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.
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:
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.
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.
Here is a video about an old farm building inspection with rotten joists, as a result of various dampness problems.
Before any treatment, a thorough inspection is vital:
Early detection and prompt action are crucial in this phase to preserve the structural integrity of historic buildings.
When faced with rot, the following steps are recommended:
To prevent joists from rotting, consider the following actions:
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.
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.
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:
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.
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.
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Here are some related pages with additional technical information, giving you a more in-depth understanding of this topic.
Here are some videos related to this solution. Please unmute the videos when playing them.
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The plaster application video below explains the concept in detail.