Lean-to extensions

What a lean-to is

A lean-to is a single-slope structure that depends on an adjacent building for support. Its rafters span from new exterior columns to the columns of the main building, where they connect at a level below the main eave. Lean-tos add covered space – loading areas, offices, stores, plant rooms, open-sided sheds – at lower cost than widening the main frame, and they are one of the most common additions to a pre-engineered building.

Lean-tos are most often attached to a sidewall, but can also be attached to an endwall, in which case they connect to endwall columns or to a rigid endwall frame.

Typical dimensions and connection types

• Pinned lean-to – rafter simply supported between the main column and a lean-to column; lightest, but transfers vertical load and lateral thrust to the main frame.
• Rigid lean-to – moment connection to the lean-to column, making a partial frame that shares lateral load.
• Open lean-to – no wall cladding on the outer side, used for vehicle parking, loading or covered storage; wind on the underside of the roof must be designed for.

Lean-to options and how they attach

Each option changes the main frame differently. A pinned lean-to at mid-height of the main column adds bending in that column under both gravity and wind; a width extension at eave level changes the main frame's stiffness and drift. In all cases, the lean-to wall and roof bracing must be aligned with, or tied into, the braced bay of the main building.

When to use a lean-to

• Covered loading docks and truck bays along a warehouse sidewall.
• Offices, toilets, plant rooms and electrical rooms beside a factory hall, where a lower height is acceptable.
• Open machinery or material storage sheds on agricultural buildings.
• Phased projects where additional floor area is needed later at minimum cost.

Design checks and limits

The critical point is that a lean-to changes the loads on the main building. The main columns receive additional vertical load, a horizontal reaction from the lean-to rafter, and – in snow regions – drift load against the higher main wall onto the lean-to roof.

• Plan it at the start. If the lean-to is known at design stage, the main columns are sized for it at small extra cost. Adding a lean-to to an existing building requires the main frames to be checked and often strengthened.
• Snow drift. The step between main roof and lean-to roof collects snow drift, which can be several times the uniform snow load.
• Drainage. The lean-to roof needs its own gutter; water from the main roof should not discharge uncontrolled onto it.
• Bracing. Longitudinal bracing is needed in the lean-to wall line or the lean-to must be tied into the main bracing system; see bracing systems.
• Flashing. The junction between lean-to roof and main wall needs a carefully detailed flashing to stay watertight.

A lean-to wider than about 12–15 m, or one carrying cranes or mezzanines, is usually better designed as a separate single-slope building or as part of a multi-span main frame.

Lean-tos from World Steel

World Steel supplies lean-to frames, secondary members, cladding, gutters and flashings together with the main building, with the main frame designed for the lean-to loads. For extensions to existing buildings, WSC engineers can check the existing frames from drawings or survey data. See the lean-to frame highlighted in the 3D model and the pre-engineered building range.