9 September 2026

Installing a flat-bottom “panettone” polyethylene tank: technical guidelines for foundations and fittings

In the hydraulic and civil plant engineering sector, the flat-bottom (“panettone”) high-density polyethylene (HDPE) tank is a frequent choice for storing potable water or first-flush rainwater at atmospheric pressure.

Its distinctive low, wide shape and extended support surface provide high stability and make installation easier. However, correct installation requires strict compliance with precise dimensional and structural criteria concerning the supporting foundation and the drilling and fitting methods used on the shaped walls. 

Failure to observe these installation requirements can cause abnormal flexing of the polymer, triggering mechanical stress on the base and leak points near the threaded fittings. 

Preparing the support foundation: mechanical and flatness requirements 

The flat-bottom tank distributes the load of the stored fluid over a large horizontal surface. Since water density corresponds to a load of 1000 kg/m3 (1 kg/dm3), a full tank exerts constant, uniform pressure on the surface beneath it. 

The support foundation must meet the following specifications: 

• The support surface must be built in reinforced concrete, perfectly levelled, smooth and free of depressions or irregularities that could concentrate stress at single points of the polymer matrix. 
• Flatness tolerance must not exceed 2 mm per linear metre, in order to prevent deformation of the shaped base under hydrostatic load. 
• The dimensions of the concrete slab must exceed the tank’s footprint diameter by at least 10-15 cm on each side, preventing overhang or flexing of the perimeter edges. 

It is strictly forbidden to place the tank directly on uneven ground, loose gravel, wooden sleepers or pallets. Any settling of the ground would cause the polyethylene to bulge, leading to fatigue cracking of the material. 

Hydraulic connections and drilling of the shaped walls 

The geometry of flat-bottom tanks features flat areas alternating with structural reinforcement ribs. Correctly applying flanged joints or threaded bulkhead fittings requires care in selecting the drilling points. 

Fittings must be installed exclusively on the flat areas provided by the manufacturer during the rotational moulding process. Drilling holes on reinforcement ribs or areas with a reduced curvature radius prevents proper adhesion of the EPDM or NBR sealing gaskets, causing leaks that cannot be resolved simply by mechanical tightening. 

For connections to the water network and other fittings, flexible piping sections or rubber anti- vibration joints must be fitted near the tank’s inlet and outlet. Rigid PVC or polypropylene pipes subject to thermal expansion or slight structural settling could exert a reinforced bending moment on the brass or plastic nipples, damaging the thread or tearing the tank wall. 

Maintenance guidelines and technical support 

Before final filling, it is recommended to carry out a visual hydraulic test, keeping the tank full for 24 hours to check the seal of gaskets and fittings. 

In the case of complex configurations or doubts regarding the compatibility of the fittings to be applied to the shaped walls, Di Camillo’s technical support provides the necessary assistance and suitable components, as well as on-site installation to ensure a perfect seal and system compliance.