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Rotting Joists - Causes, Diagnosis, Repair

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Old and listed buildings exude charm and historical significance, but their structural components often face unique challenges. Among these, rotting joists or the deterioration of timber elments pose a significant threat.

Causes of Rotting Joists

Joists rot primarily due to moisture, either liquid moisture infiltration or very high air humidity. Specifically, when joists rest on damp masonry, wet rot can occur. Here are the key reasons causing joists to rot:

  1. Sideways moisture infiltration: joists rot in contact with external earth-touching walls, especially in older buildings where the soil outside is often higher than the floor level. In this scenario, soil moisture gradually infiltrates the masonry, directly affecting the timber. The consequences are twofold: first, the continuous exposure to dampness accelerates timber decay, leading to rot; second, as the joists weaken, the building’s stability can become compromised, leading to floor sagging and wall shifting.

  2. Leaks: leaks within a building can lead to isolated rot spots. These leaks may originate from overflowing gutters, leaky pipes or cracks in the pointing (the mortar between the bricks). Swiftly identifying and addressing these leaks is essential to prevent further damage and maintain the structural integrity of the joists and surrounding materials.

  3. Rising damp: when the property lacks a damp-proof course, groundwater can rise all the way up to the joists. Non-breathable materials like cement or membranes around the timber exacerbate this issue, channeling enough moisture into the timber to cause rot over time.

  4. Condensation and poor ventilation: pose significant risks to timber structures, especially in historic and listed buildings. When air bricks beneath suspended timber floors become blocked, vapour moisture accumulates. This trapped moisture leads to condensation, which, over time, saturates the timber. Proper ventilation and adequate airflow under the floors is crucial to prevent this dampness cycle, reducing the risk of rot.

Here is a video about an old farm building inspection with rotten joists, as a result of various dampness problems.

Diagnosing & Addressing Joist Rot

Before any treatment, a thorough inspection is vital:

  • Check timber: assess the joists for signs of decay, such as softness, flexing, or visible deterioration. Look for discoloured areas, fungal growth, or weakened sections.
  • Inspect the masonry around the joists: look for wet areas near the joists as they may indicate moisture infiltration. If the masonry is lower than the adjacent soil this increases the risk of rot. Soil removal near the base of the wall can alleviate this.

Early detection and prompt action are crucial in this phase to preserve the structural integrity of historic buildings.

Intervention and Repair

When faced with rot, the following steps are recommended:

  1. Address underlying dampness problems: fix leaks, improve ventilation, and address rising damp.
  2. Repair or replace timber: use treated timber for replacements, ensuring longevity.
  3. Perform additional repairs: do additional repairs with breathable materials only.

1. Address Underlying Dampness Problems

To prevent joists from rotting, consider the following actions:

  1. Sideways moisture infiltration: have good drainage in place to channel away as much liquid moisture from the property as possible, Moreover. earth-touching walls in contact with joists can be plastered with the Rinzaffo MGN Roman waterproofing lime plaster. This is a special lime mix with volcanic sands and ashes which - due to its special pore structure - stops liquid moisture penetration while letting the underling masonry to breathe. Due to its pore structure it also regulates humidity. Replastering the base of the walls or underground areas with this Roman plaster keeps liquid water away from the joists while allowing vapours to flow and the timber to breathe, keeping the timbers dry.

  2. Leaks: leaks can cause isolated rot spots. Finding the source of leaks and remedying the dampness problems will make these areas recover. It is important to only use breathable materials during renovation.

  3. Rising damp: rising damp can be a major source of problems which can lead to slow but severe timber decay over time, especially for buildings completely lacking a damp proof course (DPC). The magnetic DPC system is a great non-invasive DPC alternative to the invasive chemical injections, being also suitable for listed buildings.

  4. Condensation and poor ventilation: to keep floor joists rot-free, prioritize proper ventilation. Unblock air bricks beneath suspended timber floors to allow fresh air circulation. These vents play a crucial role in dissipating moisture and maintaining a dry crawl space. A well-ventilated environment helps preserve the integrity of the joists and ensures the longevity of the structure.

2. Repair or Replace Timber

When it comes to repairing or replacing rotted floor joists in listed buildings, meticulous attention and adherence to guidelines are essential. For listed buildings, consult with conservation officers and follow their guidelines. Design and implement repairs that align with conservation standards. Always balance retention versus replacement.

You should consider the following points:

  1. Repairing localized rot: for localized rot, consider attaching a new joist alongside the damaged one. This reinforces the structure without complete replacement, preserving some of the historic material. Cut out decayed sections and replace them with new timber. Secure the new joist to adjacent members and the wall plate using nails or screws.

  2. Replacing severely rotted joists: if rot infestations are extensive or affect multiple joists, complete replacement may be necessary. Remove the old joists entirely and install new ones, ensuring proper alignment. Use pressure-treated timber for longevity and structural integrity. If more than 50% of the floor is affected, it is best to renovate the entire floor, including insulation, to also improve the floor's thermal efficiency.

3. Perform Additional Repairs

Always avoid cement and non-breathable membranes for replastering or when embedding timber. Use lime instead as it breathes and won't trap moisture behind it. Lime also has antiseptic and anti-fungal properties, hence it is the preferred building material to be used in timber frame buildings and around timber in general, as it keeps timber healthy.

joists-core-conservation
Joists embedded in the Betoncino MGN Roman lime mortar

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.

More Information

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

Videos

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

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Any Questions? Need Technical Advice?

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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

rinzaffo category

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.

Betoncino

betoncino-main-core-conservation

Base, Building, Consolidation, Flooring

  • 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.
  • No salt-inhibitors nor PVA bonding agents should be used before the application of the plaster. The plaster bonds well on its own.
  • 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. 
  • Application: apply the plaster in 10 mm coats.
  • Light key: give the plaster a light key using a wet brush. Do not cut into the material with the edge of a trowel.
  • 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).

Cocciopesto Deumidificante

dehumidificante-core-conservation

Main

  • 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.
  • 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. 
  • Application: apply the plaster in 10 mm coats.
  • Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.
  • Finish options: main lime coats can be optionally left without finishing. To have a finished surface any MGN finish can be applied: Calcina Fine MGN (white lime finish), Intonachino Arenino MGN (coloured lime finish) or Marmorino MGN (Venetian Marmorino finish).
  • 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).