This may not be what it seems but …. for over two years I have had two shelf supports fastened to the workshop wall with screws and plastic wall plugs. The load on the shelf has barely changed over this period.
The other night while relaxing on the terrace we heard a very loud crash and on investigation found the shelf, contents and both brackets in a heap on the workshop floor. Fortunately no major damage ensued to anything.
On investigating further I noticed that the blocks used for the wall material was warm to the touch. This is a consequence of the extraordinary long period of very hot weather we have experience in the UK. The plugs had come out of the holes intact with the screws still in place.
So my thought is that the warmth in the wall has somehow changed the characteristics of the grip in the wall. This could be the plug plastic degrading in some way i.e. going soft and having less grip, or maybe the hole in the block work has expanded in diameter enough to lose the grip on the plugs. Either way it suggests my civil engineering was marginal in the first place !
The shelf is now back up with an extra set of fixing holes and some seriously larger screws.
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