How to spot structural problems before buying a flat: the checklist

The most expensive mistake a property investor can make isn't overpaying – that can be recovered over time through the rent – but buying an asset with serious structural problems that they didn't spot before signing the deed. Rising damp that requires the foundations to be waterproofed, structural cracks in columns or floor slabs, aluminosis (the deterioration of aluminous cement) in concrete buildings from the 70s and 80s, asbestos in false ceilings or pipes, electrical wiring from the 60s that fails to meet any current regulations: all these problems can lead to remedial costs of €20,000-100,000 or even make the property temporarily uninhabitable.

This guide explains what to look for on a preliminary viewing, which signs are red flags and when it's essential to hire a professional before deciding.

The most common problems and how to identify them on a viewing

Damp: the most common problem in Spain's housing stock

• Condensation: The most common form, especially in cold areas (the north of Spain, the interior in winter). It shows up as black mould stains in the corners of rooms, around windows and along the edges of exterior walls. In many cases it can be solved with proper ventilation and better insulation, but you need to check that it isn't penetrating damp.
• Penetrating damp from the roof or terrace: Damp stains on the ceilings of top-floor flats or flats with a terrace above them are a sign of water coming in through the roof. Repairing a flat or tiled roof can cost €10,000-40,000 and is usually the responsibility of the owners' association, but it can leave the flat uninhabitable until it's repaired.
• Rising (capillary) damp: The hardest to solve. Water rises up the walls from the ground by capillary action. It shows up in the bottom 60-100cm of basement or ground-floor walls as mineral salt stains, efflorescence and paint flaking off. The solutions (injecting water-repellent products or perimeter drainage) are expensive (€15,000-50,000 depending on the building) and not always fully effective.

Cracks: when they're cosmetic and when they're structural

Not all cracks are the same. The basic distinction:

• Surface hairline cracks in the render or plaster: Not structural. They appear as the mortar or plaster shrinks, through normal hygrothermal movement. They're fixed with filler and paint. Low cost.
• Cracks in partition walls or walls between homes: Usually not structural if they're vertical or run at 45° from the corners of windows and doors. They can indicate moderate differential settlement of the building that has already stabilised.
• Cracks in columns, beams or floor slabs: SERIOUS WARNING SIGN. Any horizontal crack in a reinforced concrete column, a crack where a beam meets a column or cracks in the floor slab (the underside of the ceiling) that follow the line of the reinforcement require an urgent inspection by an aparejador or arquitecto técnico (a building surveyor) before you continue negotiating.
• Through-cracks in load-bearing walls: If the crack goes right through the wall (you can see light from the other side or slide a card through it), it's an active crack that requires a structural assessment.

Aluminosis: the hidden problem in thousands of Spanish buildings

Aluminous cement was used in Spain between the 50s and the 70s in many residential buildings. With time and moisture, aluminous cement goes through a mineralisation process that reduces its strength by as much as 50%. Buildings with severe aluminosis can be declared uninhabitable. Risk indicators: a building constructed between 1950 and 1975, a coastal area or one with high humidity, floor slabs made of prefabricated cement joists (not timber or concrete poured in situ). If the building has these characteristics, it needs to be analysed by a specialist firm before any purchase.

Asbestos: a compulsory check before refurbishing

Asbestos was used in Spanish construction until it was banned in 2002. In buildings from before 2002, it may be present in: fibre cement in roofs or skylights, drainage pipes, boiler insulation, plaster false ceilings containing asbestos fibres. If you're going to refurbish a flat in a building from before 2002, you must have an asbestos survey done before starting the works. If there's asbestos, it has to be removed by an approved specialist company, at an extra cost of €2,000-15,000 depending on the amount and where it is.

When to hire a professional before buying

Hiring an aparejador or arquitecto técnico to produce a preliminary report on the condition of the property is essential when:

• The building is more than 40 years old.
• There are visible signs of cracks in structural elements or damp of uncertain origin.
• The planned spend on refurbishment is over €20,000 and you want to make sure the building's structural condition can take the works without surprises.
• The purchase price is significantly below comparable properties with no apparent explanation.

A preliminary technical report costs between €300 and €800 for a standard flat. That cost can save you tens of thousands of euros in hidden structural problems.

Related resources: For the full due diligence checklist before making an offer (which covers the structural condition alongside registered charges and planning issues), the property due diligence checklist is the reference guide. To calculate the ROI of a refurbishment that builds the cost of structural repairs into the budget, the article on refurbishment ROI sets out the method. For the impact of structural problems on the bid price at auctions (where you can't always get inside before bidding), the guide to court auctions includes the risk assessment of the property's condition.