ᐅ Screed over underfloor heating – is wall plastering required before applying the screed?

Hello everyone,

I am renovating an old house (built around 1900) and want to install underfloor heating on the ground floor. The old floor has already been removed, and a new concrete slab has been poured. The electrical wiring and pipes are already in place, so the next step is the underfloor heating.

At the moment, we cannot start plastering immediately due to various delays. Because of this, my contractors are currently in disagreement about whether the screed can be laid directly on top of the underfloor heating now, or if the walls need to be plastered first. I have to decide whether to plaster the interior walls first, but I am unsure.

The screed would abut an old brick wall that is not perfectly even. The house stands on a raised foundation, so everything seems dry from below. The floor will be tiled afterwards.

I understand that plastering later will cause some mess, which I would try to manage by covering the floor. But in your opinion, is there any significant reason not to lay the screed directly against the “open” wall and plaster afterwards?

Kind regards,
Tom

The typical process is to plaster the wall first, then install the edge insulation strip around it, and finally pour the screed. For soundproofing reasons, the wall and the screed must not be in direct contact. If you plaster afterwards, you must not plaster all the way down to the screed; a gap must be maintained. You will have the least trouble if you avoid having to remove plaster residues from the screed later on.

I don’t think it’s possible to cover it that well... especially if the pipes are already installed.

And the underfloor heating is quite sensitive... I wouldn’t walk on it!

Depending on the exterior wall construction, the interior plaster serves as the airtight layer of the wall. Therefore, plastering is done up to the floor slab first to ensure airtightness.

T_im_Norden schrieb:

Depending on the external wall construction, the interior plaster serves as the airtight layer of the wall.
Therefore, plaster is applied up to the floor slab first in order to achieve airtightness.

I would like to ask for the relevant technical standards that define interior plaster as the airtight layer!!
I am eager to learn!
Regarding the actual question:
Yes, it is indeed a problem if the wall plaster is applied after the screed.
Just do it this way:
Typically, for heated screeds, an expansion gap with a thickness of 10mm (0.4 inches) between the screed and the plastered wall is required.
When installing the screed, it is best to use 20mm (0.8 inches) thick (double) screed edge strips, each 10mm (0.4 inches) wide, placed continuously along the brick wall. These must be fixed securely to prevent displacement. When using wooden strips, these are removed after the screed mortar has hardened and replaced by 10mm (0.4 inches) thick edge strips before applying the wall plaster.
The double edge strips remain in the expansion gap chamber later, and the plaster should only be applied up to the top edge of the screed.
For this purpose, during the application of the wall plaster, 5mm (0.2 inches) thick screed edge strips or MDF strips can be placed on the floor-to-wall transition (meaning they lie on the screed surface for protection and are covered by the plaster extending to the brick wall).
This approach is somewhat more elaborate but will work!
-----------------------------------------
Best regards, KlaRa

I cannot say whether there is a specific technical regulation for this. However, it is often pointed out that, for example, with brick walls, the interior plaster serves this function if no exterior plaster is applied.

For example, articles in DBZ, ikz, from the Architects’ Chamber BW, and others.