CSR conceded its takeover of Viridian may have been a dud, after the struggling glass division dragged down first-half earnings at the building products maker.

Viridian Glass posted a $6.9 million first-half loss as the company as a whole reported a 21 per cent drop in earnings before interest and tax (EBIT) to $92.8million.

However, CSR posted a net profit from continuing operations (after significant items) of $34.9million for the six months to September 30, up 13 per cent from the previous corresponding period.

The company's shares were up 5c at $2.41 yesterday.

Chief executive Rob Sindel blamed weak residential and commercial construction sectors for the weaker earnings. Mr Sindel expressed frustration with Viridien, which was bought for a much-criticised price of $690 million in 2007.

When asked if it was a poor purchase and whether extraneous factors were to blame - Mr Sindel, who was not chief executive when it was bought - said ''probably a bit of both''.

The company recently laid off 110 Viridien workers.

The company declared a fully franked interim dividend of 6c a share.