Part of the "Megastructures: The Visual Encyclopedia" book project, on sale now at http://www.neilblevins.com/megastructures/megastructures.htm
A Worldhouse Roof would involve the construction of a habitable enclosure which would grow until it covered an entire planet.
The enclosure would be a thin, strong, flexible, and transparent roof, or membrane, held up at least 1 km above the planet’s surface. Its purpose would be to allow sunlight in and keep the atmosphere from leaking out.
Once constructed, the interior of the membrane would then be pressurized with a breathable atmosphere, allowing you to terraform almost any planet or asteroid. At smaller sizes, this could also be thought of as a domed city. The membrane would be held up by towers and cables spaced regularly along the planet’s surface, or could potentially be held up using atmospheric pressure.