By Deepa Seetharaman

DETROIT (Reuters) - Ford Motor Co is adding a week of production at 20 North American factories this year to build an additional 40,000 vehicles and help sustain its gains in U.S. market share.

The second-largest U.S. automaker said on Wednesday that those factories will be idled for just one week this summer instead of the traditional two-week "summer shutdown" period.

This is the second year in a row that the automaker has taken this step. The added capacity comes at a time when Ford sales are outpacing the overall U.S. auto market.

Strength in the U.S. housing market has increased demand for trucks like the F-150. Newer models offer better mileage than the average U.S. vehicle, which is a record 11 years old.

"There's a lot of pent-up demand, particularly in trucks, as the age of the fleet is as old as it's ever been in North America," said Jim Tetreault, head of Ford's manufacturing operations in North America.

Ford has also been adding shifts to meet rising demand. Currently, about 75 percent of Ford's plants in North America are operating on more than two shifts a day, Ford said.

The added shifts coupled with the shorter shutdown period will help Ford build an additional 240,000 vehicles a year.

Ford sales are up nearly 13 percent for the first four months of the year, while the broader market is up 7 percent.

The move to pare down the shutdown period affects six assembly plants, including a factory in Hermosillo, Mexico, that makes the Ford Fusion and Lincoln MKZ midsize sedans.

Chicago Assembly Plant, which makes the Explorer sport-utility vehicle, and Michigan Assembly Plant, where the Ford Focus compact car is built, are also affected.

(Reporting by Deepa Seetharaman; Editing by Eric Beech)

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