If you consistently follow all the recommendations in this article, almost any homeowner can build a foundation for an extension on their own. What’s more, doing it yourself means the construction cost will be significantly lower than hiring workers from an ad. The ideal scenario is if you remember the key details of the original construction and have the project plans with geodesic survey data. All of this will greatly simplify and speed up the process of building the foundation.
Tying the Main and Extension Foundations Together
When planning an extension to your main house, it’s important to understand that you can’t just tack it on. Any construction project must be approached thoughtfully and with careful calculations. Otherwise, you risk damaging your existing home. The new extension will exert pressure on the ground, which could cause movement and, in turn, damage the structure. Therefore, when building an extension, you must connect its foundation to that of the existing house.
A foundation can be tied in several ways:
- A rigid connection. This method should be used for homes that were built long ago and whose foundations have fully settled. Additionally, you should perform simple calculations to ensure that the new load won’t cause the old foundation to deform.
- Installing special expansion joints.
As a brief aside, a rigid connection should never be used in areas with non-heaving soils.
Let’s take a more detailed look at each method for building a new foundation for an extension to a wooden house.
The Rigid Foundation Connection Method
Before you start pouring the foundation for the extension, you need to remember what type of foundation the original house was built on. The most common types are strip or column foundations; slab or pile foundations are less common.
In addition to the foundation type, you need to determine several other factors that could affect the future construction, namely:
- the depth to which the foundation was laid;
- the area it occupies under the house;
- the types and kinds of building materials used in its construction.
It’s not a disaster if some of this information is lost, as it can be recovered through new investigations. To do this, dig a trench along the old foundation to get an idea of its size. Its depth can be determined with a simple tool: a piece of rebar with an end bent like a poker. This end should be slid under the foundation so that it lies flat against its underside.
Next, rotate the rebar to a horizontal position and pull it until it hits the back of the foundation, then make a mark where the wall ends. Remove the probe and repeat this process along the entire wall to get a fairly accurate picture of the foundation’s dimensions.
The new foundation must be the same type as the old one. This should be done after calculating the soil’s potential for frost heave at the construction site. These calculations are necessary in any case—excuses like “I’m only building a small extension” or “it will be made of lightweight materials” are not acceptable. Additionally, the new foundation should not be deeper than the old one; they must be at the same level.
In the next stage of construction, we’ll need to drill holes for reinforcing rebar. The holes should be slightly larger than the diameter of the bars themselves. Inexperienced builders often ask: How deep should the rebar be embedded? There’s a universal formula for this: the embedment depth should be 35 times the bar’s diameter.
Sometimes, the width of the house’s foundation isn’t sufficient for drilling holes to the required depth. In this case, you can cut vertical slots into the face of the slab and insert special wedges. This system is called an anchor fastening and will create a strong bond between the new and old foundations.
It’s very important to correctly calculate the required number of rebar rods. The strength of the entire structure depends on this. If you install too few, the foundation will be weak; if you use more than the technology requires, it’s a waste of money. As a rule, they are installed at a rate of five per quarter meter. You can cut the rods to the required length with an angle grinder or order them pre-cut. To enhance the effectiveness of the installed rods, you can weld small pieces of metal or nuts onto their free ends. These ends will be encased in concrete, giving the structure additional reinforcement.
Properly Installing Expansion Joints
An expansion joint helps prevent negative consequences from seasonal and daily temperature fluctuations. To create a so-called expansion joint, leave a small gap (2 cm for small structures and 5 cm for buildings taller than one story) between the new and old foundations when pouring. A board of the required thickness, wrapped in plastic film and placed between the two foundations, will help you create a proper, even joint.
Digging the Trench and Pouring the Foundation
Before you start digging the trenches for the future foundation, you need to clearly define its location and mark its boundaries. This will significantly simplify all subsequent work. For a clearer layout, you can use stakes and a spool of twine. The diagonals laid out at the corners of the trench will help you check if the future foundation is square. Using a plumb bob can help you achieve straight, vertical walls, which is another important factor affecting the strength and quality of the future structure.
If the extension is small, you can dig the trench yourself; otherwise, it’s better to use specialized equipment. After the pit has been dug to the required dimensions, you’ll need to prepare its base by laying down a cushion of sand and gravel. Then, you should plan for all necessary utility lines and communications before you can start pouring the foundation.
When making concrete, you should follow a few mandatory rules:
- Choose a frost-resistant cement for the foundation.
- Strictly follow the sequence for adding the concrete components: water, gravel, fine-grained sand, and only then cement.
- Deliver the finished concrete to the worksite quickly, as it hardens fast and becomes unsuitable for pouring.
- Pour concrete directly into the trench; if the foundation needs to rise above ground level, build the formwork for that section on top. Do not remove the formwork for at least a week after pouring.
- Before building the extension, you must lay waterproofing on the foundation.
By following all these conditions, you can successfully build a quality foundation for your future wooden house extension with your own hands. The cost of the construction itself will come down to the price of building materials and rented tools.