UPDATE 1-Strabag Q1 loss widens, outlook cautious

* Q1 net loss 129.3 mln eur, in line with Reuters poll

* Output grows 14 pct, order backlog steady

* Expects 2009 earnings in line with 2008

(Adds detail, background)

VIENNA, May 29 (Reuters) - Austrian builder Strabag said on Friday its (STRV.VI) first-quarter net loss widened, in line with market expectations, due to a seasonal lull in construction and weaker business in its home market and the Czech Republic.

Its net loss widened 59 percent to 129.3 million euros

($179.4 million). Output rose 14 percent on the year, mainly thanks to acquisitions made last year.

Builders' first quarters are often weak because the winter halts construction.

"We...do not share the budding optimism; rather, we believe that the external factors will continue to develop negatively," Chief Executive Hans Peter Haselsteiner said in a statement.

He said the company expects to end the current year at the same levels as the year before, thanks to a national economic stimulus programme.

Analysts polled by Reuters had forecast a net loss of 130 million euros on average, while output was forecast to rise 17 percent.

Strabag has 60 percent of its business in civil engineering, an area where public investment programmes launched to stem the economic downturn help. It has less exposure to residential construction, a sector which is struggling.

But it is also exposed to many countries hit particularly hard by the crisis, such as Germany, Russia and other emerging European countries. This has weighed on its shares this year.

Its stock has lost 6 percent this year so far to close at 15.16 euros on Thursday. That is around 15 times next year's estimated earnings per share -- more than rivals including France's Vinci (SGEF.PA) at 13 times or Germany's Hochtief (HOTG.DE) at 14.

The DJ Stoxx Construction and Materials index .SXOP has gained 7 percent this year.