Mortgage rates on apartments and commercial developments moved upward in April for the fifth time in the past six months.

Citicorp Real Estate, Inc., reported that its monthly survey showed increases ranged from less than 1/8th percent on apartment mortgages to 1/4 percent on mortgages secured by strong tenant credit. Rates on speculative office and industrial building or shopping centers without a dominating tenant increased by only 1/8 percent.

Philip Kozloff, president of CRE, said that the trend reflects a need for mortgage rates to remain competitive with rising corporate bond yields.

"There is no shortage of mortgage funds," he said. "For one thing, this year's life insurance company income-mortgage allotments will be at least 15 percent higher than last year's record, in the range of $6.5 to $7 billion."

The upward trend on mortgage rates was particulary strong in Houston, which has had a big year in new building construction and housing.

Meanwhile, Federal Home Loan Bank Board chairman Robert H. McKinney told a group of home mortgage insurance executives here that demand for single-family houses will continue at nearly the record-high rate (1.48 million) set in 1977.

However, McKinney predicted that the net savings flow into savings and loan institutions this year will be $37 billion, compared with $50 billion last year. He also said that heavy demand for credit would place upward pressure on market interest rates, forcing savings and loan associations to seek external sources for funds. S&Ls usually borrow on a short-term basis from the FHLBB.

McKinney said that the bank board must have the ability to keep S&Ls reasonalbly confident of a supply of funds so that credit rationing, which occurred in 1974, will not be repeated.