Dry or Wet Screed for a Renovation

In an older apartment, choosing dry or wet screed should start with the load the floor structure can take, the height available at doorways and the condition of the existing base. A wet screed can form a continuous, robust base; a dry system reduces water on site, shortens later stages and may be the more cautious option over lightweight structures. The final finish alone should not drive the choice. Before ordering flooring, confirm the existing build-up, actual level differences, services to be accommodated and the complete work sequence.

Contents

1. What is the practical difference between wet screed and a dry system?
2. When does structural load stop being a minor detail?
3. What should you check before selecting a system?
4. How do you avoid footfall noise and cracks at the edges?
5. How should the work be sequenced without holding up the renovation?

What is the practical difference between wet screed and a dry system?

Wet screed is a site-applied layer, commonly made with binders and aggregates, which is spread, levelled and needs time to harden and release moisture before receiving certain finishes. It may be bonded to the substrate or installed over separating layers, insulation or services, depending on the specified system.

A dry system achieves levelling with loose fill, rigid boards, floor panels or a combination of these materials. It does not rely on the same drying period and can allow some later stages to proceed sooner. In return, it needs a stable substrate, properly treated joints, perimeter movement gaps and compatibility between boards, insulation and the final floor finish.

Do not compare only the stated thickness. Compare the full build-up: existing slab or timber floor, levelling layer, insulation, acoustic layer, boards or screed, adhesive, finish and edge details. That total decides whether a door opens, kitchen units sit at the intended level and a balcony threshold still keeps water out.

For existing residential buildings, rehabilitation work must take account of applicable requirements for the elements being altered. Portugal’s acoustic framework for existing homes sets specific rules and allows justified solutions where the existing building imposes constraints. Read Portaria no. 305/2019 on acoustic requirements in existing residential buildings before turning a straightforward floor replacement into a noise problem.

When does structural load stop being a minor detail?

In Lisbon buildings with timber floors, older slabs or alterations added over several decades, adding a heavy new layer without diagnosis can lead to deflection, cracking or incompatibility with the existing structure. The situation is more sensitive where screeds have already been added over timber, unknown fill is present, new partitions are planned or wet areas are being moved.

Removing the old finish does not prove that the base is suitable. Look for movement underfoot, sagging areas, decayed boards, recurring wall cracks, moisture coming from below or earlier cuts made for services. Lisbon City Council notes that floor repair and strengthening can be relevant in rehabilitation work to reduce vulnerabilities, while making clear that repair alone does not automatically provide seismic strengthening. See the city’s guidance on floor repair and strengthening.

A useful rule is simple: if the new assembly materially changes loads, requires cutting joists, corrects substantial sagging or reveals structural damage, pause the commercial decision and obtain technical assessment. Do not keep adding mass to an uneven floor until it looks level.

Coordinate this choice with any suspended ceiling planned in the room below and with services routed through the floor. Dealing with each element in isolation often uses up height twice.

What should you check before selecting a system?

Take levels before demolition and take them again once the necessary areas have been opened. A small difference at the entrance can become decisive in a kitchen, at a sliding door or where the floor meets a bathroom.

1. Set out levels: mark the required finished floor level in every room, including landings, thresholds, entrance doors and balcony transitions.
2. Identify the substrate: establish whether you have concrete, timber over joists, old tile, weak mortar or several poorly bonded layers.
3. Locate services: water, drainage, electrical wiring, heating and floor boxes need defined routes and access before levelling begins.
4. Choose the finish: timber, vinyl, ceramic and stone have different requirements for flatness, stiffness and base moisture.
5. Resolve transitions: doors, skirtings, kitchen furniture and shower trays should not depend on last-day improvisation.

If the work goes beyond a straightforward internal finish, the need for design packages depends on the operation and disciplines involved. Lisbon’s list includes, among others, structural design, thermal performance, acoustic conditioning and internal water and drainage systems. Check the City Council’s specialist design requirements before finalising the quotation.

How do you avoid footfall noise and cracks at the edges?

A new base may feel firmer underfoot and still transmit noise to the apartment below if it is rigidly connected to walls, columns, pipes or thresholds. A resilient layer works only when the continuity of the system is respected. A perimeter strip interrupted by mortar, a board pressed tight against a wall or a pipe fixed through a floating layer can create an acoustic bridge.

With wet screed, confirm separation between the floating layer and vertical elements, as well as the location of joints required by the system. With dry construction, confirm that the fill is level, boards have been installed with the specified stagger and there are no voids that can lead to vibration or noise.

Avoid three frequent mistakes:

• bonding ceramic tiles to a base that is still damp or unstable;
• filling a structural crack with mortar without understanding why it appeared;
• cutting the acoustic edge strip by the walls before the finish and skirtings are complete.

If reducing noise between apartments is the main aim, do not buy an underlay based on thickness or advertising alone. Ask for the full assembly, declared performance for that construction and details of the junctions. For existing buildings, the regulations recognise that flanking transmission matters too; this is why a solution applied only in the centre of a room can disappoint.

Where floors are being renewed alongside walls or plumbing, also plan fire compartmentation at pipe and cable penetrations rather than leaving it until after finishes are installed.

How should the work be sequenced without holding up the renovation?

The safest sequence starts with controlled demolition and cleaning of the substrate. Only then should local repairs be confirmed, finished levels set and the chosen system installed. Buying the finish before this confirmation narrows the options and often leads to cut doors, raised thresholds or improvised trim profiles.

In an apartment building, coordinate deliveries of bags, boards and skips with the condominium. Occupation of public space may require notification or a licence, and the city distinguishes internal work that does not affect stability from interventions that go beyond finishes. Check Lisbon’s rules on public-space occupation for works exempt from prior planning control if you need a skip, stored materials or organised unloading.

Before accepting the base as complete, carry out a simple check with the contractor:

• levels checked at doors and wet areas;
• flatness suitable for the selected finish;
• perimeter decoupling maintained where the system requires it;
• services photographed before they are covered;
• moisture and curing time confirmed for wet screed;
• boards, joints and support checked for dry construction.

The right decision is not the abstractly fastest option. It is the build-up that fits the structure, respects the available height, supports the finish and allows doors, skirtings and services to be completed without expensive corrections.

Key takeaways

• Set out all finished levels before choosing thicknesses or ordering the floor finish.
• In older floors, confirm the structure and existing layers before adding mass.
• Acoustic underlay works only if rigid bridges at walls, pipes and thresholds are avoided.
• Record services and sign off the base before it is covered by the final finish.

If you are renovating a Lisbon apartment and need to coordinate floor build-up, services and finished levels, ask Obra Lisboa for a renovation proposal that identifies the base system within the scope of work before appointment.

Sources and further reading

• Portaria no. 305/2019 on acoustic requirements in existing residential buildings
• guidance on floor repair and strengthening
• specialist design requirements
• public-space occupation for works exempt from prior planning control

Editorial image; it does not depict an Obra Lisboa project.