Thread: Island bench reinforcement

Just from the description given I can see some issues with basic material selection, initial and corrective.

25mm MDF in its own most likely has insufficient rigidity to support the stone top plus any external loads applied to the overhang. This means that the overhang will flex and will cause cracking of the stone as the stone is not getting the support it needs to remain completely stationary.

The L brackets that you have attached to the substrate and carcass would appear to be basic L brackets (i.e. unbraced) as there is no evidence of the braces in the attached picture. If left unbraced, or ineffectively braced, loads on the stone and substrate that deflect them will also deflect the L brackets as the are not totally rigid because they are inadequately braced.

There is no mention of the wall thickness of the 25x50 tube that has been added to stiffen up the edge of the overhang, or its orientation. Wall thickness is important to the rigidity of the beam and thinner wall thickness sections can deflect over the length you are trying to support just from their own weight. Since the beam is hidden in the picture, one would assume that it was placed on the flat rather than on edge. For a given RHS size, it is more rigid if placed on edge than on the flat, provided that faces are firmly constrained to maintain the rectangular profile. If poorly constrained, the walls can flex allowing the tube to flex under load.

If there is any potential for the substrate and support structures like brackets and beams to flex under load, there is a probable crack in the stone overlay somewhere in the future, the hardest part is predicting when it will occur.

the wall thickness was mentioned
6mm

if its flexing now, short of putting a vertical pole in the middle from the underside of the bench to the floor... its always going to flex.

Or like malb mentioned you need to turn the tube so its in the vertical position and sufficiently attached at both ends to anchor it down so it doesn't flex.