What it looks like and where to find it

Seen from below, without plaster or a suspended ceiling, joists are the parallel pieces repeated at regular intervals, with clay or concrete infill blocks between them. Inside a home they are usually hidden; you see them best in garages, basements, storage rooms or the roof space.

They can be reinforced concrete, prestressed concrete, steel or, in old buildings, timber. The CTE's Basic Document SE-AE treats ribs and joists as secondary elements resting on primary ones: beams, walls and columns.

Why it matters when you buy

In some older buildings, the precast joists were made with high-alumina cement, which hardens in hours but, as Spain's RC-16 cement instruction (Royal Decree 256/2016) notes, tends to lose strength over time. When that deterioration progresses, it is known as aluminosis.

A damaged joist is not fixed inside your flat: it is part of the shared structure, and strengthening it is a job for the community of owners paid for by all of them. So, before the deposit contract (arras), it is worth knowing whether the building shows warning signs and whether the latest building inspection mentions cracks or rust in the floors.

What the HOUSINGFAX report tells you

Check 03, «Aluminosis», combines the year of construction from the Spanish Cadastre with what the registered building inspection says, where one exists, and gives the likelihood of high-alumina cement joists in words, from very low to very high. Check 01, «Building condition assessment», lists the defects the inspector recorded in the Valencian Community, the Basque Country and Madrid.

Every figure comes with its official source and date, in a report of 22 checks. See the sample report and check your home for free.

What no record can tell you

Neither the building's age nor a photo confirms which cement a joist is made of. That takes the design documents, the manufacturer's paperwork or a professional who examines it and, if needed, takes samples. The report tells you what to look into, but it does not replace a technical inspection of the building.

Short answers

Is a joist the same as a beam?

No. A beam is a main element that collects loads and carries them to columns or walls; a joist is a secondary part of the floor that rests on beams or walls.

How do I know whether the joists are high-alumina cement?

Only from the building's design documents, the manufacturer's paperwork or a professional survey, with samples if needed. The year of construction and the building inspection point the way but do not confirm it.

Where can I see a building's joists?

In garages, basements, storage rooms or the roof space where there is no suspended ceiling. Inside homes they are usually covered by plaster.

Which signs should worry me?

Cracks along the joist, rust stains, pieces of concrete coming away or a ceiling that bows. If you see them, have a professional check before you sign.

Official sources you can consult

The links let you review the primary source. Their inclusion does not widen their purpose or turn a piece of context into a diagnosis.