Calculating Foundation Depth For Two Storey Extensions

Posted by Rebecca Foster on Jan 13, 2020 10:19:00 AM
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Calculating Foundation Depth For Two Storey Extensions

Foundations are undoubtedly the most important part of a building, and there are strict regulations in place to ensure foundations are adequate for the type of building or extension being proposed. If you are having a two-storey extension added to your home, it is absolutely critical that the foundations are done correctly. There is no one-size-fits-all solution for this, so let's look at the factors that need to be considered.Click here to download our FREE essential guide to extending your home!

What Is The Common Recommendation?

As a base point of reference, a two-storey extension will need strip foundations with a width of around 600mm. Strip foundations involve a wide-base concrete or block strip inserted into the trench, rather than the solid concrete involved in trench fill foundations. The strip dimensions are in proportion to the width of the wall; as a minimum, they should be equal to the wall's thickness (thicker is often preferable).

You'll be pleased to know that these are actually a lower-cost solution than some other types of foundation, because they minimise brick-laying below ground and require little specialist equipment. The minimum foundation depth for two-storey foundations is 200mm, to ensure they have the necessary strength to fulfil their purpose.

Factors To Consider When Calculating The Depth Of Foundations For A Two-Storey Extension

The first key factor that can affect the required depth of the foundations is the soil type. Different soils react differently to things like heavy rain and dry summer months. For instance, in clay soils, you need deeper foundations, since clay can expand and contract significantly with prolonged changes in the atmosphere. Meanwhile sand, chalk and various other soil types can be susceptible to frost, so it's necessary to dig below the level at which frost can permeate.

It's also important to consider the purpose of the extension, in order to determine the weight the foundations will need to bear, as well as the shape and height. Other important factors to consider on-site include neighbouring buildings, trees and drains.

Dire Consequences

If mistakes are made with any of these factors, the consequences can be severe. Poor ground conditions may mean more expensive foundation types are required. If construction is too close to a garden tree, the tree's roots can compromise the foundations and cause the extension to subside. If there is an old well or tank that goes unnoticed, the excess water can cause the foundations to become unstable. These are all reasons why it is important to have a quality, thorough inspection of the area to determine the correct type and dimensions of the foundation for your two-storey extension.

BDS Architecture have all the expertise you need to ensure the foundations for your two-storey extension are done right. If you have a question, get in touch today and our friendly team will let you know how we can help.

The Essential Guide To Extending Your Living Space

Topics: Extensions, Two Storey Extension

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