Loft conversion steels

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Hi

I live in a very standard 1930s semi-detached property.

I'm in the midst of trying to get a loft conversion done with a dormer.

I have had some building control plans produced by a firm. They've used a structural engineer for the calculations, steels, etc.

I have a question which I will approach them with after the bank holiday but just wanted to crowd source also.

The engineer has specified 5 steels:
  1. Ridge steel 152x152 UC 30kg/m.
  2. Two floor steels running from side to side at the positioned towards the front and back of the property: 152x152 UC 37 kg/m S355.
  3. A floor steel inline with the ridge: 152x152 UC 30kg/m.
  4. A floor steel, acting as a header, that connects the 2 front and rear steel, transferring the load from the floor steel that runs inline with the ridge since the staircase means that this steel can't continue to the wall: 152x152 UC 30kg/m.
He's specified 47x200 C24 floor joists at 400mm centres.

My questions are:
  1. If the steels are 152 and the timbers are 200 then the timbers will need notching to fit into the web of the steels. Am I saving anything by not having taller steels that the timbers will fit into without notching?
  2. There's roughly 5200mm between the front and back steels. I'm assuming that the reason for the floor steel inline with the ridge is because this distance is too long for the floor joists to run without support? If it was possible to span this distance then I could save on the cost of 2 steels.
Thanks for your help.
 
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Why does it need a ridge beam? Are you having a dormer built?

1. You can either notch the timbers into the beam web or timber pack the beam and use joist hangers.
2. You could span 5.2m with a single joist but you'd need a pretty big joist, and any lengths of timber over 4.8m start to get much more expensive and hard to get hold of, especially for such a large section. What is the actual maximum span of the joists taking into account the midspan steel? 47 x 200 joists for a span around 2.5-3m is completely over the top.
 
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With 152 deep steel beams, you can use 47 x 175 joists seated on the bottom flanges, as long as you notch them carefully.

The trick is to ensure that there is sufficient timber left above the top flange of the steel to ensure that the subsequent inevitable shrinkage of the joists doesn't end up below the top of the steel, otherwise you will get a hump in the floor.

As mentioned above, 47 x 200 is way over the top for such small spans.
 

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