Renovating a Bathroom in an Old Building: Pitfalls in Wilhelminian Houses in the Bergisches Land

Renovating a bathroom in a Wilhelminian house has little in common with a bathroom renovation in a new build. The rooms were rarely planned as bathrooms from the outset, the floors are usually timber, the pipework comes from several eras, and depending on the building the heritage authority has a say. This article describes the points that regularly cause delays in existing buildings and where they have to be clarified before material is ordered. It cannot and does not wish to make statements about the load-bearing capacity of a specific floor – that is a matter for a structural engineer.

Typical Building Fabric in the Bergisches Land

Haan lies in the Kreis Mettmann, the Mettmann district, between Düsseldorf, Solingen and Wuppertal, at the western edge of the Bergisches Land. Two building traditions shape the region. First, the Bergisch half-timbered house with black timber framing, light-coloured infill panels and green shutters, frequently with slate cladding on the weather-facing sides and a plinth of greywacke from regional quarries. Second, the development of the Wilhelminian era, which arose with industrialisation along the river Wupper – Wuppertal, for instance, has in the Briller Viertel a quarter regarded as the largest contiguous villa district of the Wilhelminian era in Germany.

What of this is present in a specific house can only be established on site and through the documentation; the cities' building archives often provide information on this. Little can be generalised. Residential buildings from the period between around 1870 and the First World War generally have solid external and central walls of brick masonry, timber joist floors between the storeys, vaulted or jack-arch ceilings over the basement and high rooms. Added to this are conversions, additional storeys and post-war reconstruction from later decades, which can make the build-up differ from floor to floor.

One observation is particularly important for bathroom planning: in many of these flats no bathroom was originally provided at all. Bathrooms were retrofitted into small chambers, pantries or partitioned-off ends of hallways, often in several waves over decades. This is why floor plans, pipe routes and floor build-ups in old buildings are rarely what a standard plan assumes. The first step is a survey of the existing fabric with openings at suitable points – and not the choice of fittings.

Such a survey is worthwhile because it determines the sequence of the trades. It makes sense to review building documentation and, if available, old floor plans, to open up the floor build-up at an inconspicuous point, to determine the position of the risers and to include the neighbouring rooms – often the kitchen lies behind the bathroom wall and its connections use the same riser. Only after that can questions such as a level-access shower, the position of the bath or the tile format be sensibly answered. Planned in the wrong order, every one of these decisions has to be reopened later.

Structural Engineering and Floor Loads With Timber Joist Floors

The decisive question in an old building is: what may go onto the floor? Only a structural engineer can answer that, depending on the case in consultation with a carpentry firm or an expert. This includes recording the joist cross-section, the span, the bearing situation and the condition of the timber – particularly at the bearings in the external walls, where moisture and pest damage occur most frequently. Without this clarification, any statement about permissible build-ups is speculation, including ours.

The reason for this care is weight. A wet screed with a tiled covering puts many times the surface weight of a dry construction onto the floor, and to this are added the filled bath, masonry pre-walls, natural stone or large-format slabs. This is why lighter build-ups are frequently used in old buildings: dry screed elements suitable for wet rooms, levelling fills instead of additional screed mass, pre-wall systems in dry construction instead of masonry installation walls. Whether that is sufficient in a specific case or whether the floor has to be strengthened is decided by the structural analysis.

Load-bearing capacity is not the only criterion here. Timber joist floors deform under load and vibrate; for rigid coverings such as tiles that can be too much even when the floor is adequate in calculation terms. Common answers to this are load-distributing sub-constructions, decoupling membranes beneath the covering and, where necessary, strengthening or stiffening of the floor. How such a substrate is to be checked before tiling is described in our article on checking old screed substrates.

The second obligatory point on a timber joist floor is the waterproofing. DIN 18534, the German standard on waterproofing indoors, assigns interiors to water exposure classes; a bathroom with a bath and without a floor drain is classified differently from one with a level-access shower, and from class W2-I onwards moisture-insensitive substrates are required. A bonded waterproofing beneath the tiled covering with cleanly executed junctions to walls, drain and penetrations is not an extra on a timber construction but a prerequisite. Impact sound insulation is also to be clarified early, because timber joist floors are the more difficult starting position here – in a multi-occupancy building it may be contractually owed.

Pipe Routing in Old Buildings

Drinking water pipes in old buildings are often a layered model of galvanised steel, copper, plastic and – in very old stock – lead. Lead pipes are no longer permissible: the Drinking Water Ordinance, the Trinkwasserverordnung, required them to be removed or permanently decommissioned by 12 January 2026; extensions were only possible on application and in exceptional cases. Anyone opening things up in the course of a bathroom renovation anyway should have the condition of the risers documented. Galvanised steel pipes narrow over the decades due to corrosion products, which makes itself felt as declining pressure.

On the waste water side, the fall is the real constraint. A WC cannot be moved around the room at will, because the connecting pipe needs a minimum fall to the soil stack and the available build-up height in an old building is limited. Pre-wall installations create room for manoeuvre here because they accommodate pipes and falls – but they cost floor depth, which in already small old-building bathrooms is a trade-off that belongs in the planning early on. Cast iron soil stacks are often better acoustically than their reputation suggests; their condition still has to be checked.

Chases and openings are the point at which things get expensive. Rules apply to chases in load-bearing masonry which limit depth and width depending on the wall thickness; where these limits are approached or where there is uncertainty, the structural engineer is again responsible. In half-timbered walls, cutting into posts and rails is fundamentally off limits. The route for the extract air from an internal bathroom – shaft, core drilling through the external wall, ducting above the roof – should also be settled before the planning, because it raises both structural and design questions.

That leaves the electrical installation. In unrenovated old buildings, protective conductors and residual current devices are frequently missing, and the number of circuits does not match what hangs in a bathroom today. That is a matter for a qualified electrical contractor and concerns not only the distribution board but also equipotential bonding and the protective zones in the bathroom. How such points affect the scope and calculation of a project is something we have broken down in our article on the cost factors of a bathroom renovation.

Closely connected with this is the question of indoor air. A new bathroom brings moisture into a part of the building that has not previously had to remove it in this quantity. In old buildings with cold external wall corners, old window junctions and without mechanical ventilation, these are the places where mould shows up later. It therefore has to be clarified early how the moisture leaves the room: through a window, through an extract system with a supply air opening, or through ventilation with heat recovery. If internal insulation on the external wall is also under consideration, it belongs in the same assessment, because it changes the temperatures within the building element. Doing without a good ventilation solution and hoping instead for the right airing habits rarely ends well in an old building.

Heritage Protection: When the Building Authority Has a Say

Heritage protection applies in two ways: a building can be entered in the heritage list as an individual monument, or it lies within a conservation area defined by by-law. In North Rhine-Westphalia, the lower heritage authority of the city or municipality is responsible. The legal basis is the Denkmalschutzgesetz NRW, the state's heritage protection act, in the version in force since 1 June 2022. Whether a particular house is affected is answered more quickly and more reliably by a phone call to the authority than by any research of your own.

For listed buildings the following applies: removal, alteration, relocation and change of use of a monument or part of it require permission. Repair works affecting only parts that are irrelevant to the heritage value may be exempt from permission – but it is not the client who draws that line, it is the authority. Permission is to be granted if there are no heritage protection grounds against it or if an overriding public interest requires the measure; the interests of housing construction, climate protection, the use of renewable energies and accessibility are to be given appropriate consideration.

A widespread error is to assume that protection ends at the front door. Interiors can also form part of the protected fabric – stucco, historic doors, staircases, floor coverings and, more rarely, room layouts. A bathroom that alters room-defining structures, requires openings in the façade or affects historic surfaces is therefore a matter for the authority. In practice this means: make contact early, document the existing fabric photographically, agree the project – and do so before material is ordered.

Two further distinctions. First, a heritage permit does not replace a building permit; whether a project requires approval under the state building regulations is a separate question for the building authority. Second, even without heritage status, municipal design or conservation by-laws can impose requirements, for instance on windows and façades. For listed buildings there are also special tax rules and funding routes; which of them come into question in an individual case is something the heritage authority and a tax adviser clarify, not a builders' merchant.

What This Means for Choosing Materials

From the preceding points follows a sequence that has proven its worth in old buildings: first clarify the existing fabric and the structural situation, then define build-up heights and pipe routes, then select materials. The other way round produces the typical site stoppages – the bath that does not fit through the stairwell, the drain that builds two centimetres too high, the tile format for which the substrate is not flat enough.

In concrete terms, old-building conditions affect above all these decisions:

• Weight: tile format and thickness, bath material and natural stone feed directly into the floor load
• Build-up height: extra-flat shower trays, flat drains and shower channels as well as thin-bed build-ups create room for manoeuvre
• Substrate: smaller formats forgive unevenness far more readily than large-format slabs with narrow joints
• Moisture: waterproofing, adhesive and grout as a matched system, suited to the water exposure class
• Accessibility: plan inspection openings for pipes and fittings while the wall is still open
• Long view: a level-access shower and sufficient circulation space can hardly be retrofitted later

Two practical points come on top, which arise more often in old buildings than in new ones. First, the transport route: narrow stairwells, winding flights and narrow doors have a say in which bath or which washbasin size can get into the flat at all – if in doubt, measure beforehand rather than saw afterwards. Second, the dimensional tolerance of the existing fabric: old-building walls are rarely plumb and rooms rarely square, which is why prefabricated elements, exactly fitting niche solutions and flush-planned fittings require more surveying. Both are arguments for having the contractor take measurements before ordering.

The selection itself is hard to make on screen. Formats, surfaces, slip resistance and colour effect are better judged from samples; our showroom in Haan on Düsseldorfer Straße is there for exactly that. The sanitary ware category in the shop gives a first overview of washbasins, baths, shower areas and WCs. If you would like to know how we work and which areas we cover, you will find that on our about page.

One final note: only order once the measurements, build-up height and connection positions are settled and the contractor has confirmed them. Experience shows that in old buildings these dimensions still shift as soon as the first wall is open. If you have a plan or a materials list, do send it to us in advance – then we will go through it together before anything is ordered. If insulation work is due at the same time, for instance internal insulation on the external walls, our article on insulation values and WLS classes helps in making sense of the key figures.