Loft Conversion joist question.

12 Aug 2021
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United Kingdom
I'm currently in the process of converting my loft in to a 4th bedroom. I've replaced the joists with 7x3" C16 spaced 400 apart. The majority of these are supported by a supporting wall running through 3/4 of the middle of the property.

My question is: Can I use two non supporting walls (currently constructed with old hallow blocks from early 1940's) running across the floor joists on the first floor to support the loft joists across the span?

I've attached a quick drawing to help make my question easier to grasp. (Not dimensioned or to scale).

  • The red line is the current supporting wall.
  • The green line is a section of the supporting wall that is being removed to open up the landing to allow for the new staircase.
  • The two blue lines are the hallow block walls which I'd like to use to support the new joists over the span. (I am considering replacing these with stud walls.)
I did consider running an RSJ from the supporting wall across to the external wall on the right but as shown by the orange line in the image there is a window with a concrete lintel which sits directly in line with where this RSJ would run and I'm not sure the lintel would be sufficient to sit the RSJ on even though the load wouldn't be overly heavy.


  • Floor.png
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I'd just replace the lintel with a steel (if the SE said the existing wasn't adequate). If you use the block walls surely you're just transfering the loads to the first floor joists?

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