In this article on the topic of “Substrate for pedestals” we would like to give you information on the installation of pedestals on different substrates.
The different surfaces
In this article we will cover the different substrates for laying our terrace substructures.
It is important to plan carefully in advance to avoid any surprises.
We treat the following surfaces:
- Ground contact subsoil – subsoil made of gravel / crushed stone
- concrete subsurface
- Flat roofs with thermal insulation and sealing membranes
ground-level surfaces
When laying cables in direct contact with the ground, the biggest enemy is water – in frozen form!
Ground frost occurs regularly. The water in the ground freezes and expands.
This causes the ground to rise, which must be avoided.
The left picture shows the ground frost in Germany on January 23.1.2023, XNUMX.
You can see that almost all areas – marked dark red – were affected by ground frost.
And this in a very mild winter.
Procedure:
The soil must be excavated to the frost depth (usually 80cm) and a layer of gravel / gravel must be added.
To improve the load-bearing capacity of the gravel/crushed stone layer, we strongly recommend the use of concrete slabs.
The concrete slabs were placed on top and tapped into the gravel bed with a rubber hammer.
In the photo above you can see an installation by one of our customers on compacted gravel with the 'Professional' pedestal
concrete subsurface
Experience has shown that the concrete subsurface causes the least problems.
If problems occur, they are only caused by potentially severe unevenness.
The best way to compensate for minor unevenness is with our building protection pads.
With a thickness of 4mm, these are strong enough to ensure that the pedestal supports stand firmly.
At the same time, the relatively low thickness of 4mm ensures that this building protection mat is not compressed too much when walked on, which would create an unpleasant spongy walking feeling.
Subsurface for pedestals – flat roofs / terraces with thermal insulation and sealing membranes
The subsurface for pedestals should be solid and stable. This also applies to the thermal insulation.
That is why we will deal with this topic first.
Since there is no literature on these topics, the data below were determined by us through research
The resilience of thermal insulation
The most important point in thermal insulation is the material of the thermal insulation and the resulting load values.
The permissible compressive stress for a continuous load of 50 years and a compression of 2% of the insulation material is the central value according to the standard DIN EN 1606
This value is given in kPa. 100 kPa corresponds to a maximum pressure of 10 t/m² = 1 kg/cm²
For example, a large manufacturer specifies a maximum compressive stress of 130 – 250 kPa (depending on the type) for its pressure-resistant XPS® insulation elements. This corresponds to a maximum load of 1,3 km per cm²
Typical characteristics of insulation elements according to material type:
| Material insulation | Max. compressive stress in kPa | max. pressure load in kg / cm² |
|---|---|---|
| EPS facade insulation panels | 0 | 0 |
| EPS (expanded polystyrene) pressure-resistant | 55-60 | 0,55 - 0,6 |
| PUR (polyurethane rigid foams)² | 20-30 | 0,20 - 0,30 |
| XPS (extruded polystyrene) | 120-250 | 1,2 - 1,4 |
² Information according to IVPU Industrieverband Polyurethan-Hartschaum eV
Which loads affect the thermal insulation
So what about the load?
Let us assume the typical case:
a – Porcelain stoneware 2cm (surface weight approx. 48kg / m2)
b – Pedestal support surface 150cm² (e.g. pedestal support ‚Professional‚)
c – point load 200 kg (flower trough with an additional person on a plate with 4 pedestals)
d – Total weight: 248 kg. These 248kg are distributed over 4 pedestals -> 62kg load input per pedestal
From these data the pressure load results:
Load input per pedestal according to point d / contact area of the pedestal according to point 2
for pedestal bearing 'Titan': 62kg / 150cm² = 0,41kg per cm²
Now you know the load that actually acts on the insulation, namely in this example 0,41 kg per cm².
This load situation would be suitable for pressure-resistant EPS and XPS – but not for polyurethane rigid foams!
The load-bearing capacity of sealing membranes
In DIN EN 12730, sealing materials (bitumen / EPDM / etc.) are tested for their compressive strength.
The bitumen sheet is exposed to a weight of 20 kg for 24 hours.
This weight is transferred to the bitumen sheet via a ball with a diameter of 1 cm.
The tightness of the bitumen sheet is then subjected to a detailed test.
If a waterproofing membrane is tested, it can withstand a pressure of 20 kg per cm² stands!
If you compare the values from the determination of the actual load in practice of approx. 0,4 kg per cm² with the maximum load capacity of waterproofing membranes (20 kg/cm²), you can see that these are never the critical element.
The critical element is and remains insulation!
Gernot Gusenbauer, Mag.
Managing Director of Ceratrends GmbH
Altenberg, January 19.1.2022, XNUMX