ᐅ How to Join Wooden Beams in a Garden Shed?

Hello, my father and I are currently building our garden shed. It measures 2.5 x 2.5 m (8.2 x 8.2 ft). To the left, there will be an optional extension, also 2.5 x 2.5 m (8.2 x 8.2 ft).

For the roof support, two wooden beams are used, each 4 cm (1.6 inches) wide and 12 cm (4.7 inches) high. These protrude about 11 cm (4.3 inches) from the garden shed itself. The roof frame or roof box will be attached there.

Since I purchased the optional extension, two more beams will be attached to these two beams. These will be placed to the right, running parallel, sharing the 11 cm (4.3 inches) projection, so each beam is 8 cm (3.1 inches) wide there.

According to the manufacturer, each beam should be fastened with two carriage bolts. My father is not too happy with that. He suggests using two threaded rods, 10 cm (3.9 inches) long, each with washers and cap nuts instead. I think that’s fine in principle.

My question is whether there is an additional or generally better method to maximize stability?

I find carriage bolts with a cap nut and washer to be better.

Alternatively, in my opinion, it is also possible with 2-3 screws 70mm x 6mm (2.75 inches x 0.24 inches) on each side.

Okay, thanks. Are carriage bolts better than full-thread screws like Spax, with a washer head, 8mm (0.3 inches) diameter and 8cm (3.1 inches) length?