With that in mind, work on the cathedral roof was conceived with a dual purpose: to enable safe access for inspection and maintenance without compromising the identity of the ensemble, and to ensure material compatibility in the event of potential waterproofing issues.
Discreet and effective, the intervention took the form of a lightweight roof deck made with reinforced Hebel panels, supported on low walls of Ytong blocks. These materials were chosen for their structural lightness and stable behaviour in the presence of water, supported by their Environmental Product Declaration (EPD).

Compatibility and Heritage

At first, the objective seemed clear: to build a walkable, lightweight roof deck that would allow safe access by distributing loads over the existing structure, limit invasive works and keep the site organised in line with the planned programme.
However, in a climate as humid as Galicia’s, the project management team — highly experienced in heritage interventions — considered it essential that, should any waterproofing incident occur, the load-bearing layer would not release substances that could damage the heritage assets sheltered beneath the roof.
This double constraint significantly narrowed the range of possible solutions.

The Solution: Reinforced Hebel Panels as a Lightweight Roof Deck

An effective answer was found in reinforced Hebel panels: lightweight, stable even in the presence of water, and supported by an EPD documenting their suitability for conservation environments.
AAC provides passive fire protection thanks to its mineral, non-combustible nature, its stability under hygrothermal cycles and ease of on-site machining, which allows edges and junctions to be detailed cleanly. Supported on Ytong low walls — which set out the module and help transfer loads — Hebel panels bring structural lightness, prefabrication and dimensional accuracy: key qualities when every kilogram and every junction matters.
Hebel’s EPD supports the safe, predictable use of the system with no hazardous emissions and no problematic leaching when in contact with water. Structural lightness is therefore complemented by verified material safety, aligned with the prudence required in any heritage intervention.

Installation: Sequencing and Modular Layout
The solution was organised in two steps: first, supporting low walls built with Ytong blocks were placed on the existing structure and adjusted at junctions with parapets and other singular roof elements.
Second, reinforced Hebel panels were dry-laid on those low walls, forming a continuous plane suitable for maintenance traffic, inspections, and routine cleaning operations.
The work was carried out under temporary protection appropriate to a heritage setting, with full process traceability and safety as priorities.
Results: Lightness, Precision and Serviceability
The result is a stable, homogeneous surface, with orderly joints and controlled tolerances, combining low self-weight compared with conventional solutions and sufficient load-bearing capacity for inspection and working routes. The stable behaviour of autoclaved aerated concrete in the presence of water adds an extra layer of confidence for heritage preservation.
Prefabrication brings pace and predictability to an intervention that demands meticulous planning, while the mineral, non-combustible nature of the material enhances safety.

Conclusion
On the cathedral’s ancient stone, between gargoyles and buttresses, a lightweight prefabricated solution based on reinforced Hebel panels and Ytong low walls delivered a walkable roof deck for maintenance—precise and respectful of the historic structure.
In heritage contexts, designing these roof decks means balancing weight, stability and compatibility while accounting for every singularity: this case shows how lightweight AAC systems can resolve complex situations with precision and respect for the original structural logic.
A silent intervention, unnoticed by most visitors, yet essential to ensuring the monument can continue to be cared for with the rigour its history demands.