* Offering proceeds to retire debt

* Shares fall almost 4 percent in after hours trading

HOUSTON, Sept 21 (Reuters) - Louisiana Pacific Corp (LPX.N: Quote, Profile, Research), which makes building materials said on Monday it plans a public offering of 18 million shares to retire debt and said it expects operating results to improve compared with a year ago.

Shares of Louisiana Pacific fell nearly 4 percent to $6.95 in trading after the New York Stock Exchange close.

The Nashville company said it granted underwriters a 30-day option to buy up to 2.7 million additional shares.

Funds from the public offering will be used to retire up to 35 percent of its senior secured notes due 2017 and for general purposes.

In the third quarter, the company said it expects an operating loss of $8 million to $10 million in its oriented strand board unit and a loss of $8 million to $10 in its engineered wood products unit. In its siding unit its expects to have operating income of $14 million to $16 million.

In the year ago period the company had an operating loss of $27.7 million in it oriented strand board unit and an operating loss of $11 million in its engineered wood products unit. Its siding unit had operating income of $4.6 million. (Reporting by Anna Driver in Houston; editing by Carol Bishopric)