Last month’s slowdown, following an upward turn in December, could be attributed to the winter weather, according to AIA chief economist Kermit Baker.
The index fell to 49.9 in January from 52.6 in December. A reading above 50 indicates an increase in billings.
The index, based on a survey of U.S. architects, reflects the roughly nine to 12 months between architecture billings and construction spending.
"This easing in demand for design services is a bit of a surprise given the overall strength of the market over the past nine months," AIA Chief Economist Kermit Baker said.
Industrial companies and their investors track the monthly index as an indicator of future demand for machinery and components used to construct buildings.
The score for an indicator to measure trends in new design contracts at architecture firms in January was 51.3.
The new projects inquiry index rose to 58.7 in January from 51.3 in December.