Woodworking

I need/slash want to build a few pieces of furniture for my house.

After visiting a shop, I had the carpenter go on a long tirade on how my choice on OSB was poor, when compared to the standard plywood the shop uses for their work, being weaker and not as "clean" looking.

We personally like OSB, here. We already have a few pieces and the rough look of the material is our style.

But is it really a poorer choice when compared to plywood?

The next project will be a bookcase and it will have to withstand a heavy load.

I brought a small strip of plywood home with me, as a sample. The material is made up of five layers. If necessary, I can upload a picture later.

I intend to use 15mm material for the sides of the boxes that will compose the bookcase, with 9mm for the back. These specifications exist for OSB; on plywood, I was told the closest is 13mm and 6mm.

Can someone share some advice and knowledge in what can be the better choice to build this project?