Why 2 cavity trays for one head / lintel?

[WWilts]

Are two cavity trays necessary in this situation? How important is the lower tray?

[PeterW]

That’s an MI detail for a specific stone / recon stone lintel that requires a slip plane. Means the outer stone isn’t structural in any way and takes all the loading off it. Only works if you can get a concrete (or box lintel) the correct size and dimensions to match the stone on the inside leaf. 
 

[WWilts]

1 hour ago, PeterW said:

Only works if you can get a concrete (or box lintel) the correct size and dimensions to match the stone on the inside leaf. 

Tks.

 

But why the lower cavity tray? Necessary?

Edited June 20, 2021 by WWilts

[PeterW]

Stone product is potentially porous so may allow water into the cavity and behind the window / door and inside the building.