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Hello all, Our current 1st floor consists of 40x210mm joists (unstamped but assumed to be C16) spaced at nominally 600mm centres. Based on the increased dead load from removing the old paramount partitions, replacing with 38x89mm CLS studwork with some acoustic insulation, replacing the chipboard subfloor with this floating acoustic subfloor and adding an overlay underfloor heating system, a structural engineer has recommended reinforcing the floor by adding additional joists in between the existing joists by building into the external walls to bring the nominal spacing down to 300mm centres. What would be the best way of installing these new joists? The existing joists are either built into the external walls or rest on masonry joist hangers on the party wall. I was initially considering raking out the mortar joints, filling with fresh mortar with a mortar gun and installing masonry joist hangers to match the existing joist hangers but a civil engineer work colleague recommended against this. He was uneasy about there not being adequate compression of the mortar if retrofit this way. I really want to avoid building these new joists into the wall as I doubt I would be able to cut the holes at the right height and size without making a mess of it and I expect it would take a really long time. I wouldn't want to do this to the party wall anyway as it would compromise the acoustic performance of the party wall. The other solution I'm considering (pictured below) is installing ledger boards between the existing joists and installing joist hangers off those, but I'm not sure if that would be a good idea since the external walls consist of hollow concrete blockwork. From my online searches I can only seem to find American examples of fixing ledger boards to cinder blocks to build outdoor decking. Does anyone have any views on this?