be OK?

JOE ZERMANI, Rockport

A. So far you're on the right track, in wanting to keep a fence level. Any fence that follows the contours of the property will look dumb. If your property slopes, step the fence, making each section at a different height, but level.

You can use a string level, a little gizmo with hooks that attach to a taut twine. Buy surveyor's twine, a tough twine that pulls quite tight. You can't get it not to sag a little, but this may not be important when leveling a rigid board. A regular level, such as a four-foot mason's level, will be more accurate. You can use the top rail of a fence section as a level by taping the level on it.