I'm looking at building a desk but have been reading a lot of conflicting info about which material to use. My main concern is having the desk sag at all. I will also be painting/coloring the desk. Which material is stronger while bearing weight? Which would be the best choice for building a desk without sag?
Any sheet goods are going to be lacking in strength in terms of deflection so your construction will have to compensate for this. You'll want to create rigidity using geometry of some kind like this torsion box or maybe by adding solid wood stretchers and aprons. As far as materials go, I'd be partial to MDO for a project like this but here's your options.
- Plywood is the strongest of the sheet goods and takes mechanical fasteners by far the best. But it's either a terrible surface to paint (if you get construction grade) or too nice/expensive to cover up (if you get cabinet grade).
- MDF paints well but is monstrously heavy, is the most prone to sagging, and will not take fasteners in its edges.
- Particle board is inexpensive but it's flimsy and is almost impossible to seal up well enough to protect it from water damage unless you cover it with laminate but then you've just got a counter-top.
- Melamine has an durable, stain proof film on both faces but as such it cannot be painted. It's also susceptible to water damage as it's just particle board in the center.
- MDO is a sheet of plywood covered in a thin layer of MDf. This gives it the strength and versatility of plywood while keeping the weight down, but it also takes paint just like MDF. It's only draw back is that the edges are not finish grade in the traditional sense so they'll need to be banded or captured in some fashion, and it's a bit more pricey than MDF or CDX.
All wood sags. How much depends on the type of wood, the dimensions and the weight that will be put on it. Generally particle sags more than MDF, MDF sags more than OSB, OSB sags more than Plywood. The addition of an edge strip helps reduce sag considerably.
Check out WoodBin's Sagulator which can estimate the amount of sag given your exact conditions using different materials.
OSB takes forever to sand smooth enough for a desk. My vote is plywood, for that reason alone. You can plywood with one side smooth for not too much more money. Without a really good sander and patience, you won't be happy with the splintery OSB as a desk surface.
Another option is laminated pine. I've built some cheap desks out of it with good results.
Experience: I've built some desks and platform beds and cabinets and stuff. Not a real carpenter.