The Module Joint And How It Is Sealed

A factory built module is complete when it leaves. The one thing it cannot be is joined to the module next to it.

Which makes the joint the most important detail in the building, because every system in it crosses the same line.

Structural connection

Modules are bolted together at designed connection points so that the assembly behaves as one structure rather than as separate units sitting beside each other.

Those connections carry lateral load, wind and, in a stacked arrangement, the load path from the module above down through the one below. Which is why the connection points are engineered with the frame rather than detailed on site, and why [the steel frame and why it is over-engineered](/blog/the-steel-frame-and-why-it-is-over-engineered) is not marketing language.

In a stacked building the joint is doing more and the tolerance is tighter, which is the subject of [two-storey and stacked modules](/blog/two-storey-and-stacked-modules).

The weather seal

Where the joint occurs in an external wall or a roof, it is a weather detail, and it is the detail most likely to leak if it is treated as a gap to fill.

A designed seal has layers: a compression seal between the modules, a drainage path that takes any water that gets past the first layer and returns it to the outside, and a cover or flashing that protects both. Not a single bead of sealant, which has a service life and no redundancy.

The roof junction is the governing case, because that is where water arrives with gravity behind it. Which is why [the roof on a modular building](/blog/the-roof-on-a-modular-building) and the joint detail are designed together, and why the membrane work in [waterproofing in a factory-built building](/blog/waterproofing-in-a-factory-built-building) has to be continuous across the interface rather than stopping at each module edge.

Fire and acoustic continuity

A fire rated or acoustically rated separation between two modules is only as good as the joint.

A rated wall with an unsealed gap at the module line is not rated, and an acoustic separation with a flanking path through the joint does not perform as tested. Which means the joint detail is part of the rated system, tested as an assembly, and inspected rather than assumed.

That continuity requirement is the practical half of [fire and acoustic performance in modular](/blog/fire-and-acoustic-performance-in-modular).

Services crossing the joint

Every service that runs through the building crosses the joint at least once: electrical, data, water, waste, ventilation, and the fire detection interconnection.

Each crossing is a connection made on site, which means three things.

**It is designed to be accessible,** so the connection can be made and later inspected without opening finished work.

**It is tested after connection,** per system, rather than relying on the factory test of each module.

**It maintains the rating** of whatever it passes through, with the correct penetration treatment, because an unsealed service penetration defeats the fire and acoustic work either side of it.

That coordination is the subject of [services coordination in modular delivery](/blog/services-coordination-in-modular-delivery).

Closing out the finishes

Internally, the joint is covered: the lining is completed, the junction set and painted, the flooring continued, and the skirting run through.

Done properly, a visitor cannot tell where the line is. Done as an afterthought, it is the first thing anybody notices, and it becomes the detail that cracks first because the joint moves slightly as the building settles.

Which is why the closeout is a specified scope with a tolerance attached rather than a patch, and why the finishes either side are selected knowing they have to meet there, per [internal linings and finishes selection](/blog/internal-linings-and-finishes-selection).

When it happens

All of it happens in the days after the lift, on site, which is the part of the programme described in [what happens on installation day](/blog/what-happens-on-installation-day).

Which means the site has to be ready for finishing work, under weather protection, with access and power, because the joint is completed in site conditions regardless of how much of the building was not.

Why fewer joints is usually better

Every joint is a connection, a seal, a set of service crossings and a closeout.

So the module layout is designed to put joints where they do least work: not through a wet area, not through a rated wall if it can be avoided, not through a ceiling hoist track. Which is one of the real inputs into [how many modules make a dwelling](/blog/how-many-modules-make-a-dwelling).

MADEmodular / MADEbetter.

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