The addition is a small room 3mX3m and at the exterior corner (the other tree corners are attached to the house) there is a sinking - like 2 cm in 12 years.
How can this be repaired?
While 2cm is less than an inch, a foundation should not sink at all.
Likely, the foundation was built on disturbed soil. If it is a pier (post hole) that was dug and poured in a sonotube it is quite common as people rarely clean out the hole prior to the pour and this almost always results in settlement.
If it is a poured footing and foundation it may have been dug too deep and then had soil thrown back in the bottom of the forms to block off the gaps at the bottom of the form boards, or was built on frozen soil.
Regardless, you can either jack up the corner (carefully) and shim the post, however this is not as simple as it sounds, the post to framing connection must be reestablished and secured.
The other option is to jack up the corner and excavate below the footing and underpin to solid bearing.
If you post your project on the site, you will get qualified contractors to bid on the repair.
Hope this helps.
The Cedarbrook Group
Without seeing the situation it is hard to figure out what is causing the sinking. But if it is sinking then we would need to get down to the basics and start with checking out the support/floor joists/foundation. The sizes and how the addition is supported must be checked out and the sizes of the support must be confirmed that there sized to support the addition.
There could be issues like a) the size of the support corner, b) does the support have frost coverage, c) how is the floor/walls supported d) how is the addition supported at the 3 attached corners
A contractor who is familar with structural sizing or an architect or a professional structural engineer should be consulted.
The sinking could be repaired by adding a foundation support or increasing or repairing the existing support(s). But a site inspection is required to give an opinion of what is required or/and a direction to proceed.
Sinking foundations can become a problem because the Contractor constructed footings that are now insufficient in size (15") to accommodate your specific soil's support capabilities in all conditions. This problem has become very evident in areas like Ottawa Valley that has a variety of seven (7) soil types and is exacerbated because of droughts and roots of trees close-by that remove all moisture during ht latest droughts we've all experienced. Drought-like conditions may get worse over the coming years, as the trees grow larger and our summers get hotter and dryer.
It appears you may have to do invest in some serious foundation work ($30K - $65K) to extend your old footings to 30" in width to accommodate dry spells, coming in the years ahead. As well, have the Contractor protect the walls of the house at the same time.
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