MANY older houses feature outdoor basement entrances covered by sloping wooden doors that heavy loads of snow and prolonged dampness eventually destroy. Even if your doors survived the winter, you can virtually rid yourself of a maintenance task by replacing them with metal doors.

Building supply stores and large home centers carry or can order basement door kits. Costs average $200 to $300 depending on size and on whether the kit includes sloping metal side walls that support the door. If the existing entrance well has masonry side walls in good condition, metal side walls are not needed. The largest maker of basement doors is the Bilco Company in New Haven (203) 934-6363. The Gordon Corporation of Southington, Conn., (800) 333-4564, makes kits without side walls.

To determine the size of door to buy, get a brochure from a dealer and follow the instructions for measuring the entrance well. Usually, this involves measuring the inside length and width of the well and the height of the doorway into the basement.

A kit without side walls is usually easier to install. First remove the wooden doors and their hinges, then fill the holes left in the side walls with a concrete repair compound.

If the entrance well is too wide or too narrow to accommodate the new doors, or if the side walls are made of brick or mortared stones, the next step is to fasten flat strips of metal called foundation plates to the tops of the side walls. These are supplied with the kit.

Saw the plates to length with either an electric circular saw fitted with a metal-cutting abrasive disk or a saber saw equipped with a hacksaw blade. Cut across the ends that will fit against the house, so they can be concealed with caulking compound for a neat appearance.

After sawing, drill three or four evenly spaced holes about a quarter of an inch in diameter through each plate. Position the plates on the side walls as they should go, then use the holes as guides for marking the surfaces of the side walls beneath them.

Remove the plates, drill holes into the side walls with an electric drill and a carbide-tipped masonry bit, and then fasten the plates to the side walls with masonry anchors. When drilling into brick or stone side walls, be sure the holes enter mortar joints.

Whether or not foundation plates are used, the next step is to assemble the door frame and mount the doors. Arrange the pieces of the frame on top of the side walls or foundation plates and fasten them together with the bolts provided in the kit.

A part of the frame called the header is fastened against the house wall above the entrance well. On a house with siding, use screws that penetrate the sheathing underneath the siding to attach the header. If the wall is masonry, fasten the header with masonry anchors, installed as described above.

Attach the doors and their hardware to the frames by following the kit's instructions. With the doors closed, adjust the frame by pushing against the sides and corners until the doors operate without binding. In this position the bottom edges of both doors should form a straight line.

Drill holes into the side walls or foundation plates, using the prepunched holes in the door frame as guides. If plates are not used, fasten the frame to the side walls with masonry anchors. If plates are used, fasten the frame to them with self-tapping screws or with small bolts, lock washers and nuts, provided the undersides of the plates are accessible.

Finally, apply caulking compound to all seams where parts of the door frame or foundation plates meet the side walls and house. Complete the project by installing any remaining hardware.

Door kits with metal side walls are chiefly designed for new construction where they can be fastened with masonry anchors to the flat upper edges of precast entrance wells installed as part of the foundation.

These kits can also be used to replace existing basement doors whose side walls are crumbling. This is achieved by demolishing the existing side walls to a height about level with the ground and then capping them with a smooth layer of concrete about two inches thick.

The process sounds more difficult than it often is. Deteriorated brick or stone masonry can usually be dismantled with a small sledge hammer; demolishing concrete requires initial blows with a heavy sledge, but once the concrete is cracked it can be broken away fairly easily with lighter blows.

Building a form for pouring the concrete is probably the most challenging step, but the key is to work methodically, lining the upper edge of the entrance well on the inside and outside with strips of lumber or plywood. These are braced on the outside by stakes driven into the ground and on the inside by horizontal boards spanning the well.

Detailed instructions for building forms and for pouring and smoothing concrete are supplied with door kits. You will probably need about three bags of premixed concrete (to which you must add water). A bag typically contains one cubic foot.

Some doors are assembled and then carefully positioned and braced in place before the concrete is poured. With others, the door is assembled and fastened with masonry anchors to the fresh surface after it has hardened.

On walls with siding, a rectangular section above the basement doorway must be cut away so that the rear edges of the side walls can be pushed against the house and sealed.

After mounting any remaining hardware, use caulking compound to seal all seams where the side walls and door frame meet the house and entrance well.

Some doors are supplied already painted; others come with only primer applied. To paint doors that are primed, apply one or two coats of alkyd-base metal enamel inside and out.

After installing any door kit, test it for leaks by spraying it with a garden hose. Aim the nozzle upward so the spray falls like rain.

Diagrams: Metal side walls are an option on replacement doors. Foundation plates allow for adjustments. Assemble the door frame on the top of the side walls or foundation plates, and attach the header to the house (A). Next, install and adjust the doors to fit the frame (B). Then, fasten the door frame to the existing side walls (C). Build a form for concrete cap. Horizontal boards can brace inside strips and ground stakes outside. Apply caulking around the finished doors. Test seams for tightness by spraying them with a garden hose. (Ed Lipinski)