Bathroom in a panel building

Umakart core: paint, restore or replace with masonry

The umakart core is still present in many prefab blocks of flats from the time of the building's construction. If it is solid and dry, it can be painted and will last for years. If the walls are bending, there is moisture behind the umakart, or you need new installations, it is worth replacing it with a masonry bathroom. We will show both paths and the replacement process step by step.

Prepared by: the TGB Kúpeľne team, bathroom showroom in Senec since 1991updated 9. 11. 20266 min read

Illustrative image: original umakart prefabricated bathroom core in the bathroom of a prefab block of flats

The prefabricated bathroom core connects the bathroom, WC, and installations into one unit, which was installed at once during the construction of the prefab block of flats. After decades, it is decided whether it is sufficient to restore it or to replace it. A comprehensive view of the bathroom in a prefab block of flats – layouts, soil stacks, and permits – can be found in the article Bathroom in a prefab block of flats.

What is an umakart core

Since the 1960s, prefabricated bathroom cores have been installed in panel houses. The bathroom and WC were separated from the apartment by thin walls made of umakart on a metal or wooden frame, behind which the water, waste, and electrical installations were routed. Cores were installed approximately from 1958 to 1992 and were marked B2, B3, B4, B6, B7, and B10. The most common was core B3, which was produced from 1961 to the early 1980s and had walls made of umakart. Core B4 from 1964 to 1972 was modular made of panels. You can find out what core your building has from the administrator or in the project documentation.

After years, the cores sound similar:

• thin walls – cannot support heavier tiles or wall-hung cabinets and bend when touched,
• moisture behind the umakart – joints are not sealed, and moisture and mould accumulate behind the wall,
• old plumbing – steel pipes clogged with limescale and old aluminium cables,
• poor sound insulation – everything from the bathroom can be heard throughout the apartment.

Paint, restore or replace?

Paint for umakart: procedure

Umakart is smooth and low-absorbent, so regular paint does not adhere well. Preparation is key. Paint manufacturers recommend thorough degreasing, fine sanding, and a primer that improves adhesion for smooth and old surfaces. The usual procedure:

1. Check the condition – the walls must be solid and dry, free of mould and loose sections.
2. Wash and degrease the surface, especially around the sink and bath in the bathroom.
3. Lightly sand the entire area and thoroughly dust off.
4. Apply adhesive primer, as specified by the manufacturer, even on umakart or smooth plastics. For unknown substrates, manufacturers recommend a test coat on a small area.
5. Two coats of top paint for humid environments, with high resistance to wet abrasion according to EN 13300.
6. Renew the silicone around the bath, sink and in the corners with sanitary silicone.

Paint should not be used in areas with constant water contact, such as in a shower without a screen. For advice on which paint to choose and when it fails, see the article Paint for wall tiles. Further modifications without demolition can be found in the article Reconstruction without demolition.

Replacing the umakart core with a brick one step by step

1. Measurement and inspection. Dimensions, position of the soil stack, water meters, and shut-off valves, condition of the waste and electrical supply to the bathroom.
2. Administrator and office. Work schedule for the administrator, notification with the project in case of layout changes, date for water shut-off.
3. Protection of the building. Hallway, staircase, and lift covered, notice for neighbours with the demolition schedule.
4. Dismantling the core. The electrician will switch off and disconnect the circuits, the plumber will shut off the water and disconnect the fixtures. Then the laminate parts, frame, and original door frame are dismantled. If there is suspicion of asbestos in the waste or boards, stop work – only an authorized company can remove it.
5. New installations. Water, waste with a fall to the soil stack, and electricity according to zones in the bathroom – these are done by a professional, and the electrician will issue a report.
6. Partitions and installation wall. Lightweight partitions made of aerated concrete or moisture-resistant plasterboard, installation wall with an installation frame for the wall-hung toilet.
7. Floor. A thin leveling layer instead of a thick screed, flat shower tray – there is often little space for a fall in the floor of a prefab block of flats.
8. Waterproofing, wall tiles, and grouting in the same order as in any bathroom.
9. Doors, fixtures, and ventilation. New frame and doors, installation of sanitary ware, fan with a backdraft damper into the shaft.
10. Handover. Electrical installation report, photos of the installations, and leak test.

The entire bathroom reconstruction in a prefab block of flats usually takes 2 to 4 weeks. You can find a detailed procedure for each step in the article Bathroom reconstruction step by step.

What to build new walls from

Umakart is replaced with lightweight partitions – most commonly made from aerated concrete blocks 5 to 10 cm thick or moisture-resistant plasterboard. Building administrators usually prefer lightweight partitions; heavy masonry and thick concrete screed would unnecessarily burden the ceiling. Large-format wall tiles can also be adhered to the new walls, and cabinets can be hung. New partitions have a different thickness than umakart, so the bathroom size may change by a few centimetres. What to watch out for with plasterboard is described in the article Plasterboard in the bathroom, and how to utilise 3 m² is covered in the article Bathroom 3 m² in a prefab block of flats.

Permissions and building administrator

Always inform the administrator – core replacement will affect the soil stack and common installations. Many building authorities consider core replacement as a building modification that requires notification; if the layout changes or the bathroom is combined with the toilet, notification is necessary. According to Act No. 25/2025 Z. z., the office will process the notification within 30 days. What exactly to arrange is described in the chapter What the administrator and building authority will allow. An indicative budget can be calculated using the bathroom reconstruction calculator.

Common mistakes

• Painting without preparation – the paint peels off after a few months.
• Painting on damp or warped walls – the problem behind the umakart remains.
• Heavy masonry and thick screed during replacement – unnecessary burden on the ceiling.
• Covered closures and water meters – without inspection doors.
• Demolition without agreement with the administrator and without a water shut-off date.

Sources: types and periods of production of prefabricated bathroom cores according to surveys by architects and building administrators (as in our article Bathroom in a prefab block of flats), technical data sheets for Primalex paints (degreasing and sanding old coatings) and Hornbach (adhesive primer, test coat), EN 13300 (resistance to wet abrasion), Act No. 25/2025 Z. z. (notification of building modification, 30 days).

Terms in the glossary: Soil stack · Prefabricated bathroom core (umakart core) · Sanitary silicone · Fall (gradient) · Installation wall · Moisture-resistant plasterboard