When project manager Volker Wolf first descended into the intermediate level between the Hohenzollern Crypt and the Church of the Sermon at Berlin Cathedral, he thought he had seen it all. He had not.
There are construction sites, and then there are construction sites. Most involve scaffolding, open skies, and the occasional tricky corner. Very few involve crawling on your hands and knees through a pitch-dark space barely half a metre high, directly above the resting place of Prussian kings and queens who shaped European history for five centuries.
Welcome to one of KAEFER Germany’s most extraordinary passive fire protection projects to date.
A cathedral, a crypt, and a very tight deadline
The Berliner Dom, or Berlin Cathedral, is not just a church. Built between 1894 and 1905 under Kaiser Wilhelm II as a monument to the Hohenzollern dynasty, it is the largest Protestant church in Germany and one of the most visited landmarks in the country. Beneath its 114-metre dome, which gazes across the UNESCO World Heritage Museum Island, lies the Hohenzollern Crypt: one of the most significant dynastic burial sites in Europe, home to 91 burials spanning five centuries of Brandenburg-Prussian history.
After more than ten years of planning and six years of construction, the crypt reopened to the public on 1 March 2026, fully renovated, newly accessible, and brought into the 21st century. Behind that milestone was a vast team of architects, engineers, restorers, and specialist contractors. Among them: the KAEFER Germany team, tasked with a comprehensive passive fire protection upgrade that was as technically demanding as it was historically sensitive.
From left to right: Crypt before and after the construction
“I thought I had seen everything”
Volker Wolf, project manager at KAEFER Germany, has worked in construction long enough to have encountered most things. But when he first stepped into the intermediate level between the crypt and the Church of the Sermon above it, he paused.
“I thought I had seen a lot in my career,” he says. “But this was something very special. A new kind of challenge, even for me.”
The intermediate level is the hidden space sandwiched between the floor of the Church of the Sermon and the vaulted ceiling of the crypt below. It is not a room in any conventional sense. The working height ranges from just 0.5 to 1.5 metres. There is no standing. There is no walking. There is only crawling, on all fours, through a confined space that runs for the length of the building, carrying tools and materials as you go.
And there were approximately 1,800 metres of steel beams in there, all of them in need of attention.
The challenge: getting in, getting it done
The first question was simple and immediately complicated: how do you get people, equipment, and materials into a space you can barely fit a person through?
The answer: through two ceiling openings. That was it. Two access points for the entire intermediate level.
“The logistics were the trickiest part of the planning phase,” Volker explains. “The material transport of pre-cut fire protection boards from the KAEFER workshop in Butzbach, the extremely confined working conditions, and only two openings to work through. That was the moment we knew this would be more complex than expected.”
Everything had to be passed through those two openings, lowered onto scaffolding, and then moved by hand across the space. Once inside, the team worked to remove rust from the steel beams, apply a primer coat, and then add a double layer of corrosion protection. After that came the fire protection boards, applied on three sides of each beam to achieve F90 fire resistance classification. The level was then permanently sealed again, in close coordination with the building services team.
It was painstaking, physical, and slow. And it was done with precision.
Left: F90 steel cladding for cap ceiling
Right: Access opening in the intermediate level
Thirty metres down, no light, no ladder worth trusting
If the intermediate level was challenging, the utility shafts were something else entirely.
The cathedral’s supply shafts run approximately 30 metres deep, packed with vertical and partially horizontal pipework serving the entire building. A fixed iron ladder ran down each shaft. Or rather, it used to. By the time the KAEFER team arrived, the ladder had seen better days: unchecked, partially defective, and missing rungs in places.
“You could just about turn around on your own axis if you were hanging on the rope,” Volker recalls. “And there was no light at all in the entire shaft.”
This is where KAEFER’s rope access specialists came in. Descending on ropes into the shafts, they carried out a full photographic survey with height measurements, identifying every fire protection deficiency and proposing solutions for the client. What they found was significant: penetrations of heating, sanitary, ventilation, and electrical lines that were either incorrectly sealed or not sealed at all.
“Well,” Volker adds with characteristic pragmatism, “that is partly why we were there.”
The rope access and fire protection teams then worked together to remediate every deficiency they had documented, sealing penetrations and bringing the shafts up to standard.
Access to and view of the 30-metre service shaft
Fifty kilograms through a hole that was too small
Among the roughly 50 new elements installed during the project, including T90 and T30 steel fire doors, motorised smoke extraction flaps, and inspection doors, a few stand out as particularly memorable.
The F90 inspection flaps needed to be moved from the crypt level up into the intermediate level. Each one weighed around 50 kilograms. And in several places, the ceiling openings were simply too small to pass them through.
“We had to partially open the vaulted ceiling ourselves,” Volker says. “And in the worst cases, the flaps had to be transported via detours across the top of the vault.”
Then there were the doors. Four pallets arrived from the supplier in two partial deliveries. Inevitably, some doors were missing. Others arrived without the correct accessories. The team had to work out which accessories belonged to which door, while also managing the fact that the only available storage space was in the escape and rescue route, meaning everything had to be moved again within two days.
And the lift from the ground floor to the crypt level? It kept breaking down.
“So that meant waiting for help,” Volker says, with the tone of someone who has learned to find these things mildly amusing in retrospect.
Old building, modern standards, careful hands
Working in a listed monument that is over 120 years old brings its own set of considerations. Every decision about materials, colours, and finishes had to be weighed against heritage protection requirements. Where modern fire doors were installed in publicly visible areas, the team worked carefully to ensure that old and new sat in harmony, matching colour tones and door fittings to the character of each space.
“We were mainly active in non-public areas,” Volker explains. “But where it did matter visually, we made sure that everything was adapted and aligned with the building.”
When asked whether he ever felt the weight of history while working there, whether he ever thought about the craftsmen who had stood in those same spaces a century before, Volker pauses.
“Not really, in the moment,” he admits. “But now that you ask, it is actually quite remarkable, and a little moving, that you get to work in places like this.”
View of the finished suspended ceiling
The final push
The project did not end without its challenges. Delivery delays over the Christmas and New Year period put the team behind schedule, with the reopening of the cathedral drawing closer. There was pressure on all sides to get back on track.
KAEFER’s response was straightforward: additional installation teams, extended working hours, and weekend shifts. The delay was recovered. The deadline was met.
“That phase, just before the reopening, was when the team really showed what it was made of,” Volker says. “In the end, a shared solution was always the goal. We laughed together, we argued, internally and externally. But we got there.”
What Grandpa built
Ask Volker what he will say when he visits the Berliner Dom with his family in ten years, and he does not hesitate.
“I will point at the ceiling when I walk through the Hohenzollern Crypt. And I will point at the floor when I move through the rest of the building. And then I will hopefully tell my grandchild: Grandpa did all of that. And then my grandchild will hopefully ask: all of that? And then we will go into the detail.”
It is a good answer. And it captures something important about what this project represents: not just a technical achievement in a difficult environment, but a contribution to a place that will be visited, admired, and preserved for generations to come.
The passive fire protection systems installed by KAEFER will not be visible to the tourists who descend into the Hohenzollern Crypt to stand among five centuries of history. The sealed beams, the new doors, the remediated shafts, the work done on hands and knees in the dark: none of it will be seen.
But it will be there. Quietly doing its job. Just as it should be.
Note: Written by humans – with a little help from GenAI. This article combines human understanding and intelligent digital tools to bring KAEFER stories to life.