RSJ Pocket Size and Packing

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20 Jan 2026
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I am prepping for installing an RSJ next week and about to prep the pockets. I’m in a Victorian terrace, the steel will be wall to wall on the ground floor (4.2m) and the structural engineer has prescribed a 152x152x37 UB steel sitting on a 500x100x40mm spreader plate.

Due to how the joists on the floor above lie, removing full bricks gives me either a 190mm or 280mm pocket. The minimum size I need is 161.8mm + 40mm = 201.8mm for the RSJ and spreader.

What would be the best way to make up the just under 80mm gap?

I’m assuming 80mm is too big for mortar only?

Other RSJs I’ve installed have been for completely new structures and am waiting to hear back from BC so just made me stop and think to ask about best practice here.

(First time poster but avid stalker and user of many great pieces of advice in this forum :)
 

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