The Architecture Billings Index, an indicator of U.S. non-residential building activity, rose in December, the American Institute of Architects (AIA) said on Wednesday.

The index rose to 52.2 in December from 50.9 in November, making it ten months that the index had risen in 2014.

A reading above 50 indicates an increase in billings.

“Particularly encouraging is the continued solid upturn in design activity at institutional firms, since public sector  facilities were the last nonresidential building project type to recover from the downturn,” AIA Chief Economist Kermit Baker said.

The index, based on a survey of U.S. architects, reflects the roughly nine to 12 months between architecture billings and construction spending.

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 December was 49.9, down from 54.9 in November.

The new projects inquiry index slipped to 58.2 in December from 58.8 in November.