It should come as no surprise that commercially available roof snow load monitoring systems use beam deflection as a proxy for direct measurement of snow weight. It should also be no surprise that (it appears) there are no such systems for wooden structures.

As I specifically mentioned above it would be straightforward to use the structural beam as the mounting point for a strain gauge—but not the beams of a residential wooden truss as they wouldn’t have enough consistency to either be calibrated for kg/m² or for maximum safe deflection.

Using a bathroom or other scale doesn’t change that. You still need to know the meaning of what it reads out. The TS actually proposed monitoring beam deflection in the original post, but using a laser is a vague way. The idea could be made to work but it is a lot of extra effort to do something that has fundamental shortcomings.

In my followup (#6) I said:



I still can’t find any evidence anyone has done this commercially, and I can see why. I also suggested that retrofitting a system has many more problems than doing one designed to be part of a particular roofing system. This would seem the most fruitful direction and offers many possibilities.

While it would certainly be possible to incorporate such a system into roof supporting structures designed for the purpose, it might even be possible to do something that could be installed under the roofing shingles, slates, or tiles for a roof being refurbished.

As a commercial product, I would keep it simple, as this suggests. A flat packaged solar cell/battery/bluetooth/weight gauge that could be affixed in a strategic location, will not get the actual total live load, but depending on placement, gauge load could be sufficient to warrant action.

 

I missed getting in at the start of this thread, but now that aa bunch of discussion of methods have been published any system you develop may not be able to gain patent protection. The other bad news is that wood construction roof deflection measurement will be very unstable with weather humidity, temperature, and age.
But if the intention is to warn of an approaching collapse, I recommend a variation of the laser beam excess deflection method, for the reason that it can be arranged for one beam to sense excess deflection across a whole roof segment. So the hardware cost will not be that much but the installation and calibration costs will be the show-stopping problem. That is because every roof is different, even when made from the same drawing.

 

I missed getting in at the start of this thread, but now that aa bunch of discussion of methods have been published any system you develop may not be able to gain patent protection. The other bad news is that wood construction roof deflection measurement will be very unstable with weather humidity, temperature, and age.
But if the intention is to warn of an approaching collapse, I recommend a variation of the laser beam excess deflection method, for the reason that it can be arranged for one beam to sense excess deflection across a whole roof segment. So the hardware cost will not be that much but the installation and calibration costs will be the show-stopping problem. That is because every roof is different, even when made from the same drawing.

How would you calibrate it so it would be able to indicate anything useful?

 

The "calibration" will consist of, after the laser beam is in place and functional, attaching "flags" to every roof supporting rafter and beam so that some small amount of displacement will break the beam, and thus trigger the warning. The "flags will need to be stiff enough so that any air currents will not move them, and opaque enough to block the laser beam. Of course, this will require a visible laser, not an IR device. The calibration will consist of establishing the correct gap between the edge of the flag and the laser beam. I anticipate that will be

The concept is that support members will deflect under load a small amount prior to collapse. Wooden structure will deflect more than steel, but they all deflect a bit. I anticipate that the gap will vary between 1/8 inch in older structures to over 1/4 inch in those structures built with the really flimsy wood used in some localities. Certainly some skill will be required, as well as insight and experience.

 

…so that some small amount of displacement will break the beam, and thus trigger the warning.

Calibration requires determining that “small amount” so your warning means something, and therein lies the trouble. What is the basis for selecting the “small amount”? Is every roof similar enough to use one calibration? If not, how do we know which to use? How much deflection is too much? Does it change with the age of the wood? If so, how do you manage that?

… &c.

 

Calibration requires determining that “small amount” so your warning means something, and therein lies the trouble. What is the basis for selecting the “small amount”? Is every roof similar enough to use one calibration? If not, how do we know which to use? How much deflection is too much? Does it change with the age of the wood? If so, how do you manage that?

… &c.

Certainly "Y" is quite correct in that deciding that "small amount" of deflection will not be a minor action at all. But it might be as simple as having an assistant walk on the roof while the person calibrating watches what happens inside. There is no question about the need for a great deal of understanding of both roof construction and load bearing abilities. I make no representation that it will be a simple call.
But I also can assure everybody that doing the process with fully calibrated instrumentation with adequate resolution will not be any simpler regarding the decision to deliver a warning..

Consider the mechanism of a roof collapse under a snow load: First a fairly uniform deflection as the load increases, then at some point one segment yields a bit more, which increases the load on adjacent segments, which results in greater deflection. ( It is slightly similar to a beam buckling under compression loading. ) At each increment of deflection that load distribution changes, leading to more deflection. (Second semester Structural Dynamics class)

 

Throw a screwball into the mix, what if the roof is made with a truss structure? In certain conditions, certain truss members may "lift" rather than drop under mild load, yet change again under heavier load.
Temperature makes a difference as well, for in a long truss, the long bottom member will shrink more than other members, causing the lower member to appear to lift. (I've seen it on a 40 foot long roof truss).
Bottom line, there is no "reliable" method of measuring deflection as a direct indication of snow load unless the roof members are constant (same material, same length, etc.). Some roof rafters may be 2x6, others 2x8, some can be trusses. short roof span could even be 2x4. The length of the roof will determine overall snowload and how much it deflects. Example would be a 10 foot roof span with 20lb/sq ft will deflect a lot less than a 16 foot roof span with the same load of 20lb per sq ft and made of same rafter material.

 

Throw a screwball into the mix, what if the roof is made with a truss structure? In certain conditions, certain truss members may "lift" rather than drop under mild load, yet change again under heavier load.
Temperature makes a difference as well, for in a long truss, the long bottom member will shrink more than other members, causing the lower member to appear to lift. (I've seen it on a 40 foot long roof truss).
Bottom line, there is no "reliable" method of measuring deflection as a direct indication of snow load unless the roof members are constant (same material, same length, etc.). Some roof rafters may be 2x6, others 2x8, some can be trusses. short roof span could even be 2x4. The length of the roof will determine overall snowload and how much it deflects. Example would be a 10 foot roof span with 20lb/sq ft will deflect a lot less than a 16 foot roof span with the same load of 20lb per sq ft and made of same rafter material.

My response indicated that there would be a whole lot of skill and insight involved, and while that may be an understatement indeed, the point was that a deflection detector will be the simple scheme that will not cost more than replacing a roof. The biggest concern would probably be in the older structures that do not include the fabricated truss arrangements that are indeed assembled with an inferior technology that IS subject to catastrophic failure because of the very limited strength and sudden yield point. There may possibly, now, exist some more trustworthy truss construction methods, MAYBE.