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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.mber in general, as it keeps timber healthy.

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. Use the Right Lime Plaster for 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.

For low level areas, near ground level or underground, subject to high levels of moisture and salts, there are special Roman lime plasters with volcanic sands - also known as pozzolans -  that can tolerate high humidity and salts without breaking down. These plasters are also highly breathable, preserving timber elements, causing no rot long-term. 

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 product variations are possible as we have different types of plasters and paints, depending on your needs or application. Please get in touch to discuss additional options.

Work Specifications

Here are the recommended work specifications for this solution.

rinzaffo category

Base, Waterproofing

Application Guide

Rinzaffo Controspinta MGN

[15-20 mm] ABOVE GROUND: Lime base coat, applied as:

  • █ [5-10 mm] Levelling / Repair: level, repair and point uneven walls first. The material consumption here can vary, depending on substrate condition. Very uneven walls (e.g. after cement plaster removal) can take up more material.
  • █ [10 mm] Protective base coat: apply a 10 mm coat.

[25-30 mm] UNDER GROUND: Lime waterproofing coat or for very damp wall sections, applied as:

  • █ [5-10 mm] Levelling / Repair: level, repair and point uneven walls first. The material consumption here can vary, depending on substrate condition. Very uneven walls (e.g. after cement plaster removal) can take up more material.
  • █ [10 mm] Waterproofing coat 1: apply a first 10 mm coat.
  • █ [10 mm] Waterproofing coat 2: apply a second 10 mm coat to complete the waterproofing.
betoncino-main-core-conservation

Base, Consolidation, Flooring, Building

Betoncino Consolidante MGN

[20-40 mm] Structural Consolidation lime plaster, in 20 – 40 mm or more thickness, applied as:

  • █ [10 – 15 mm] Consolidation plaster, coat 1
  • ▒ [Mesh] Structural reinforcement mesh (20 x 20 mm, 40 x 40 mm): embedded between coats for additional reinforcement and flexibility.
  • █ [10 – 15 mm] Consolidation plaster, coat 2. Etc.
dehumidificante-core-conservation

Second coat

Cocciopesto Deumidificante MGN

[15-20 mm] Lime second coat, with 15 mm for internal, and 20 mm for external walls. Applied usually in 2 coats as:

  • █ [8-10 mm] Second coat, first half, for vapour control.
  • █ [8-10 mm] Second coat, second half, for vapour control.

Plaster Application

Here are some key application points about the application of each plaster, as well as links to the full application guides. Please have your plasterer read / watch the application instructions before applying the product. If any questions, please contact us, happy to help.

rinzaffo category

Base, Waterproofing

Application Guide

Rinzaffo

VERY IMPORTANT POINTS

There are a few very important points that MUST be respected for the correct application of this plaster. These are highlighted in red.

  • 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. The easiest way to achiever his is to level (dub out) the wall first, then apply your required thickness. 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.
  • Close all pores, no matter how small. Treat and apply the plaster as a tanking-grade material. Cover the whole wall surface thoroughly leaving no holes or gaps behind, no matter how small. Smoothen the surface, giving it a light key with a damp brush.
  • Reinforce dark patches. The thickness of the plaster must be adapted to the (damp) condition of the underlying wall. Damp wall sections do evaporate out a lot of moisture, creating dark plaster areas as a result of condensation. These areas need extra thickness, more material. Apply an extra coat over darker areas.

– – – – – 

  • 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.
  • Cement backgrounds: the Rinzaffo MGN base coat can be applied and will bond well to cementicios backgrounds. The removal of cement plasters is recommended for the vast majority of cases to restore the breathability of the wall fabric. However, in some (rare) cases, when the removal of cement would be too invasive, too costly or would not beneficial the underlying masonry, if the cement backing is stable, the Rinzaffo MGN can be applied over it for waterproofing purposes (e.g. to waterproof cement floor bases). 
  • Salt-inhibitors or PVA bonding agents are NOT needed and should not be used before the application of this plaster. This natural lime plaster bonds very 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. 
  • IMPORTANT: Dry to wet mixing: plasters with hydraulic ingredients should always be mixed from dry to wet (workable) state. Add your powder to a bit of water, start mixing and gradually add more water till the right consistency is reached. This is the right way. Mixing the other way – wet to dry mixing – by starting with lots of water then gradually adding powder to the right consistency, is incorrect. Reason: excessive wetting weakens the plaster, alters its hydraulic set resulting in less performant, more friable and shorter lasting plasters.
  • 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.
  • 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.
betoncino-main-core-conservation

Base, Consolidation, Flooring, Building

Betoncino

  • 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.
  • Salt-inhibitors or PVA bonding agents are NOT needed and should not be used before the application of this plaster. This natural lime plaster bonds very 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. 
  • IMPORTANT: Dry to wet mixing: plasters with hydraulic ingredients should always be mixed from dry to wet (workable) state. Add your powder to a bit of water, start mixing and gradually add more water till the right consistency is reached. This is the right way. Mixing the other way – wet to dry mixing – by starting with lots of water then gradually adding powder to the right consistency, is incorrect. Reason: excessive wetting weakens the plaster, alters its hydraulic set resulting in less performant, more friable and shorter lasting plasters.
  • 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.
  • Drying time: the plaster dries quite quickly, typically in 1-2 days, depending on ambient conditions. Once it has hardened, the next coat can be applied.
  • 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).
dehumidificante-core-conservation

Second coat

Cocciopesto Deumidificante

  • 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. 
  • IMPORTANT: Dry to wet mixing: plasters with hydraulic ingredients should always be mixed from dry to wet (workable) state. Add your powder to a bit of water, start mixing and gradually add more water till the right consistency is reached. This is the right way. Mixing the other way – wet to dry mixing – by starting with lots of water then gradually adding powder to the right consistency, is incorrect. Reason: excessive wetting weakens the plaster, alters its hydraulic set resulting in less performant, more friable and shorter lasting plasters.
  • 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.
  • Drying time: the plaster dries quite quickly, typically in 1-2 days, depending on ambient conditions. Once it has hardened, the next coat can be applied.
  • 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).

Videos

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More Information

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

Any Questions? Need Technical Advice?

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