M-Shaped Roof

An M-shaped roof (also called a double-pitched or twin-gable roof) consists of two parallel gable roofs built side by side, with their outer slopes resting on the outer load-bearing walls and their inner slopes meeting in a central valley. Seen from the gable end, the profile forms the letter M. The arrangement allows a deep building to be roofed with two shorter-span gables instead of one very tall roof.

Where it is found

• British terraced and older houses: many deep Georgian and Victorian houses use double-pile roofs with a central valley gutter, sometimes hidden behind a front parapet. A related form with an inverted V is the London roof or butterfly roof.
• Industrial and agricultural buildings: multiple-span roofs, sometimes extended to three or more bays, are used on warehouses, barns and factories; with glazing on one face of each span they become sawtooth roofs.
• Contemporary houses: the M-shape is used as a deliberate design feature.

Drainage

The central valley must collect water from two inner slopes and carry it to the ends of the building or to internal outlets. It is commonly formed as a lined valley gutter or box gutter, traditionally in lead in the UK, sized for the combined area. Because the valley is enclosed by the roof, blockages or undersized gutters can overflow into the building, so access for cleaning, adequate fall and overflow outlets are important.

Advantages and drawbacks

• Reduces overall roof height and rafter length on deep plans.
• Allows two smaller attic spaces or vaulted ceilings.
• The central gutter is a long-term maintenance point, and snow can accumulate in the valley in cold climates.