Motorhome Waterproofing Check: When, How and What It Costs

A motorhome that lets in water is a sure way to get mould in the walls, corrosion under the bodywork, and repair bills that can reach €2,000. For example, for a 5-year-old vehicle parked in Brittany, a check in October prevents storing a damp motorhome after autumn rains. Here’s how to do it yourself, when to call in a professional, and how much it costs.

Why check your motorhome’s waterproofing?

There are many weak points: a roof seal that comes unstuck after a high-pressure wash, a loose fixing screw on a window after a mountain road trip, or an ageing waterproof membrane on an 8-year-old motorhome. Moisture seeps in, lingers, and eventually rots the wooden panels or oxidises the metal reinforcements.

Here’s what happens when you ignore these signs:

• Persistent musty smells, even after 48 hours of airing with a dehumidifier.
• Black stains on walls or ceilings, especially around skylights.
• Swelling of the interior lining, a sign of trapped water behind the panels.
• Abnormally high electricity consumption, as the dehumidifier runs continuously.

On a 5-year-old vehicle, an undetected leak can reduce its resale value by 30%. Consequently, if you rent out your motorhome through a platform like Wikicampers, poor waterproofing can lead to an insurance refusal in case of a claim.

When should you carry out the check?

Two key times for a motorhome used all year round:

1. October, before winter storage — after a season on the road, seals have endured sun, cold, and vibrations. A check prevents storing a damp vehicle, especially in rainy regions like the Pays de la Loire.
2. April, before the holiday season — ensure winter hasn’t weakened the waterproofing. A single night in the rain with a leaky motorhome can let moisture set in for good.

Other situations call for a check:

• After a high-pressure wash, as seals can come unstuck under pressure, even on a motorhome less than 3 years old.
• If you’ve driven through heavy rain, as water can seep in through roof openings or vents.
• Before a long road trip to Northern Europe, to avoid emergency repairs in Germany or the Netherlands.

How to check waterproofing yourself

No need for fancy tools for a first check. Here’s how to do it, step by step:

1. Visual inspection:
   • Door and window seals — look for cracks, peeling, or mould, especially on rubber seals from 2010s models.
   • Roof — check fixings on skylights, aerials, and air conditioning units. A loose screw after a motorway trip is a common cause of leaks.
   • Lower bodywork — with a torch, spot rust marks or white streaks, signs of corrosion, especially on motorhomes parked near the sea.

2. Water test:
   • With a garden hose, wet the motorhome section by section. Inside, a second person watches the walls and ceiling for drips or dark patches — a sign of water absorption.
   • Tip — don’t aim the jet directly at seals, as this can loosen them. Use a moderate flow at a 45° angle.

These two steps catch most problems. For trickier leaks, other methods exist.

3. Absorbent paper test:
   • Place sheets of absorbent paper along suspect seals and corners, such as around windows, doors, and skylights.
   • Leave for 24 hours. If the paper is damp or warped, there’s a leak.

4. Smoke test (for invisible leaks):
   • Use a smoke generator, available for hire at €50/day, or a smoke bomb to detect draughts.
   • Light the smoke outside near suspect areas. If it’s drawn inside, there’s a breach.

These methods spot 80% of problems. For the remaining 20% — like leaks in walls or through electrical cables — professional tools are needed.

When to call in a professional

If you’ve found a leak but can’t pinpoint it, or if your motorhome is over 10 years old, a professional check is essential. Here’s what they offer:

Method	Principle	Price	Duration
Pressure test	A fan draws air out of the motorhome to create a vacuum. Leaks are detected with smoke or a draught detector.	€80–120	1h
Thermal imaging camera	An infrared camera detects temperature variations caused by moisture — damp areas appear colder.	€100–150	30 min
Moisture meter	A sensor measures moisture levels in walls. Above 15%, there’s a risk of mould.	€50–80	20 min

Where to find these services?

• Specialist motorhome maintenance centres, like Norauto Pro or Feu Vert networks, or independent garages.
• Motorhome shows, where stands offer free or discounted diagnostics, like the Salon des Véhicules de Loisirs in Paris.
• Manufacturers like Hymer or Dethleffs during annual services — a waterproofing check is often included in winter storage packages.
• Specialist winter storage workshops for a full check before hitting the road again.

How much does waterproofing repair cost?

Costs depend on the severity of the problem. Here’s what to expect:

Problem	Repair cost	Duration
Door seal replacement	€50–150	1h
Partial roof membrane replacement	€300–800	1 day
Skylight replacement	€200–500	2h
Anti-corrosion treatment (lower bodywork)	€150–400	3h
Leak repair in walls (partial dismantling)	€800–2,000	2–3 days

If your motorhome is still under manufacturer’s warranty — typically 5 to 7 years — check if waterproofing is covered. Some policies cover leak repairs, provided you’ve had an annual check. For example, Hymer covers roof membrane repairs for 6 years if the vehicle has been serviced within its network.

How to prevent waterproofing problems

A few simple steps can prevent nasty surprises, especially if you often drive in the rain or near the sea:

• Clean the seals — once a month, wipe door and window seals with a damp cloth to remove dust and silicone residue. Use a specialist seal spray like CRC Joints to keep them supple, especially before winter.
• Check fixing screws — after every wash or outing in the rain, tighten screws on skylights, aerials, and air conditioning units. A simple cross-head screwdriver can prevent leaks after a motorway trip.
• Apply a water-repellent product — on the roof and lower bodywork, a water-repellent spray (€50 per litre) extends the life of waterproof membranes. Reapply every 2 years for a motorhome used year-round.
• Avoid high-pressure washes — the pressure can loosen seals or force water into gaps. Opt for a manual wash or roller, especially on older models.
• Store your motorhome dry — if possible, park under cover or use a breathable tarpaulin to avoid condensation. A motorhome left in full sun in summer and rain in winter ages twice as fast.

For a weekend camper van, these steps are enough. However, for a 10-year-old motorhome touring Europe, an annual professional check is a must.