Chapter 7 — Section 6: Body, structure and corrosion

A vehicle's structure determines how it behaves in a crash and how stable it is mechanically. Section 6 is about finding corrosion or damage severe enough to reduce that strength. It isn't a cosmetic check: the focus is on prescribed areas, places where structural failure could mean losing steering, losing braking, or a major component coming loose. Surface rust rarely fails a car; what matters is whether corrosion has crossed a specific threshold near a load-bearing point.

01What counts as a prescribed area

A prescribed area is any part of the structure within a set distance of a critical mounting point, because failure there would be serious. Prescribed areas typically include the zone around:

• Suspension mounting points, including struts, spring seats and wishbone pivots.
• Steering gear mounting points, such as the rack or box bolts.
• Braking components, including master cylinders and servo brackets.
• Seat belt anchorage points and seat mountings.
• Pillars, sills and cross-members that support any of the above.

For most modern monocoque cars, the inner sills, floor pans and wheel arches count as prescribed areas if they're within 30cm of a mounting point. A hole, or a cluster of holes, caused by corrosion within that radius will usually fail the car. Testers often call this the '30cm rule', though applying it properly needs a real understanding of how load moves through the body.

02The 30cm rule and load-bearing judgement

The 30cm rule assesses whether corrosion threatens a mounting point's safety, and it's measured following the contour of the metal rather than in a straight line. If the tester finds a hole that the MOT probe passes through, or metal thin enough to collapse under thumb pressure, they measure its distance from the nearest load-bearing point.

Corrosion more than 30cm away can still fail a car if it risks a component detaching, or leaves a sharp edge that could injure a pedestrian. But holes in the middle of a large floor pan, or in the lower skin of a door, are often recorded as advisories rather than failures, provided they don't reach a seat belt mount or a structural pillar. Judging whether structural integrity has been significantly reduced is partly down to the tester's experience, though the 30cm distance gives a clear line for a mandatory fail.

03Underseal, dirt and the limits of a visual check

Underseal and heavy coatings are a genuine grey area. The MOT is a non-destructive visual inspection, so the tester can't scrape away underseal, paint or mud looking for rust. They can only tap and press with a blunt corrosion assessment tool. A car arriving with a thick, fresh layer of underseal can put the tester in a difficult position.

If the coating is thick enough to stop the tester assessing a prescribed area properly, they can refuse to test the car. More often, if the underseal is peeling or bubbling, that gives the tester good reason to press it with the probe; if the metal gives way, it's recorded as a fail. Fresh underseal doesn't hide problems as well as owners sometimes think; an unusual-looking new coating tends to draw closer attention, and testers are trained to spot the texture of rust trapped under a layer of paint.

04Assessing repairs

Where a car has previously failed on corrosion, the repair method matters. Structural repairs to load-bearing members should be continuous seam welding. Spot or stitch welding is only acceptable if that was how the panel was originally built. Pop rivets, glass fibre or body filler used in a prescribed area is an automatic fail.

• Repair panels should be a suitable thickness, matching the original gauge.
• Welds should be left visible, not covered in sealant, until after the retest so the tester can check penetration and quality.
• Braze welding or soldering isn't acceptable on structural components.

A patch only tacked in at the corners is not considered structurally sound. The tester needs to be satisfied that a repair restores something close to the component's original strength, with the new metal properly integrated into the surrounding structure.

05Body panels and projections

Beyond the structure, Section 6 also covers the general external condition of the bodywork. Corrosion that's left a jagged or sharp edge fails if it could injure a pedestrian or other road user, commonly at the lower edge of a front wing or the end of a sill. A temporary fix like gaffer tape over a sharp edge isn't good enough, since it isn't secure or permanent.

Panel security is checked too: a bumper or wing at risk of falling off through corroded brackets fails. Doors are also covered, at least the driver's and front passenger's must open from inside and outside using their normal controls, so occupants and rescuers can get in and out.

06Where this leaves the owner

Section 6 sets a minimum safety baseline, not a guarantee of longevity. The 30cm rule gives testers a clear way to fail perforated metal near suspension or steering mounts, but it's still a non-destructive check. A car can pass with only minor advisories while corrosion under factory linings goes unseen. A pass simply means the prescribed areas were sound enough on the day to meet the requirements of the Road Traffic Act.