The Roof On A Modular Building

The roof is the part of a modular building most constrained by how it gets to site.

A module travels on a truck, under bridges and power lines, and the total height is capped. Which means the roof pitch is not a free design choice, and the way around that constraint shapes the building.

How transport height decides the pitch

The module floor sits on the trailer deck, and everything above it counts toward the legal height.

So a low pitch roof travels completed. A steeper pitch either exceeds the limit or requires a permit, an escort and a route assessment, which is the territory covered in [how is a module transported and installed](/blog/how-is-a-module-transported-and-installed).

Three ways around it.

**Keep the pitch low** and design the building to suit, with the roof completed in the factory. Simplest and the most common.

**Fold or hinge the roof,** travelling flat and raised on site. Works and adds a site operation and a weather window.

**Frame the roof on site** over installed modules. Gives any roof form the design wants, and puts significant work into the site programme with the weather exposure described in [weather and programme risk](/blog/weather-and-programme-risk).

Which one is right depends on the planning context. A development consent requiring a pitched roof to match a streetscape pushes toward the third option, and that is a cost and programme consequence of the planning pathway rather than a construction preference.

What is built in the factory

For a conventionally pitched module, most of the roof is complete before it leaves.

Structure, insulation, sarking, the roof sheeting or tiles, the ceiling lining below, and the service penetrations. Done inside, measured, inspected, and with none of the weather risk of building a roof in the open.

That is a real quality difference. A roof built in a controlled environment has its insulation installed without gaps, its sarking lapped properly and its penetrations flashed before anything else goes over them, and all of it is inspected under the regime in [factory quality control and tolerances](/blog/factory-quality-control-and-tolerances).

What has to happen on site

Three things, and they are the three places a modular roof leaks if they are rushed.

**The junction between adjoining modules.** Two modules side by side have a joint running the length of the building, under the roof. It is flashed, sealed and covered on site, and it is the most important roof detail on the project.

**Gutters, downpipes and fascias,** which travel separately because they would be damaged in transit and because they have to be continuous across modules.

**Flashings at every change of plane,** at parapets, at abutments to a higher element, and around anything penetrating.

Each of those is a site operation with a weather dependency, which is why the sequence after installation moves straight to making the building watertight, as in [security and lock up after installation](/blog/security-and-lock-up-after-installation).

Drainage decides more than it looks like

Roof water has to get off the building and then off the site.

Which ties the roof to the civil design: gutter sizing for the catchment and the rainfall intensity, downpipe positions that work with the module layout rather than cutting across a wall, and a connection to the stormwater system or to tanks.

On a site with on-site detention or rainwater harvesting, the roof is the collection surface and its area and material decide what is possible, as in [water tanks and on-site detention](/blog/water-tanks-and-on-site-detention).

A gutter that overflows at the wall is a sub-floor and facade problem rather than a roof one, and it is the most common cause of water damage on an otherwise sound building.

Roof space and services

The space between the ceiling and the roof carries insulation, ducting, wiring and sometimes mechanical plant.

Which has to be planned, because a low pitch roof has very little of it, and a services route that assumes a generous roof space does not exist on a module. That coordination is the work in [services coordination in modular delivery](/blog/services-coordination-in-modular-delivery).

It also decides whether solar is straightforward. Orientation, pitch, available area and the structural capacity for the mounting all come from the roof design, which is the starting point for [solar, batteries and all-electric dwellings](/blog/solar-batteries-and-all-electric-dwellings).

Stacked modules

A two storey building has a roof on the upper module only, and the floor-to-ceiling junction between levels becomes the critical detail instead.

Which changes the problem rather than removing it, and the detailing of that interface is covered in [two storey and stacked modules](/blog/two-storey-and-stacked-modules).

How we deliver. Discuss a project.

MADEmodular / MADEbetter.