Building dimensions and terminology

Why definitions matter

Two suppliers can quote "a 30 m wide building with 8 m eave height" and mean different buildings. In the metal building industry, dimensions are measured to defined reference lines, and misunderstanding them is a common source of clashes with foundations, cranes, racking and doors. This article uses the conventions of the MBMA Metal Building Systems Manual, which most PEB suppliers – including World Steel – follow unless a project specifies otherwise.

Main dimensions

Always confirm whether a quoted height is eave height or clear height. The difference at the haunch can exceed one metre on wide spans.

Walls, roofs and frame lines

• Sidewall – a wall parallel to the ridge; endwall – a wall at the end of the building, perpendicular to the ridge.
• Gable – the triangular upper part of an endwall on a double-slope roof.
• Ridge – the high line of the roof; eave – the line where roof meets sidewall; rake – the sloping edge of the roof at the endwall.
• Frame lines are usually numbered 1, 2, 3… along the length; column lines are lettered A, B, C… across the width.
• Module – in a multi-span building, the distance between adjacent column lines.
• End bay – the bay between an endwall and the first interior frame; interior bay – any bay between two interior frames; braced bay – a bay containing the diagonal roof and wall bracing.
• Step in eave height – a change of eave level along the length or across a lean-to, which needs its own framing and flashings.
• Width extension – a lean-to whose roof continues the main roof plane at the same slope; a normal lean-to sits below the main eave.
• Haunch or knee – the deepened corner where column and rafter meet; see rafters.

Endwall types and roof slope conventions

The endwall is defined both by geometry and by how it is framed, and the choice affects cost and future expansion:

• Post-and-beam endwall – endwall columns (posts) at about 6–8 m spacing support a light endwall rafter that spans between them. It is the lightest solution but cannot be removed for extension without adding a frame.
• Rigid frame endwall – a full interior-type frame placed at the endwall, with endwall posts only supporting girts. It is heavier, but the wall can later be removed and the building extended.
• Expandable endwall – a post-and-beam endwall designed so that a future frame can be inserted, or a rigid frame detailed with girt clips for future bays.

Roof slope is written in several ways. The table converts the most common values; slopes below about 1:20 are generally reserved for standing seam roofs, while through-fastened roofs usually need a steeper minimum set by the sheet manufacturer.

Eave height conventions vary: most PEB suppliers measure to the top of the eave strut, but some specifications refer to the top of the column or the underside of the rafter at the steel line. State the definition on the enquiry to avoid a mismatch.

Typical ranges

Most PEBs fall within the following ranges. Values outside them are possible but should be discussed with the designer early because they affect member sizes, cladding and transport.

Dimensions that drive cost

Width and eave height have the largest effect on frame weight because they govern bending moments and wind loads. Bay spacing is a balance: wider bays mean fewer frames but heavier purlins and girts. Roof slope influences drainage, snow drift and the choice between through-fastened and standing seam roofs. See how a PEB carries loads for why these dimensions matter structurally, and contact the WSC design team to test options during concept design.