Fibrocement (asbestos-cement) roof waterproofing: safe, long-lasting options

Fibrocement (asbestos-cement) roof waterproofing is a common request when leaks appear in older buildings, plant rooms, or industrial units. The right solution must be compatible with the substrate, durable, and—most importantly—safe (given the material’s condition and the potential presence of asbestos in older sheets).

Before waterproofing: a quick diagnosis (and why it matters)

On corrugated fibrocement roofs, leaks typically come from:

• Poor or deformed overlaps between sheets.
• Fixings with aged or failed washers.
• Cracks from movement, impacts, or long-term weathering.
• Junctions at upstands, skylights, flues, and gutters.
• Blocked gutters causing overflow.

A proper diagnosis prevents “coating over” a fixing/overlap/detail issue that will reappear with the next heavy rain.

Safety and compliance: asbestos risk

Many older “uralita” roofs contain asbestos. This directly affects:

• How access and work on the roof must be managed.
• What operations are allowed (cutting, drilling, removal, etc.).
• Whether licensed/authorised procedures are required if the material is disturbed or removed.

If composition or condition is uncertain, the safest route is a technical inspection first and a plan built around safe working practices.

Waterproofing options for fibrocement roofs (often without removing sheets)

If the roof is structurally acceptable and you want to minimise disruption, these are the most common approaches.

1) Liquid-applied membrane (polyurethane or hybrid) with reinforcement mesh

A widely used solution for corrugated roof refurbishment because it follows the profile.

• Pros: seamless waterproofing, strong performance at overlaps and micro-cracks, targeted reinforcement with mesh.
• Watch-outs: surface preparation (cleaning, priming), detailing at fixings/overlaps, verified film thickness.
• Best when: there are local and widespread leaks but the sheets remain stable.

2) Acrylic/elastomeric coating (budget-friendly, but with limits)

Can work as maintenance or a shorter-term improvement if the roof is in relatively good condition.

• Pros: straightforward application, lower cost.
• Limits: lower resistance to long-term ponding and typically shorter service life than premium systems.
• Best when: you want a maintenance solution and accept periodic re-coating.

3) Adhered or mechanically fixed sheets (where geometry allows)

In some roofs (or associated flatter areas) a sheet membrane (EPDM/TPO/PVC) may be feasible with robust detailing.

• Pros: high durability when correctly installed.
• Limits: on fully corrugated areas, folds, terminations, and fixings can make it complex.
• Best when: there are suitable zones, or as part of a combined detailing strategy.

4) Overcladding (new metal roof over a subframe)

This is more than “waterproofing over the top”: it creates a new outer skin.

• Pros: very robust, improves appearance, and can incorporate insulation.
• Limits: more construction work, load checks, more complex junctions.
• Best when: the existing roof is heavily degraded or an overall upgrade is required.

Details that make or break the job (where repairs often fail)

Fixings and penetrations

• Replacing degraded washers/seals.
• Compatible sealants and reinforced patches with mesh.

Overlaps and junctions at upstands/skylights

• Continuous reinforcement along overlap lines.
• Termination details using compatible bands/accessories to avoid weak “cut edges”.

Gutters and drainage

• Cleaning and correcting blockages/overflow points.
• Repairing gutter joints and leak paths.

When it’s better not to waterproof (and to consider replacement)

Consider replacement (or overcladding) if:

• Sheets are extensively cracked, deformed, or repeatedly failing.
• Fixings are too compromised for a reliable repair.
• You need better insulation, safety, or energy performance.
• The roof presents a risk for access and maintenance.

A common Madrid scenario (residential blocks and units): what tends to work best

Around Madrid we often see fibrocement roofs on residential communities (plant rooms, rooftop enclosures) and older industrial units. In practice, the most reliable outcomes usually come from:

• Fixing/overlap/detail repairs before any coating.
• A properly specified seamless system (liquid membrane with reinforcement).
• An annual maintenance plan: gutters, terminations, and shaded areas.

Quote checklist (so you can compare like-for-like)

• Inspection and photos of key details (fixings, overlaps, junctions).
• Surface preparation scope (cleaning, priming, repairs).
• Reinforcement: where it’s applied and mesh weight.
• Guaranteed consumption/verified thickness.
• Gutter/drainage scope.
• Written warranty and maintenance conditions.

If you’re dealing with leaks on a fibrocement roof in Madrid or Toledo, Impermyna can identify the real cause and recommend the safest, longest-lasting option for the roof’s actual condition. Get in touch for an assessment.