How many screw piles are needed for a building?

Why is there no “one screw per X square metres”?

Because the foundation does not support square metres. The foundation supports the structure.

Suppose we have two buildings of 60 m². The first is a modular building with two rigid longitudinal beams, so the loads can be transmitted through two main support lines. The second also has 60 m², but is made of several modules, has load-bearing interior walls and several structural joints. Three or four support lines may be needed.

Where do we start when determining the number of screws?

The basic question is: through which element does the weight of the building reach each screw?

A foundation screw must sit under a structural element that can transmit the load: a beam, a longitudinal member, a frame, a column, a load-bearing wall, a base plate or a steel distribution structure.

A steel pile installed somewhere under the building does not automatically become useful just because it is under the building's footprint. If there is no element above it capable of transmitting the load, the point is positioned wrongly.

In a modular house, the position of the screws must be correlated with the beams and the joints between modules. In a container, the intermediate points must be placed under longitudinal members, cross-members or distribution beams, not directly under the floor sheet. On a terrace, the distance between supports must allow the beams to work without excessive deflection or vibration.

How many screws are needed, indicatively?

For a first discussion we can use a few indicative ranges. They are not execution configurations and they do not replace design.

These ranges are useful for the order of magnitude. A building can fall outside them if it has heavy loads, significant spans, several storeys, difficult ground, level differences or significant lateral loads.

How many screws are needed for a container?

Here the idea comes up very often: “A container has four corners, so four screws are enough.” Sometimes that may be true, but it is not a rule for all containers.

The original shipping container is designed for handling and stacking through its corner fittings. The situation can change when the container is converted into a dwelling or office and large cut-outs, windows and doors appear, several containers are joined, or interior walls, services, finishes, terraces or other loads are added.

How many screws are needed for a modular house?

For a modular house, the number of points is linked first of all to the modulation of the floor structure and the position of the load-bearing walls. The main beams, the joints between modules, the load-bearing exterior and interior walls, large openings, glazed façades, the roof, terraces and areas with concentrated loads all matter.

As a preliminary benchmark, a single-storey modular house of 50–60 m² can start, in certain configurations, from about 15–24 points. For about 80–100 m², 20–36 points may frequently appear. For larger areas, ground-plus-one storeys, large glazing, heavy roofs or large spans, the configuration must be analysed separately.

How many screws are needed for a terrace?

On terraces, the number of points is strongly influenced by the beams and the comfort of the floor. A terrace can be strong enough and still have a floor that vibrates or feels springy when the supports are too far apart.

For a terrace of about 20–30 m², indicatively 9–16 points may appear. For a 4.5 × 5.5 m terrace, a 3 × 3 grid may be too sparse for certain timber sections. A 4 × 3 grid can be a more suitable starting point, and in other configurations 15–16 supports may appear.

The number can increase if the terrace includes a hot tub, an outdoor kitchen, heavy planters, steps, several levels, reduced-section beams, a great height above the ground, or finishes sensitive to deformation.

How many screws are needed for a pergola, gazebo or carport?

For a simple pergola, the first thought is often: number of posts = number of screws. Sometimes this can be the solution. For example, a simple pergola with four posts can have four points if the structure, the bracing, the foundation and the site conditions allow it.

But a light roof does not automatically mean small loads. Wind can produce uplift and significant moments at the base of the posts.

What is the distance between foundation screws?

There is no universal distance of 1.5 m, 2 m or 3 m that is correct for any building. The spacing is controlled simultaneously by two things: the structure above and the foundation in the ground.

The beam between two points must be able to span the distance without excessive deformation. At the same time, the screws must not be positioned without taking into account how each one mobilises the surrounding ground and any group effects.

Does a bigger screw mean we can put fewer points?

Sometimes yes, but not automatically. If we go from an M76 to an M89 and the screw can take a higher load, that does not mean we can simply remove every second point.

The beam or the chassis must be able to span the new distance between supports, the connections must be suitable, the reactions at the remaining points increase, and the displacements must be checked. Two M76 screws are not automatically equivalent to one M89.

If we put more screws, does the foundation become safer?

Not necessarily. More points do not automatically mean a better foundation. Each additional screw must be set out, installed, brought to level, connected and checked.

A grid that is too dense means more products, more connections, more installation time and a greater risk of differences between points. In addition, screws that are close together can influence the same volume of ground.

How does the ground influence the number of screws?

Very much. A good, relatively uniform layer can allow a simpler configuration. A site with uncontrolled fill, very soft ground, water, coarse gravel, boulders or shallow rock may require a completely different approach.

Sometimes the solution means longer screws. Other times a modular system. Sometimes testing is necessary. And in certain situations foundation screws may not be the right solution.

If a screw has a lower capacity than assumed, the answer is not automatically “let's put two more next to it”. A change of length, model, grid or even the foundation solution may be needed.

What loads must be taken into account?

The weight of the building is only one of them.

Indicative example: a modular house of about 80 m²

Let's take a simplified example: a single-storey house of about 8 × 10 m. Suppose the structure allows three longitudinal support lines, one on each edge and one in the centre.

How do we set a preliminary MARTETIC configuration?

Can the number of screws change during installation?

It can happen that a point provided for in the design cannot be installed exactly in the desired position or to the desired depth. Buried concrete, a boulder, an underground network, an old foundation or lack of machine access may appear.

But the solution is not to move the screw “a bit further over” and carry on. If the position of a support must change, it must be checked how the load will be transmitted to the new position. It may be necessary to move the point, change the product, introduce a distribution beam or modify the solution.

What do we check during installation?

A foundation is not finished just because all the screws have gone into the ground.

• the position relative to the axes;
• the top level;
• the verticality;
• the integrity of the product;
• the flanges and the connections;
• the parameters available from the installation;
• the documentation of the work;
• testing, if provided for.

These checks are especially important when the prefabricated building must arrive later and fit exactly over the installed points.

What do you need to send us to estimate the number of points?

For a first analysis, the following are useful: the building plan or the chassis plan, the dimensions, the number of storeys, the building system, the position of the beams and load-bearing walls if known, the weight or reactions of the structure if available, the locality, photographs of the site, the level differences, the geotechnical study if it exists, and information about access and utilities.

With this information we can discuss a preliminary support grid, the products that can enter the analysis and the installation conditions.

The right number is a result, not an assumption

For some buildings the answer may be 4 or 6. For others it may be 12, 24, 40, 50 or more. The figure alone says very little.

What matters is where each point is, what structural element it supports, what load it receives and how it transmits that load into the ground.

A well-conceived foundation does not start with choosing the biggest screw, nor with copying a grid seen on the internet. It starts with the building. Then we determine the supports, analyse the ground, choose the right products and prepare the installation.

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