Damp Patches on Plasterboard

Dot and Dab Plasterboarding Problems

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Plasterboarding

Plasterboarding or drylining represents an attractive and cost-effective alternative to wet plastering. This method became popular starting from the 1950s and gradually replaced wet plastering or traditional Victorian “lath & plaster” technique in many applications.

Application Methods

Here are the most common plasterboard application methods.

1. Dot and Dab

The dot and dab technique, also known as drylining, is a very popular due to its low cost, ease of finishing and fast installation. Walls finished in this way can be painted almost immediately. It involves attaching the plasterboard directly onto the wall using “blobs” of drywall adhesive.

While a budget-friendly option, it should be avoided on external walls (due to condensation and penetrating rain) or generally ground floor walls - usually prone to damp and salts (e.g. walls with rising damp or following a rising damp treatment) - as over time the moisture will gradually seep through the glue into the plasterboard, ruining it.

2. Timber Battening

This method involves fixing the plasterboard onto a wooden framework attached to the wall. Vertical timber battens are fitted to the wall at approx. 400 - 600 mm intervals, followed by screwing the plasterboard to the battens with drywall screws. Wall insulation can also be fitted behind the plasterboard in-between the battens.

This system allows better load resistance (e.g. for fitting shelves or cabinets) than the dot and dab system.

To protect the wooden battens from rot, insulating membrane is often fitted between the wall fabric and the wooden battens. However, if no membrane is used or the fastening screws pierce the waterproof membrane, the battens over time become susceptible to rot.

3. Metal Battening

Using metal battens or studs is the most modern method of plasterboarding and it is recommended for walls affected by dampness and salts.

Instead of using wooden battens, the more resistant galvanized steel is used and the plasterboard is fastened with screws onto the metal studs. Galvanized steel won’t rot or rust and offers long-term (50-75 years) protection in corrosive or salty environments.

Here are some examples of the various plasterboarding methods.

The Problem - Damp Patches on Plasterboard

After the plasterboarding is completed, damp patches can appear on the surface of the plasterboard under certain conditions. The damp patches often have a roundish shape. Mould spores can also appear on damp plasterboard surfaces.

These damp patches occur when the plasterboard is dot-and-dabbed or glued onto the underlying wall fabric. The plasterboard glue permits the transfer of moisture and salts from the underlying damp fabric into the plasterboard, making that damp. As glue patches under the plasterboard often have a round shape, many damp patches are also round.

patches-core-conservation — Damp patches and mould as a result of dot and dab plasterboarding

The Solution

To prevent the appearance of damp patches, the dot-and-dab plasterboard mounting method should be used sparingly or entirely avoided. Dot and dab should not be used on any ground floor walls or any external upper walls as all these walls are potentially subject to rising damp and/or wind driven rain.

Dot and dab can be used on the internal walls of upper floors only, these not being subject to rising damp wind driven rain. However avoiding dot and dab altogether and using other plasterboard mounting methods, such as timber or galvanized steel battens, is a much safer option or better practice.

Keeping the Plasterboard Cavity Dry

For the longevity of the plasterboarding and the health of the internal environment (prevention of mould growth) it is important to keep the plasterboard cavity as dry as possible.

Damp walls constantly evaporate moisture. Salts on the surface of the plasterboard can further accumulate and retain a lot of moisture as a result of hygroscopic effect. This can lead to mould grouwth behind the plasterboard, leading to mould spores inside the living environment.

Due to ongoing temperature and humidity changes, old walls constantly crumble, filling up the cavity with rubble. This over time can lead to moisture bridging, allowing the transfer of humidity from the damp walls into the plasterboard near the base of the walls.

To prevent all of the above from happening and to keep the plasterboard cavity dry and rubble-free, it is recommended to plaster the surface of the walls in the cavity with the Rinzaffo MGN Roman plaster in a thin (8-10 mm) coat. This Roman mortar coat applied under the plasterboard has the following benefits:

Mechanically stabilizes the wall surface: prevents crumbling, keeping the cavity free of debris.
Keeps the cavity dry: by acting as a liquid water barrier, on external walls affected by rain keeps liquid water away from the cavity.
Keeps the cavity salt-free: by acting as a salt barrier it provides a salt-free surface, keeping vapour humidity low in the cavity.
Regulates the internal humidity in the cavity: being a microporous lime plaster, it will not transfer sudden humidity surges from the wall fabric to the cavity, evening out humidity variations. keeping the moisture content of the cavity relatively even.

under plasterboard — Roman plaster under plasteboard

Here are some project application examples.

Recommended Products

Here are the typical recommended materials / products for this solution. Other plaster variations are possible as we have different types of main coats (normal or thermal) and finishes (smoother, grainier, coloured etc.) depending on your needs or application. Please get in touch to discuss additional options.

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Roman Waterproofing Lime Base Coat

More Information

Here are some related pages with additional technical information, giving you a more in-depth understanding of this topic.

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The Effect of Salts in Old Buildings

The Role and Importance of Salts

Videos

Here are some videos related to this solution. Please unmute the videos when playing them.

Normal lime plaster vs. a Roman salt-resistant lime plaster

Plaster Application

Here are some key application points about the application of each plaster, as well as links to the full application guides. Additional documents can be found on the individual plaster pages.

Rinzaffo

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Base, Waterproofing

Application Guide

Clean the masonry: the plaster must be applied on a cleaned and uniformly wet substrate. All crumbling and loose parts must be removed by brushing so the masonry is free of dust, salts and oils. If possible, also wash the walls with a pressure washer; this will clean and wet the walls in one go. Close larger holes with lime mortar and pieces of bricks.

Must be the first coat: the Rinzaffo MGN plaster must be the first coat on any wall. It should not be applied on top of other parge coats or plasters because when those fail the performance of the Roman base coat will also likely to be affected.

No salt-inhibitors nor PVA bonding agents should be used before the application of the plaster. The plaster bonds well on its own.

Masonry paints should be removed (which occasionally can be challenging) so the plaster can firmly adhere to the underlying stones or bricks. Remove at least 50% of the existing paint – the more is removed, the better.

Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained.

Wet the wall fabric abundantly before applying the plaster, as well as in-between each subsequent coat. Lime plasters need moisture as they set slowly in a damp environment. If the walls are already damp, there is no need to wait to become drier, you can proceed with the application of this plaster.

Level uneven surface first: very uneven walls (e.g. stone walls or crumbling old brick walls) are recommended to be patched up and levelled first before the application of a continuous coat, to ensure the consistency and required thickness of the base coat.

Application: apply the plaster in 10 mm coats.

IMPORTANT: Respect the thickness: do not under-spec the material. Apply min 10 mm (1 coat) for above ground level walls. Apply min 20 mm thickness (in 2 coats) for underground or extremely damp or salty walls. If more than one coat is applied, embedding a 10 x 10 mm fibreglass mesh is recommended between the coats – a standard practice in the industry.

IMPORTANT: Close all pores, no matter how small. Treat and apply the plaster as a tanking-grade material. Attention should be paid to compact it on the wall closing off all pores, no matter how small, leaving no gaps or holes where salts or liquid water could come through. Once an area has been completed, recheck that here are no missed holes, not even small ones.
The plaster application video below explains the concept in detail.

Light key: give the plaster a light key using a wet brush. Do not cut into the material with the edge of a trowel.

IMPORTANT: Dark patches. Allow the plaster to dry for 48 hours. Ideally, the whole surface should dry out uniformly to light brown, however you might notice some dark or damp-looking areas exhibiting surface condensation. Dark areas indicate insufficient thickness of the plaster in raport to the amount of moisture behind it. This occurs in areas where the underlying wall fabric is very damp, the intense evaporation causing surface condensation.
The fix is easy: apply extra material over such dark areas increasing the thickness of the plaster, closing off all pores. The increased plaster volume dilutes the vapour flow allowing the surface to dry.

Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.

Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).

IMPORTANT: Please watch the plaster application video below before applying the material. Unmute the video if it plays without sound.