Should there be a washer on both sides of a through bolt?

[Villa1949]

If you are going to fasten two beams together with a through hex bolt, should there be a washer on both sides, i.e., both at the head and the nut?

 

[ToRy]

I would say it depends. Among other things, it depends on the material of the beams; wood or metal? It depends on the size of the hole and the screw/screw head. It depends on whether there is an aesthetic aspect to the whole thing.

If it's wood and the screw head and nut are of normal size, and if it's not completely ruled out due to aesthetics, I would undoubtedly have washers on both sides. It would take a lot for me to choose not to have washers.

 

[Villa1949]

Ok. Thanks for the response. It's about a patio roof. From an aesthetic aspect, I have considered using carriage bolts instead, but I have other hex bolts available.

The posts are 90x90 glulam that will be attached together with a beam in 42x180 glulam. I plan to make a notch at the top of the posts like an L, place the beam on it, and fasten the screw through the post and beam.

 

[ToRy]

Then I would have probably routed a recess on both sides, large enough for the washer to fit in, and deep enough so that the screw head aligns with the surface of the post/beam. I think this gives a neat result that looks professional. Note that it might not be the best solution if the routing is exposed to rain.

Carriage bolts look nice, but there will be a problem IF you need to remove the screw and nut without damaging the wood. There's a high risk that the nut is stuck, and then you're stuck...