Joint weakness

[JamesO]

Hi

This is a very basic question but I can't find anything about it online.

To what extent does removing wood for a joint, e.g. Half the width for a cross lap joint, affect the strength of the timber? Obviously it creates a weak point. But how much does the other piece of wood that fills the hole make up for it?

I'm trying to choose to joint two pieces of 38mm x 63mm softwood in a T-shape in the horizontal plane, and needs to be fairly strong for a dining table. I am considering two 5mm screws, or a T-lap joint (or a M&T but can't see any advantage over the lap joint). Screws would be quicker but may not have as much shear strength, and presumably would also weaken the timber. The joint will be hidden so the aesthetics are irrelevant.

Many thanks

 

[transatlantic]

It all depends on the direction of stress that joint will be under. For example, when comparing a half lap T, with a butt joint T (glued and screwed), the half lap will be stronger to rotational stress (trying to pull the joint out of square), but weaker to vertical stress (stress on the long grain where material was removed). The butt joint will be weaker to rotational stress, but stronger to vertical stress along the 'top' or the T.

In other words, you want to be looking at the joint itself and figure out where it will be under most stress and improve that situation, which may involve removing material from a direction of little stress.

 

[JamesO]

Thanks

They'll be no rotational stress but quite a lot of vertical stress, so perhaps taking wood from the long grain of the top of the T is a bad idea. That price will have the most vertical load.