Everyone is familiar with the old saying: “Repair your roof in the summer.” It’s best to fix problems before rain, frost, and snow set in. And the same actually applies to measuring: if you want to know for sure what’s happening on a roof in the winter, you need to install the solution while it’s still dry and calm.
Last winter once again demonstrated why this is important. At various times, there was a lot of snow on Dutch roofs. As soon as snow begins to accumulate, the same questions arise almost automatically. How heavy is that snow, actually? How much weight is on the roof? Is it still safe? Do we need to take action? And when does snow suddenly become much heavier due to thawing and rain?
Just looking outside won't give you much of an answer. Ten centimeters of fresh, dry snow is very different from ten centimeters of wet, compacted snow. So the depth only tells part of the story.
A question that comes up again and again
Ever since we started working with sensors in and on roofs, we’ve been asked regularly whether we can also measure pressure or weight inside the roof.
After extensive testing and research, it appears that this is not a reliable solution. There are simply too many factors that influence the measured value, such as the roof structure, insulation, material stiffness, roofing material, and construction. As a result, it is not possible to objectively measure the actual load. And it is precisely that objective value that matters most.
That's why we started taking a different approach.
“Taking measurements in the roof makes sense, but when it comes to weight, there are simply too many factors that influence the result.”
Not inside the roof, but on top of the roof
Last summer, we were working on developing a new measurement concept. The basic premise was deliberately simple:If we want to know what’s on a roof, we have to take measurements there.
Not the distortion caused by the layer somewhere deeper in the roof. Not a derived force in the insulation. But directly the two properties that really matter:
How tall is the layer, and how much does it weigh?
The concept has now been finalized, extensively tested, and is being manufactured for installation before winter.
To that end, the setup combines two different measurement principles.
1. Ultrasound: How much is there?
An ultrasonic distance sensor is located above the measuring plate. It continuously measures the distance between the sensor and the surface below it. When the roof is clear, we know the reference distance. Suppose the sensor is positioned exactly 50 centimeters above the plate. As soon as snow falls on the plate, that distance decreases.
For example, if the sensor measures 38 centimeters, then there is:50 − 38 = 12 centimeters of snow.
The same principle can be applied to water or other accumulations on the roof. The major advantage of ultrasonic measurement is that it requires no mechanical contact. The sensor hangs above the measurement surface and, as it were, continuously looks down.
But simply measuring height isn't enough.
2. A load cell: How much does it weigh?
A load cell is therefore located beneath the same measuring plate. A load cell is, in fact, a highly accurate force sensor. Under load, a metal measuring element undergoes microscopic deformation. Strain gauges are embedded in the element to electrically record this deformation.
The distortion is extremely small, but can be measured with great precision.
This allows us to determine how much extra mass is on the plate. Our measuring plate has an area of 35 × 35 centimeters: 0,35×0,35=0,1,225 m²
If the load cell then measures, for example , 6.125 kg of additional weight, we can immediately convert that to load per square meter: 6,125/0,1225=50 kg/m2
That is fundamentally different from a pressure sensor located inside the roof. Here, we measure the actual weight of the layer resting on a known surface. This results in a reproducible and easily interpretable measurement value:kg/m².
“Snow depth is visible. Snow load is not.”
Height and weight together tell the real story
It is the combination of both sensors that makes the concept particularly interesting. Suppose the ultrasonic sensor measures 25 centimeters of snow and the load cell measures a load of 50 kg/m². In that case, in addition to height and load, we can also determine the average density of the snow:
50/0,25=200 kg/m³
A day later, there may still be about 25 centimeters of snow on the ground, while the load has by then increased to, say, 80 kg/m². Due to rain, thawing, refreezing, and compaction, the snow has become considerably heavier. Using only a snow depth gauge, that difference would remain virtually invisible.
And that is precisely where the added value lies for us.Height tells us how much is there. Weight tells us what that layer actually means for the roof.
A standalone measurement system on the roof
The unit is installed on the roof as a standalone system. The structure consists of a compact stainless steel base and upright, with the ultrasonic sensor at the top and the weighing plate with load cell at the bottom.
The weighing plate rests on the load cell's measuring system. This is important: if the plate were to make contact with the structure anywhere else, part of the force would be dissipated outside the sensor, resulting in an incorrect measurement.
The electronics are located on the outside of the upright post and transmit the measurement data to the platform via LoRaWAN. The sensors are powered autonomously by an integrated solar panel. This eliminates the need to run cables through the roof or to open the roof covering for installation.
This makes the measurement chain very clear:
Low on the roof → height + weight → LoRaWAN → data → analysis and alerts.
“If you want to know what’s on a roof, don’t take measurements inside the roof—take them on top of the roof.”
Where do you install a sensor like that?
The quality of the measurement depends not only on the sensor itself, but also on where you place it. This is because snow does not always distribute evenly across a roof. Wind, differences in elevation, roof upstands, and other features can cause snow to accumulate much more heavily in some areas than in others.
That is why it makes sense to take measurements specifically at locations where the load could become critical. These include wind corners and areas where snow can be blown up or accumulate, as well as spots near parapets, elevated structural elements, or other roof details where snow is more likely to accumulate locally.
In addition, the structural load-bearing capacity may also be a determining factor. Not every part of a roof can always support the same additional load. In areas where the structure has less reserve capacity, it may be particularly important to continuously monitor the actual load.
Therefore, the placement should preferably be based on three factors:
- the geometry of the roof;
- areas where snow, leaves, or water are likely to accumulate;
- the parts of the roof where the structural load is most critical.
For larger or more complex roofs, it may make sense to use multiple measurement points. For example, one sensor in a representative section of the roof and one or more additional sensors in critical areas.
This way, we measure not only what’s on the roof, but especially what’s in the spot where it matters most.
Not just for snow
Although snow load was a major factor in its development, the principle has broader applications.
On flat roofs, water or leaves can also accumulate in certain areas. Examples include insufficient slope, a partially clogged rainwater drain, or temporary water accumulation. In addition, debris, gravel, leaves, and other materials can cause additional stress in specific areas.
By monitoring height and weight simultaneously, we can gather much more information than we could with a single sensor. So ultimately, it’s not just a matter of whether there’s something on the roof, but also: how much, how heavy is it, and how does that change over time?
“We don’t just want to see that the tax is changing; we want to know exactly how much it is.”
Tested, in production, and ready for winter
For many roofing issues, we work with models, design values, and assumptions. These remain essential. But as soon as outdoor conditions change, up-to-date and, above all, factual measurement data is a necessary supplement. This is certainly true for snow.
Weather data can indicate how much precipitation has fallen in an area. The ultrasonic sensor measures the depth of the snow on the roof. The load cell adds the crucial piece of information: the actual weight of that layer at that moment.
We worked on that last summer. The prototype is now ready, testing has been completed, and the sensors are in production. This means that this coming winter, we’ll not only be able to see that there’s snow on a roof, but also objectively measure how much snow is actually there.
