5mm or 6mm screw for 25x31mm timber?

[Yrrol]

I am going to mount a shelf on the wall and I'm considering which screw is most suitable.
The shelf needs to withstand a demanding load. There are only provisions for two large screws, and a number of smaller ones.
The wood piece I need to fasten is 25mm wide and 31mm deep.

The picture shows a 5mm and a 6mm screw.
Is a thicker screw always better, or is there a risk of the wood "splitting" (should pre-drill)?

When it comes to nails, they should be 3 times as long as the board you're nailing down.
But screws can be shorter than that, if I recall correctly?
I think the length of the smooth section without threads should correspond to the thickness of the item you're fastening? (with the exception of this screw type which has left-hand threads that I assume are meant to press the "stud" against the wall and should therefore also end up directly in the stud)

 

[Finndjävel]

5mm is probably enough. You have to pre-drill, otherwise it will crack.

 

[Daaaaaniel]

If you pre-drill large enough, it won't split.
You should pre-drill larger for the larger screw compared to the smaller screw.

The question is what you will mount the screw into?
Concrete wall?
Wooden wall?

If it's a concrete wall, you'll need to use a plug, and the screw, including the shelf bracket, should be long enough to be longer than the plug.

 

[Yrrol]

If you pre-drill large enough, it will not split. You should pre-drill larger for the larger screw compared to the smaller screw.

The question is what are you attaching the screw to?
Concrete wall?
Wooden wall?

If it's a concrete wall, you will need to use an anchor, and the screw, including the shelf bracket, must be long enough to be longer than the anchor.

I plan to pre-drill regardless of the screw's diameter and use a countersink instead of letting the screw head do the job. The wall is hardboard. Hoping to hit the wooden studs. If I don't, I'm considering using molly anchors (and countersunk screw with metric thread) even though it’s hardboard. I find that the hardboard breaks apart too easily even though it's theoretically supposed to support most things.

 

[tttomten]

You must meet the rule, there is no tensile strength at all in masonite. With Molly you increase the strength but not enough for a shelf unless you have thick masonite (about 10+ mm)

 

[Yrrol]

You must hit a stud, there's no tensile strength at all in hardboard. Using a Molly bolt increases the strength but not enough for a shelf unless you have thick hardboard (about 10+ mm)

It is 9.8mm.

 

[Daaaaaniel]

Masoint is not fun, and unfortunately neither is molly to be honest...

To digress a bit and risk going OT, have you considered tearing down the wall and building a new one?!
It might feel a bit overkill to mount ONE shelf, but if you rebuild the wall with OSB/plywood behind the plasterboard, you get a wall strong enough to crucify yourself on, without worrying about where the studs are.
If this is a short-term residence, it might feel overambitious, but if it feels like a more permanent home, it might be worth it.

I'm biased in this matter because a few months ago I moved into a house with masonite walls and am now tearing everything down and rebuilding everything anew.