Steel roof systems on concrete or masonry structures

What a roof system is

A roof system is a pre-engineered steel roof – rafters, purlins, bracing, sheeting and accessories – supplied to sit on top of a substructure designed and built by others, usually reinforced concrete columns and beams or load-bearing masonry walls. It is common for schools, markets, sports halls, warehouses on existing slabs, and refurbishment projects where an old roof is replaced on sound walls.

The concept separates responsibilities: the steel supplier designs the roof and provides its support reactions; the owner's engineer designs the substructure for those reactions and builds it to the required tolerances. Getting this interface right is the key to a successful roof system.

Rafter types and support conditions

A simply supported rafter needs more depth than a frame rafter, because it does not benefit from moment continuity at the knees. That is the trade-off for keeping horizontal thrust off slender concrete columns or masonry walls. For long spans a portal or tied solution can be more economical if the substructure is designed for it.

Interface with the substructure

• Anchor bolts and bearing plates. Cast-in or post-installed anchors in column heads or bond beams, designed for uplift and shear. Edge distances in narrow concrete or masonry are often the governing check – see base plates and anchor bolts.
• Tolerances. Concrete and masonry are built to wider tolerances than steel. Specify level and line limits for bearing surfaces, allow grout pads, and use oversized or slotted holes where codes permit.
• Wind uplift. Light steel roofs can have net uplift. The substructure must have enough weight or tie-down to resist it.
• Longitudinal stability. Roof bracing transfers wind and seismic forces to the supports; the substructure must have braced or stiff bays to receive them – see bracing systems.
• Wall heads and gables. Masonry gables are finished with flashings and closures to the roof sheeting; see masonry and precast interfaces.

Always issue the roof support reactions – vertical, horizontal and uplift, for each load case – to the substructure engineer before the concrete is designed.

When a roof system is less suitable

A roof system is usually less economical than a complete PEB when concrete columns are being built only to support the roof: steel columns are faster to erect and integrate with the roof frame. It can also be difficult where the existing walls are weak, out of plumb or unknown, or where the building must be extended later. In those cases a complete clear-span building with independent steel frames inside the walls may be the better choice.

Roof systems from World Steel

World Steel supplies long-span steel roofs on concrete and masonry structures, with rafters, purlins, bracing, cladding and anchor bolts, and provides a clear reaction schedule and setting-out drawing for the substructure. Designs follow AISC 360-22 and ASCE 7-22, CSA S16-19 and NBC 2020, AS 4100:2020 and AS/NZS 1170, or EN 1993 and EN 1991, as required.