For the extension flat roof the SE caluclated 220x75 joists, but left a choice of 254*146 UB or 203*203 UC to support it. Pricewise they seem to be £320 vs £400, so not a lot of impact.

However my thinking is that 203x203 will be better choice as easy to conceal, whereas 254x146 would require battens to offset the ceiling boards to go flush under the beam (there is a condition that top of joist to be 10mm above top of steel - what could be the reason for that?). Extra battens and 50mm height added (and so the cost of all the extra wall materials) will cost me much more than 80 quid saved on the beam upfront - am I right?

Edited January 30, 20215 yr by Olf

Easier to conceal will be a time saving.

 

The 10mm is to allow for the shrinkage in the timber. If you install flush, when the joists shrink to adjust to the RH of the house, you may be left with a bump in the floor over the steel beam. Less of a problem with kiln dried but still good practice.