Top over lashing is the most frequently used type of lashing in the transport of goods by road as most loads are so wide that only vertical or slightly diagonal top over lashing is possible. However, the following prerequisites must always be noted for top over lashing:

• High friction between the load and load surface and between the load units (sheet transport) must be ensured.
• It must be possible to estimate the approximate dynamic coefficient of friction μ or it must be known.
• The load must be able to withstand higher prestressing (tensioning).
• The lashing points must be suitable for the higher load.
• The most important factor, the size of the tension force required, to be applied using the tensioning device, must be known.

This list shows the disadvantages and limits of top over lashing: With top over lashing the lashing equipment, the lashing points and the load itself are exposed to permanently high lashing capacity. But top over lashing only works if, as mentioned, a sufficiently large coefficient of friction exists between the payload area and the load. Steel on steel, for example, is very unfavourable, which is why timber supports, friction increasing mats (anti-slip mats) or similar are used to increase the friction. The payload area and load must of course be free of oil, dirt and ice.

How does the restraining effect occur in top over lashing?

The frictional force Fr is increased by applying the total tension force Fv in the lashing equipment (lashing chain, web lashing) by means of tensioning devices (load binder / ratchet-type load binder). The actual friction force acting, also called the restraining force, is therefore made up of the proportion resulting from the self-weight of the load with (G x μ) and the part from the tension, which is calculated from the additionally applied tension force (Fv x sin α x μ). Both values together must be larger than the force with which the load tries to move on the payload area, i.e. 0.8 or 0.5 times the load weight.

The following formula results for the total tension force Fv required:

G: Weight in daN ≈ Mass in kg
0.8: Acceleration factor in direction of travel
µ: Dynamic coefficient of friction
α: Vertical angle (angle between payload area / loading surface and means of restraint)

Calculation example:

Load consisting of precast concrete element
m= 4000 kg ≈ 4000 daN = G

Concrete on wooden payload area:
µ = 0.3

Vertical angle:
α = 60°

Fv = 7698 daN total tension force
From this the number of strapping elements n can be calculated:

The following abbreviations are used:
STF = Standard tension force (the tension force which can be achieved with a tensioning device).
1.5 = tension multiplier (STF ratchet side + STF adjustable end)
In the example, a 5t-ErgoABS web lashing with an STF of 500 daN is chosen.
11 web lashings are required to top over lash the concrete part.

To minimise the number of straps required, anti-slip mats with a dynamic coefficient of friction of 0.6µ should be used!