When homeowners or apartment dwellers think of installing their own vinyl floor covering they invariably start shopping around for tiles of some kind.

Sheet vinyl flooring that comes in wide seamless rolls has always been considered beyond the ability of the average do-it-yourself to install. Even though sheet flooring offers distinct advantages, the added expenses of having it installed professional often rules it out.

However, two new sheet vinyl flooring materials, introduced by the Armstrong Cork Co., have changed all this; they are so much quicker and easier to install that they can be put down easily by any home handyman or handywoman who can work with a sharp knife, a metal yardstick or straight-edge and a staple gun.

The first product, Treadway was specifically designed with the do-it-yourselfer in mind. It is cushion-back vinyl flooring material so flexible and easy to handle that it can actually be folded like carpet without damage to it. The second, Premier Sundial, is a higher priced version of the same type of very flexible, elastic sheet vinyl flooring, except that it has a no-wax type finish and a thicker chushion backing for added comfort and resiliency. Treadway sells for about $8 a yard; Premier Sundial costs about $13 a yard.

Both these sheet materials come in rolls either 6 or 12 feet wide (the 12-foot wide eliminates the need for seams in most rooms), and both offer all the usual advantages of sheet flooring, as well as the extra advantages of easy handling and simplified installation techniques. Like other sheet materials, these materials form a seamless floor, or at worst, one with only one or two seams, so there is no problem with dirt accumulating in seams, or with individual tiles lifting.

Also, the design flows in a continuous pattern from wall to wall, and unlike old-fashioned linoleum floor coverings (which are no longer made), they can be installed over any floor, above, below, or on grade.

Nither of these new Armstrong products have to be cemented down over the entire floor. Thus a messy and time-consuming job spreading adhesive over large areas with a notched trowel or paint brush is eliminated. The material is unrolled on the floor, trimmed to size around the perimeter, then fastened down around the edges with a staple gun (a special adhesive is available for the edges where a staple gun cannot be used).

The flexibiltiy and elasticity of these products also offers another advantage to the amateur installer - it is "forgiving" of slight errors when trimmed to size around the edges. If a bit too much is trimmed, the vinyl can actually be stretched to fit, like carpet. If you trim a bit too large and then fasten down around the edges, the slight slack or small wrinkles that remain will gradually disappear as the flooring settles down and contracts slightyly.

Another work-saving characteristic of this type vinyl flooring is that it is capable of bridging minor irregularities in the flooring over which it is applied - small cracks and holes or open seams between old tiles, for example. It can be applied over almost any type of existing floor: wood, concrete, ceramic tile, marble, and old resilient flooring materials (except other cushioned vinyls).

And unlike tiles, sheet flooring will not be affected by wood subfloors that shrink or expand. A tile floor may show open seams when this happens, but Treadway and Premier Sundial stay tight by moving slightly with the sunfloor.

To install this material, start by unrolling the vinyl and allowing the excess to lap up onto the walls. Make vertical cuts in each corner with a sharp utility knife so the material will lay flat as possible, then trim along one wall at a time (if there is a molding along the floor, it is usually easier if this is pried up first).

To trim the excess off, use a metal straightedge as a guide. Press this into the joint formed where the wall or baseboard meets the floor, then trim off the excess by cutting along the inside of the crease formed by first pushing the point of the blade through the material so it goes into the corner at a slight angle, then continuing the cut while using the straighedge as a guide.

After the flooring material has been trimmed to size around all edges, the perimeter should be fastened down within two hours - fastening within this time period makes use of the vinyl's capability to contract slightly and correct any mistakes in fitting. The edges can be fastened down with an ordinary staple gun, or with a special adhesive sold for this purpose. Stapling is the fastest and easiest method.

The adhesive is used where the staple gun cannot reach (under the toe space of a kitchen cabinet, for example), or where the subfloor will not accept staples (in concrete, for example). A special dispenser with an applicator head that spreads five ribbons of adhesive simultaneously is used for cementing the edges.

Where seaming is required because the room is too wide for one sheet of the vinyl, the seam should be formed before the vinyl is trimmed to fit around the perimeter.

To do this, one piece of material is unrolled on the floor first and positioned. Its edge is then folded back and a strip of double-stick carpet tape put down along the edge line (draw a line with a pencil first). The second sheet of vinyl is then put down so it overlaps the first piece slightly, after which a straightedge and knife are used to trim through both pieces at once, thus assuring two edges that will match perfectly.

The two cut edges are then folded back again so the paper facing can be removed from the tape, after which the pieces are folded back down against the floor so the tape holds them in place. The seam is then completed by using a special seam sealing cement that bonds the two pieces together permanently to form a continuous seamless floor.