How far apart should adjustable pedestals be placed under terrace joists? 40 cm, 50 cm, 60 cm, or maybe 70 cm?
This is one of the most frequently asked questions when designing a deck terrace on pedestals.
However, there is no single correct spacing for all structures.
The spacing of pedestals under a joist depends primarily on:
- the material the joist is made of,
- the dimensions of its cross-section,
- above all, its height in the installation position,
- the design of the profile itself,
- the planned loads on the terrace.
In practice, in typical residential terraces, supports under joists are often spaced approximately 40–70 cm apart, but for a specific joist, the correct value may be either smaller or larger.
Very importantly, when designing the structure, there are two different spacings that are often confused with each other:
A. the spacing of pedestals along the joist – depends primarily on the strength and stiffness of the joist,
B. the spacing between adjacent joists – results primarily from the requirements of the deck board manufacturer.
These are two independent values.
Raised deck terraces – installation systems and technology for professionals
1. Spacing of pedestals along the joist
The first dimension determines how many centimeters apart a single joist is supported by an adjustable pedestal.
Imagine a joist that is 4 meters long.
It can be supported by pedestals:
- every 40 cm,
- every 50 cm,
- every 60 cm,
- every 70 cm,
- or – if its structural parameters allow it – even less frequently.
However, this distance should not be chosen randomly.
A joist between two pedestals behaves like a structural element carrying a load. The greater the distance between support points, the more important its stiffness becomes.
Therefore, the main question should not be:
"How strong is the pedestal?"
but rather:
"What span can the joist used safely cover?"
The pedestal is a support point. It is the joist that must carry the load between successive support points.
Terrace system manufacturers also make the distance between supports dependent on the type and cross-section of the joist. For example, Jouplast gives a general recommendation of about 70 cm, while noting that the distance should be adapted to the type of wood and the cross-section of the joist.
What does the maximum distance between pedestals depend on?
Four factors are the most important.
1..Joist material
Terrace joists can be made, among others, from:
- softwood,
- hardwood and exotic wood,
- aluminum,
- composite,
- plastics,
- steel.
These materials have different stiffness and behave differently under load.
This does not mean, however, that an aluminum joist, for example, can always be supported less frequently than a wooden one.
The material itself is not enough to determine the spacing of supports.
Equally important, and often even more important, is the cross-section of the joist.
2. The height of the joist is of great importance
Let's compare two wooden joists:
40 × 40 mm
and
40 × 70 mm placed on edge.
The second joist has as much as 70 mm of structural height, which makes it significantly stiffer when bending.
That is why it can be supported at greater intervals.
This is an important principle:
We do not only look at the width of the joist. Its height in the direction of the load is of key importance.
That is why a 40 × 70 mm joist should not be laid "flat" unnecessarily. Placed on edge, it uses its cross-section much more efficiently.
3. The profile construction also matters
In the case of aluminum profiles, the information:
"aluminum joist 25 × 40 mm" is not enough.
What also matters is:
- which dimension is vertical,
- the profile wall thickness,
- its geometry,
- the presence of internal ribs,
- the aluminum alloy grade,
- the way the structural elements are connected.
That is why two aluminum joists with the same external dimensions can have different load-bearing capacity and stiffness.
Complete structural systems are a good example. For the DuoSpan aluminum profile with a height of 51 mm, the manufacturer specifies supports every 900 mm for residential applications, while for higher commercial loads, it reduces the spacing to 600 mm. This shows that the support spacing is a parameter of the entire structure and the anticipated load, not of the pedestal itself.
4. Terrace loading
The same joist may require a different pedestal spacing depending on how the terrace is used.
Different designs are used for:
- private balcony,
- terrace at a single-family house,
- restaurant terrace,
- hotel space,
- public terrace,
- structure for heavy equipment.
The greater the designed loads, the more often it may be necessary to reduce the distance between supports.
Therefore, the values presented below should be treated as indicative recommendations for typical terraces used by pedestrian traffic, and not as a table of maximum structural spans.
Example pedestal spacing for different types of joists
The table below shows how much the support method can vary depending on the material and cross-section of the joist.
Here is the text from the table converted into a readable list:
- Hardwood 40 × 40 mm (square cross-section)
- Approximate spacing of pedestals along the joist: approx. 50–60 cm
- Notes: Low cross-section limits the span despite the good properties of the wood
- Softwood 40 × 70 mm (70 mm vertically, "on edge")
- Approximate spacing of pedestals along the joist: approx. 60–70 cm
- Notes: Tall cross-section provides significantly greater rigidity
- Aluminium 25 × 40 mm (depending on the profile design)
- Approximate spacing of pedestals along the joist: approx. 40–60 cm
- Notes: The exact value depends on the installation height, wall thickness, and profile geometry
Important note regarding the aluminum joist 25 × 40 mm
If the profile is positioned so that 40 mm is the vertical dimension, its structural capacity will be greater than when the profile height is only 25 mm.
Therefore, for a low profile with a height of 25 mm, it is advisable to assume a more conservative support spacing, e.g., around 40–50 cm, unless the manufacturer of the specific profile allows a greater span.
However, if 40 mm constitutes the structural height of the profile, a spacing of around 50–60 cm can often be considered.
The technical data of the manufacturer of the specific joist or structural calculations always take precedence.
Three cross-sections side by side:
- hardwood 40 × 40 mm → supports every 50–60 cm,
- softwood 40 × 70 mm "on edge" → supports every 60–70 cm,
- aluminum 25 × 40 mm → supports approximately every 40–60 cm.
It would be good to show two or three supports under each joist as well as a dimension arrow between them.
Why can a 40 × 70 mm joist be supported less frequently than a 40 × 40 mm one?
This is a common source of misunderstanding.
Someone might say:
"But both joists are 40 mm wide."
Yes, but width is only one of the dimensions.
A 40 × 70 mm joist placed on edge has 70 mm of height, which allows it to resist deflection much better.
For this reason, when designing the substructure, one should always specify not only the material but also the full cross-section and the way it is positioned.
The statement:
"I have a wooden joist"
is insufficient to determine the correct pedestal spacing.
We need at least the following information:
what material + what cross-section + how the joist is positioned.
Does the spacing depend on the type of pedestal?
In a typical terrace construction, the type of pedestal does not determine the spacing between joist supports.
The pedestal must of course:
- have adequate load capacity,
- be stable,
- be designed for supporting joists,
- allow proper fastening of the profile used.
However, whether the supports are placed, for example, every 40, 50, or 70 cm is determined primarily by the joist parameters and the load on the structure.
Terrace pedestals have very high compressive strength, which is why in typical pedestrian terraces, the element requiring particular analysis is often the substructure between support points. The strength parameters of individual pedestal series should always be checked in the manufacturer's technical documentation.
Note: there is also a second spacing – the distance between joists
This is very important.
So far we have been talking about the distance:
PEDESTAL → PEDESTAL
along a single joist.
But the terrace structure also has a second dimension:
JOIST → JOIST
i.e. the distance between adjacent joists.
And this parameter is determined completely differently.
What does the spacing between joists depend on?
The joist spacing results primarily from the type of deck board used and its manufacturer's instructions.
It is the board that spans between two adjacent joists, which is why its:
- material,
- thickness,
- width,
- construction,
- rigidity,
- installation method
determine the maximum allowable distance between joists.
It is not determined by the series of pedestals used.
For example, terrace system manufacturers may recommend approximately 500 mm between joists for some natural wood boards, and approximately 400 mm for composite boards. Jouplast specifies exactly 500 mm for natural wood and 400 mm for composite, while recommending checking the board manufacturer's requirements.
Other board manufacturers require even smaller values. TimberTech specifies a maximum spacing of approximately 16 inches, i.e. approximately 406 mm, for many of its boards, while Millboard specifies a maximum of 400 mm for installing boards perpendicular to joists.
Therefore, there is no single universal joist spacing value for all boards.
Example: pedestals every 60 cm do not mean joists every 60 cm
Let's assume we are designing a terrace with:
- a joist that can be supported every 60 cm,
- a composite board whose manufacturer requires joists at a maximum of every 40 cm.
The correct layout then looks as follows:
Along the terrace:
pedestal – 60 cm – pedestal – 60 cm – pedestal
Across the terrace:
joist – 40 cm – joist – 40 cm – joist
So we get a grid:
60 × 40 cm
and not 60 × 60 cm.
It is precisely this arrangement of two dimensions that allows the terrace structure to be calculated correctly.
Do wooden and composite deck boards require the same joist spacing?
Not necessarily.
You cannot assume a single value even for the entire group of "composite deck boards."
One manufacturer may allow a spacing of 400 mm, another 350 mm, and for specific applications 300 mm may be required.
The spacing can also be influenced by:
- the thickness of the board,
- solid or hollow construction,
- installation of the board at an angle,
- residential or commercial use,
- the location of specific loads.
Therefore, when designing a terrace, you should first check the deck board manufacturer's installation instructions, and only then plan the joists based on them.
And what if the boards are laid diagonally?
If the board is not laid perpendicular to the joists, its actual span between support points increases.
That is why manufacturers often require a reduced spacing between joists in such cases.
This is further proof that:
joist spacing results from the requirements of the decking surface, not from the type of adjustable pedestals beneath the structure.
How to correctly choose both spacings?
The easiest way is to do it in two steps.
Step 1 – determine the spacing of the pedestals
Check:
- the type of joist,
- the material,
- the cross-section,
- the height of the cross-section,
- the method of installation,
- the manufacturer's documentation,
- the anticipated load.
Based on this, you determine:
how often to support the joist.
Example:
60 cm.
Step 2 – determine the joist spacing
Check the deck board manufacturer's instructions.
Based on this, you determine:
how far apart to place adjacent joists.
Example:
40 cm.
The final pedestal grid is then:
60 × 40 cm
Only once you have both dimensions can you correctly calculate the number of pedestals needed.
Where should additional pedestals be used?
Regardless of the standard spacing, the structure may require additional support points.
This applies, among others, to:
- joist connections,
- joist ends,
- terrace edges,
- steps,
- unusual shapes,
- areas of concentrated loads,
- heavy equipment elements,
- changes in the direction of the structure.
Particular attention should be paid to connections of two joist sections.
The connection should not be placed randomly in the middle of a large span between pedestals.
In such places, appropriate support should be designed according to the solution adopted for the specific substructure. System instructions also often require joining profile ends directly above a support.
Is more pedestals always better?
Reducing the support spacing increases the rigidity of the structure, but this does not mean that the maximum possible number of pedestals should be installed unnecessarily.
A properly designed structure should find a balance between:
- safety,
- rigidity,
- material consumption,
- installation time,
- the cost of the entire terrace.
If a given joist requires support every 50 cm, placing supports every 25 cm usually does not provide a justified economic benefit.
On the other hand, increasing the spacing from the recommended 50 cm to 80 or 100 cm solely to reduce the number of pedestals can lead to excessive deflection of the structure.
The most common mistake: one value for the entire terrace
We often encounter the assumption:
"I'll do everything in a 50 × 50 cm grid and it will be fine."
Sometimes such a layout will indeed be appropriate.
However, it should not be treated as a universal rule.
For example, we may have:
- a joist requiring support every 50 cm,
- a board requiring joists every 30 cm,
which gives a grid of:
50 × 30 cm.
We may also have:
- a stiffer joist supported every 70 cm,
- a board requiring joists every 40 cm,
which gives:
70 × 40 cm.
Both solutions can be correct.
Summary – how far apart to place pedestals under joists?
There is no single universal distance between adjustable pedestals under joists.
In typical private terrace constructions, spacings in the range of approximately 40–70 cm can be found, but the correct value must be matched to the joist used.
As a rough guide, you can assume:
- hardwood 40 × 40 mm – approximately 50–60 cm,
- softwood 40 × 70 mm placed on edge – approximately 60–70 cm,
- low aluminum profile 25 × 40 mm – approximately 40–60 cm, depending on the geometry and orientation.
The most important rule is:
The spacing of pedestals along the joist depends on the joist. The spacing between joists depends on the board.
These are two independent parameters, and both must be taken into account when designing the terrace.
Raised deck terraces – installation systems and technology for professionals
FAQ – frequently asked questions
How many centimeters apart should pedestals be placed under joists?
In typical private terraces, a spacing of about 40–70 cm is often used. The exact value, however, depends on the material, cross-section, height, and rigidity of the joist used, as well as on the terrace loads.
Can pedestals under the joist be placed every 50 cm?
Yes. A spacing of about 50 cm is a commonly used, conservative value for many low raised deck substructures. However, this does not mean that 50 cm is a mandatory value for every joist.
Can pedestals be spaced every 70 cm?
Yes, provided the joist used has adequate rigidity and the manufacturer or structural design permits such a span. Some systems intended for wooden terraces assume approximately 70 cm as a typical support spacing.
What determines the spacing of pedestals under the joist?
First of all, it depends on the material of the joist, its cross-section, structural height, profile geometry, and the expected loads.
Does the height of the joist affect the spacing of the pedestals?
Yes. A taller cross-section placed in the correct orientation is typically much stiffer than a low profile, which can allow for greater distances between support points.
Does the joist spacing depend on the pedestals?
No. The type of pedestals used does not determine the distance between joists. The maximum joist spacing results primarily from the requirements of the manufacturer of the decking boards used.
How often should joists be spaced for composite decking?
Always use the value specified by the manufacturer of the specific decking board. Depending on the system, this may be, for example, 300, 350, or 400 mm. In many composite systems, the maximum spacing is around 400 mm.
Can the spacing of pedestals and joists be different?
Yes – and it usually is. For example, we can support each joist with pedestals every 60 cm, while adjacent joists are spaced every 40 cm. This gives us a support grid of 60 × 40 cm.
Do you need to reduce the spacing of pedestals under heavier loads?
Often yes. A good example are tested aluminum systems, where the manufacturer allows greater support spacing for residential loads and smaller spacing for commercial applications.