The National Park Service said Thursday that it has started a 15-month project to restore the roof and clean the marble at the Jefferson Memorial.

Two of the flat roofs that circle the dome to keep the memorial watertight and dry will be replaced. Large marble slabs covering the memorial’s portico will be lifted to fix deteriorated waterproofing underneath.

Stone will be repaired under the portico and along the colonnade ceilings, the Park Service said in a statement. Roof drains, downspouts and gutters will be improved.

A black “biofilm” of algae, fungi and bacteria on the dome that has been spreading for a few years will be removed with lasers.

The memorial, on the Tidal Basin in Washington, will remain open during the project, although some areas will be inaccessible at times, the Park Service said.

The 32,000-ton memorial, which honors the nation’s third president and main author of the Declaration of Independence, rests on 634 pilings and caissons sunk down to bedrock on the south side of the basin.

It is a prime backdrop for photographers during the annual Cherry Blossom Festival.

In 2010, a $12.4 million repair project shored up the memorial’s poorly supported sea wall, which had been sinking into the basin.

The marble and limestone memorial was dedicated by President Franklin Delano Roosevelt on April 13, 1943.