Lean-To Steel Buildings and Attached Additions

A lean-to is a single-slope steel structure attached to the side of a main building, with its high side supported by the main building columns and its low side on its own columns. It adds covered or enclosed space for storage, offices, loading areas or plant rooms at low cost.

Key facts

The high end of the lean-to rafter connects to the main building columns.
The low end is supported by separate lean-to columns.
The main frames must be designed for the extra loads.
Lean-tos can be open, partly enclosed or fully enclosed.
Drainage normally runs away from the main building to a lean-to gutter.

How is a lean-to supported?

The lean-to rafter spans from the main building sidewall to its own outer columns. At the high end it bolts to the main building columns, usually below the main eave, and at the low end it sits on lean-to columns founded on separate footings.

The lean-to often relies on the main building for lateral stability. This means the main frames must be checked for the added vertical and horizontal loads, which is easiest when the lean-to is part of the original design.

Typical uses of lean-tos

Lean-tos are used for covered loading areas, vehicle parking, material storage, equipment and plant rooms, offices and amenity blocks. Open lean-tos give shelter while keeping access easy; enclosed ones extend usable floor space.

They are also an efficient way to add space to an existing building, provided the existing frames are checked for the new loads by a qualified engineer.

Roof, drainage and snow

The lean-to roof slopes away from the main building, so water drains to an outer gutter. The joint between the main building wall and the lean-to roof needs careful flashing to stay watertight.

In snow regions, snow can slide off the higher main roof or drift against the main wall onto the lean-to. These drift and sliding loads are significant and are set from the governing load code by the project engineer.

Planning a lean-to with the main building

If a lean-to may be added later, design the main building columns and foundations for it now and provide connection points. This avoids costly strengthening and keeps the future work simple.

Check clear heights under the lean-to at both the high and low sides, especially where trucks or equipment must pass. The main building eave height limits how high the lean-to can be at its connection.

Points to confirm with your engineer

Lean-to width, length and low-side clear height.
Whether the lean-to is open or enclosed.
Snow drift and sliding snow from the main roof.
Main building frame checks for the lean-to loads.
Flashing detail at the main wall junction.

Frequently Asked Questions

Can I add a lean-to to an existing building?

Often yes, but the existing frames, bracing and foundations must be checked by a qualified engineer for the new loads, and strengthening may be needed.

Does a lean-to need its own foundations?

Yes. The outer lean-to columns need their own footings, and the main building foundations must carry the additional reaction from the lean-to.

What is the difference between a lean-to and a canopy?

A lean-to has columns at its outer edge and can be enclosed. A canopy is usually cantilevered from the wall with no outer columns and stays open.

Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.