Box Section

In construction, a box section is an extruded length of metal with a hollow square or rectangular cross-section. The shape provides rigidity.

Most modern cars are not built on a chassis of welded box sections, instead the strength of the car comes from the shell, into which are sections are welded to stiffen the shell.

These are often spot welded in place, which allows enough gaps for liquids to flow inside of them, but they are enclosed enough that there is no access.

As they often run the length of the car, they can provide a channel for water to flow along. Meaning that water dripping in the boot of the car can end up in the front. 

It is often the case that when people say they can hear water sloshing around, it is water inside box sections.

Although removing the water is relatively simple, because these sections are enclosed with no access, it is not possible to physically clean inside them to treat contaminated water or spillages. This means they need to be flushed through and/or treated with steam or fog.

What it means

A box section is any hollow, closed structural part of the car made from metal panels joined to form a box. Instead of a single flat panel, you have several sides that create a tube or beam. Typical examples are sills under the doors, chassis rails running front to back, cross members under the seats and boot floor, and some roof and pillar reinforcements. These sections provide strength and stiffness without huge weight.

Why it matters

• Can trap water: If drains, grommets or seams fail, water can run into a box section and sit there, slowly rusting it from the inside where you cannot see it.
• Hidden corrosion risk: Rust in sills, cross members and rails can be advanced before there are obvious bubbles outside, affecting MOT results and overall safety.
• Water paths and smells: In leak jobs, water often travels along box sections and cross members before appearing in a footwell or boot, which can confuse diagnosis if you do not consider what is happening inside the structure.
• Harder to inspect and repair: Because box sections are closed, checking and treating them properly usually needs inspection holes, endoscope cameras and specialist rust protection products.

Where you will see it

You will see box section mentioned in leak reports, rust assessments and structural inspections. Typical comments include box section in sill holding water, rust trace inside sill box section, moisture detected in cross member box section or box sections require internal rust protection. It is often referenced when explaining why leaks and damp in one area can be caused by water entering somewhere completely different.

Context

Box sections are part of the car’s safety and strength, forming load paths in side impacts, jacking points, seat mountings and suspension pick-up areas. They also sit alongside drainage pipes, grommets, rear vents and membranes in the overall water management system. When leaks are left unchecked, water can find its way into these hollow members via seams, unsealed holes or failed grommets. A proper diagnosis and repair plan will consider whether any box sections have been exposed to standing water, how far rust and contamination have spread inside them and whether drying, rust treatment or structural repair is required as part of the job.

Common mistakes

• Assuming that because the outside of a sill or rail looks tidy, the box section inside must be rust free and dry.
• Fixing a visible leak or drying carpets but ignoring box sections that have been quietly holding water and rusting from within.
• Drilling random holes into box sections without planning, creating new corrosion points and sometimes breaking intended drainage paths.
• Applying underseal over rusty box sections without treating or inspecting the inside, trapping moisture and allowing corrosion to continue unseen.

Written by Danny Argent. Last updated 08/12/2025 19:59