"To lower the roofline" means to decrease the height or level of the upper part of a building or vehicle, specifically the part that covers and protects the inside space. Full definition
The prestige brand has settled on even numbers for its sportier models — the cars with lower rooflines and (frequently) fewer doors. (motoring.com.au)
At nearly five metres from tip to tail the Karma is almost the same length as a Mercedes S - Class, although its striking design and low roofline means its closer in spirit to «coupe saloons» like the Aston Martin Rapide. (evo.co.uk)
While the CLA is based on the A-Class hatchback, its relatively low roofline makes it less practical; if you like the styling but need more space, take a look at the CLA Shooting Brake estate, which we've review here. (carbuyer.co.uk)