When her monthly rent was raised $41 last spring, 87-year-old Ruth Jackson knew it was time to think about moving from her two-bedroom apartment in Alexandria. The increase meant her rent was only $27 less than her entire Social Security income of $442 each month.

"I'd like to get into subsidized housing," Jackson said. "Then at least I'd know my feet were on the ground."

On a heavily shaded street in upper Northwest Washington, 80-year-old Nellie Thomas sits in her bedroom, surrounded by peeling paint and cracked walls. Her red brick apartment building is being sold. The tennants are trying to buy the building, but Thomas feels that it is time to move.

"I'm too old to think about buying," she said, "and I don't have anything to buy with. If I can get out, I want out as soon as I can . . . I'm just crying and paying."

There are at least 40,000 elderly people in the Washington area who need some kind of housing assistance, according to local government officials. Squeezed by rising costs for food, clothing and housing, trying to eke out their lives on Social Security payments, they are forced to live in substandard housing and to pay excessive amounts of their income for rent.

Usually the aged are a silent suffering army in the battle for decent, reasonably priced housing. When their rent is raised, they scrimp on food. When their utility bills and tax assessments go up, they cut back on medicine or clothing. When they get a notice to vacate their apartment, they move out and find whatever they can.

"It has almost become a disgrace to be old," says Barbara Schiller, president of the Montgomery County Tenants Association. "Where do the elderly go? They need to be given the right to live out the rest of their lives with a little dignity."

The Washington area's population of people aged 65 and older grew from about 172,000 in 1970 to nearly 202,000 in 1977. The median income of families headed by an elderly person was about $13,000 for homeowners and $7,700 for renters in 1977, according to U.S. census figures, and there is substantial evidence that many of the elderly here earn much less.

D. Richard Artis, executive director of the D.C. Office on Aging, said a survey in the city two years ago of persons aged at least 60 years showed that about a third earned less than $6,000 a year. Housing was one of the four foremost problems listed by the people surveyed.

To help the elderly, local governments have set up a number of services throughout the area. In the District, for example, the elderly can get visiting nurses or homemakers or special legal services through the city and voluntary agencies. About 550 elderly persons are served home-delivered meals each day and about 3,000 participate in a lunch program at 65 sites in the city.

But it is in housing that major problems often outpace a city's ability to keep up with needs. The Washington area has about 12,500 federally subsidized units set aside for the elderly, with money committed for an additional 1,250 according to the area office of the U.S. Department of Housing and Urban Development.

Waiting lists for these units vary, but waits of at least a year and sometimes two years are not unusual.

Waiting lists also are long for efficiencies and one-bedroom apartments -- the kind most needed by the elderly -- at other subsidized housing projects that house persons of all ages.

Officials who work with the aged say people like Ruth Jackson and Nellie Thomas, who cope with rent increases that eat up much of their monthly income, are common in this area.

Jackson showed a visitor a dozen jars of coffee in her kitchen cabinet. She pays for coffee and other household goods by using discount-coupons she clips from discarded newspapers in the laundry room of the building where she has lived for 13 years. For the last few months, she has used her savings to supplement her income. But her savings are nearly gone now.

For Thomas the monthly rent of $170 is more than half her monthly government fixed income of $300 a month.

"I have no bills but my telephone bill, my food, my hospital insurance and my medicine," she said. "I eat whatever I can get -- chicken backs and noodles, things like that, vegetables and fruits. I just have to make ends meet the best way I can."

Local officials say that condominium conversions are the cause of many of the housing problems faced by the elderly.

A recently released federal survey of condominium and cooperative conversions showed that the Washington area had the highest percentage in the United States of occupied rental housing units converted to condominium and cooperative ownership between 1970 and 1979.

The study by the U.S. Department of Housing and Urban Development found that one of every eight persons who moved from converted buildings nationally was elderly, and had an average annual income of less than $12,500.

Elderly persons were more likely to feel pressured by conversions than younger persons, the report said. Along with minorities and lower-income tenants, the elderly were most likely to report they were having trouble finding new housing.

Tears glistened in the eyes of one 88-year-old woman in Northwest Washington as she talked about her fear that she will have to move if her building is converted. "I moved here because I believed it would be my last settlement. I knew I couldn't possibly have too many more years to live," she said. "I certainly don't want to pick up an move. "It's a big headache for us old people."

Conversions have kept elderly people like John and Rosemary Roberts on the move.

Last spring the Robertses left the Bethesda apartment that had been their home since 1972 because it was being converted to condominiums. They couldn't afford to pay $57,000 to buy their apartment, and they turned down an offer to remain as renters for three years.

"I figured, it would be easier to move now when I'm almost 69 than when I'm 72," Mrs. Roberts said. "Dumb me."

They moved to a Silver Spring apartment on May 1, and installed wall-to-wall carpeting and wallpaper in the kitchen.

On May 24, as Mrs. Roberts was dusting her living room furniture her husband walked in with the midmorning mail. It's going condo, he told her.

"I felt sick. I stopped dusting right then," Mrs. Roberts said. "I sat down and cried. And when I got over the shcok, I got mad."

Mrs. Roberts said that when they move this time, it won't be to another Montgomery County apartment building.

"I'm sorely tempted to move to Delaware, or else get a mobile home near Rehoboth," Mrs. Roberts said. "I'm sorely tempted to get away from this rat race. I'm betwixt and between, and I'm not the only one caught in this squeeze."

Being forced to move can have a devastating impact on the elderly, and has led to death and serious stress-related illnesses such as strokes and high blood pressure, according to Leon Pastalan of the University of Michigan's gerontology institute.

Moving can be "a burdensome and dangerous experience and must be avoided whenever humanly possible," Pastalan testified during congressional hearings last May on the effect of conversions on the elderly.

Two local governments have condominium laws designed to help the elderly.

In Montgomery County, regulations require that once a developer send notice of this intent to convert to condomiums, persons aged 65 and older must be given 360 days to move. In addition, elderly tenants displaced by conversions may get up to $3,000 to help them pay rent at their new apartments. c

Under, legislation pending in the District, tenants at least 62 years old with incomes of less than $30,000 a year would be guaranteed rent-controlled tenancy for three years in buildings converted to condominiums or cooperatives, as long as the current housing crisis continues.

The anew law also would provide housing assistance payments to low-income tenants and relocation money for those who move.

The new law is expected to take effect in September. The D.C. City Council is expected to enact the measure on an emergency basis at its meeting tonight.