DEAR SAM: Our house is a split level on a cement slab. A year ago, we had an invasion of large black ants upstairs and downstairs. I painted all the base-boards in every room with Johnson's No-Roach and they disappeared. This winter I found a few live ones in our utility room and painted the joints of walls and floors. Now I find several dead ants almost every day in that room.

Today I found two dead ones - with wings - but I feel sure they're ants, as the bodies are divided into three sections. (I understand that termites have no waistline.) There is no evidence of sawdust or other destruction. What would you recommend?

ANSWER: You are correct about the difference between ants and the more malicious termites; also, the ant's four wings are unequal in size and transparent, compared with four equal-size translucent wings for termites. Remember, too, that termites leave telltale tunnels on concrete foundations leading to the wood sills. Former hideouts may be located by poking into the sills.

You have taken some good measures in painting the baseboards where they appeared. The recent dead ones must have escaped the first painting.

In the spring, they sometimes appear again in search of food - at the sink and refrigerator. Be sure that you paint the door thresholds on both floors. For further new insecticides, check with your hardware store or write to the Department of Agriculture in Washington.

DEAR SAM: I am planning to purchase a new home from a building company that does not use solid wood floor joists but a new type of "truss," composed of 2x4's held together my metal supports, like inverted V's throughout the length. My question is whether these "trusses" are sufficiently solid, have been tested for load-bearing capability, and have passed the test of time for durability.

ANSWER: While I don't know about these trusses, the building company no doubt has the approval of your juridiction's building inpsectors to use them. The architect who incorporated them in the house plans must be knowledgeable concerning their load bearing capability and durability.

There is no harm, however, in making your own inquiry from the local building inspectors. The high cost of lumber has resulted in the increased use of prefabricated roof trusses. However the floor trusses you describe won't save much money for the builder unless they are spaced 24 inches apart instead of the usual 16.

If you prefer the usual wood joists, make you wishes known to the builider. He may be agreeable to the change, with or without additional cost. CAPTION: Illustration, The ant: spring invader.