Damp and weights: the two checks that decide a used motorhome or caravan
How to check a used motorhome or caravan for damp and weight problems: moisture meter bands, habitation history, MTPLM versus MIRO, and licence limits.
Why damp is the check that matters most
Most motorhomes and caravans do not die of engine failure. They die of water. The habitation body of a typical British-built van is a timber frame skinned with bonded wallboard, and once water finds a way past a seal it soaks into that structure quietly, sometimes for years, before anything shows on the inside. Timber rots, wallboard delaminates, and the only proper fix is to strip the interior back to the frame and rebuild. On an older van the quote for that work regularly exceeds the value of the vehicle, which is why damp, not mileage, is the single biggest financial risk when buying used. A motorhome with a sweet engine and a rotten rear panel is, in practical terms, a donor vehicle.
The second thing that quietly breaks deals is weight. Plenty of motorhomes leave the forecourt with so little usable payload that a full water tank, two passengers and a bike rack put them over their legal limit. Neither problem shows up in an advert photo. Both can be checked before you pay.
A habitation check is not an MOT
A motorhome needs an MOT like any other vehicle, but the MOT only looks at the base vehicle: brakes, lights, steering, emissions, chassis. It says nothing about the living quarters. That is the job of the habitation check (often called a habitation service), an annual inspection that takes damp readings throughout the body, tests the gas system for leaks and correct operation, checks the 12V and 230V electrics, and confirms the ventilation is safe, which matters because gas appliances in a sealed box are a carbon monoxide risk.
The catch is that a habitation check is not a legal requirement. There is no habitation equivalent of the MOT database, no reminder letter, and no penalty for skipping it. So many used motorhomes, and most privately kept caravans, have never had one, or last had one years ago. A van with a folder of annual habitation reports is telling you something about its owner. A van with none is not automatically bad, but you are buying blind on the most expensive thing that can go wrong.
How damp readings actually work
A damp check is done with a moisture meter pressed against the interior wallboard at dozens of points around the vehicle. The meter reads out a percentage, and the trade works to rough bands. As a guide, readings under about 15 per cent are normal for a vehicle that lives outdoors. Between roughly 15 and 20 per cent, something wants investigating: it may be condensation, or it may be the early stage of an ingress. Above 20 to 25 per cent you are looking at active water ingress and probable structural damage. Treat those bands as typical industry practice rather than a standard set in law; different workshops draw the lines slightly differently, and readings move with temperature and season. What matters is the pattern: one slightly raised reading near a window is a conversation, a cluster of high readings along a roof joint is a walk-away.
Where damp hides
Water gets in where the body is pierced or joined, so check:
- Window seals and their corners, especially the bottom corners where water pools
- Roof lights and vents, including inside the surrounding trim
- Awning rails and anything else screwed through the exterior skin
- Floor edges, particularly under the washroom and around the wheel arches; press with your heel and feel for sponginess or delamination
- Under and behind seat cushions, inside lockers, and behind fitted furniture where nobody looks
Use your nose the moment you step in. A musty, earthy smell in a van that has been closed up is a stronger signal than anything the seller says. Look for tide marks and staining on the wallboard, bubbling or lifting wall covering, corroded screw heads, and fresh sealant or new trim in odd places, which can mean a repair or can mean a cover-up.
Get the history in writing
Before travelling to view, ask the seller in writing for the habitation service history and a copy of the last damp report with the actual readings, not a verbal assurance that it is bone dry. A dealer who services what they sell will have this. A private seller with a well-kept van usually will too. If neither exists, price the van as if you will be paying for a full habitation inspection yourself, because you should be.
MTPLM, MIRO and the payload that is not really there
Every motorhome and caravan carries two weights on its plate. MTPLM is the maximum technically permissible laden mass, the most it may legally weigh. MIRO is the mass in running order, roughly the vehicle as it left the factory. Your payload, everything you are allowed to add, is MTPLM minus MIRO.
The trap is that MIRO is a factory figure. It typically does not include the awning, the second leisure battery, the solar panel, the tow bar, the bike rack or the extra gas bottle the previous owner had fitted, and every one of them comes straight out of your payload. A motorhome plated at 3,500 kg with a MIRO of 3,050 kg looks like it has 450 kg to play with; after common fitted extras, water, gas and passengers, the real remaining margin can be down to double digits. Some well-specced vans are effectively overweight with a family on board. Running over MTPLM is an offence, causes real trouble with insurers, and is exactly what a roadside weighbridge check finds. When viewing, read the actual plate rather than the brochure, ask what has been fitted since new, and if the sums look tight, put the van over a public weighbridge before you commit.
Licences and the 3,500 kg line
The reason so many motorhomes are plated at exactly 3,500 kg is the driving licence. A standard category B car licence covers vehicles up to 3,500 kg maximum authorised mass. If you passed your test on or after 1 January 1997, driving anything heavier needs C1 entitlement, which means an extra test and periodic medicals. Drivers who passed before that date generally have C1 grandfathered onto their licence. Manufacturers therefore squeeze large vans under the 3,500 kg ceiling, which is precisely why payloads are so tight. Check the categories on your own licence before you fall for a 4,250 kg tag-axle.
Matching a caravan to the tow car
For caravans the weight question runs the other way: can your car tow it safely? The long-standing industry guideline is that a fully laden caravan should weigh no more than 85 per cent of the tow car's kerbweight, especially for drivers new to towing. That 85 per cent figure is a recommendation from the caravanning bodies, not law; the legal limits are the car's plated towing capacity and its gross train weight. But an outfit near 100 per cent is measurably less stable, and instability at motorway speed is how caravans end up on their roofs.
Noseweight matters too: the downward load the hitch puts on the tow ball, usually best at around 5 to 7 per cent of the caravan's actual laden weight, and never more than the car or hitch limit allows. Too little noseweight is a common cause of snaking. A cheap noseweight gauge is worth taking to a viewing.
Identity checks, and the check nobody does for you
Caravans have no registration plate of their own and no MOT, but they do have CRiS, the industry-run Caravan Registration and Identification Scheme, which records the chassis identity and can flag theft and outstanding finance. Run it on any caravan you are serious about. But be clear about what it is: an identity check. No database anywhere records whether a caravan is dry. Nothing checks damp for you.
Before you pay
Two habits protect you. First, commission an independent damp and habitation inspection before money changes hands. Mobile engineers and approved workshops do pre-purchase checks, the cost is small against the downside, and a seller who refuses access has answered your question. Second, budget for tyres on any caravan: caravan tyres die of age, not wear, because the vehicle sits still for months at a time. The tread can look showroom-fresh on a tyre whose sidewalls are seven years old and cracking. Read the date code on the sidewall and price replacements by age, not tread depth.
How CompareForecourt handles it
CompareForecourt treats motorhomes and caravans as first-class categories, compared only against their own kind, so a coachbuilt motorhome is never scored against a hatchback. The extractor stores a category-specific spec for each listing, and a completeness score flags where decision-relevant information is missing from the advert, which for these vehicles usually means weights and habitation history. Caravans show n/a for mileage and MOT, because they have neither, while motorhomes get full DVSA MOT history support for the base vehicle. What the platform cannot do is verify damp or habitation condition from an advert; no tool can. What it can do is flag that a listing mentions no evidence, no damp report and no habitation history, so you know exactly what to ask for before you travel.