Rooftop additions in light gauge steel
Why the technology suits
Section titled “Why the technology suits”Building on top of an existing building means the old structure and the foundations have to carry the additional load.
Self-weight is everything here: the load from a masonry or in-situ addition often exceeds the reserve, while a light gauge steel one fits within it.
Three further advantages come with that:
Dry technology
Section titled “Dry technology”No concreting, no curing time, no mass moisture. The occupants can often stay in.
Fast site work
Section titled “Fast site work”The members arrive premade and erection is short.
Modest plant requirements
Section titled “Modest plant requirements”A smaller crane, less site storage — decisive in a dense urban setting.
What has to be settled in advance
Section titled “What has to be settled in advance”The capacity of the existing structure
Section titled “The capacity of the existing structure”Survey, and where needed trial openings and material testing. The old drawings are not enough. Surveying an existing building.
The reserve in the foundations
Section titled “The reserve in the foundations”The additional load goes right down to the foundations. If there is no reserve there, the addition requires strengthening.
The load path
Section titled “The load path”Where the load of the new structure is delivered: onto the existing walls, or onto a separate load-bearing system. That is a plan question too, because the old and the new wall layouts do not necessarily coincide.
The bracing
Section titled “The bracing”The horizontal loads of the new storey also have to be taken down. That is often the harder task than the vertical load. Designing a shear wall.
An addition can change the fire situation of the whole building: number of storeys, risk class, escape. Risk classification.
Snow and wind
Section titled “Snow and wind”A taller building takes more wind load, and the snow drifting conditions on the old roof can change too.
Where hot-rolled steel enters
Section titled “Where hot-rolled steel enters”If the load-bearing walls of the new storey do not coincide with the existing ones, a transfer structure is needed. That is generally solved with hot-rolled beams, onto which the light structure is built.
The classic case of a mixed structure.
The point
Section titled “The point”In a rooftop addition self-weight decides, and that is where light gauge steel is strongest. The difficulty of the task is not the new storey but getting to know the existing structure.