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Installation of steels


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Hi all, new to this site, and would be grateful if anyone can help me get my head around a question relating to steelwork for a loft conversion. 

We’ve had plans drawn up for a loft conversion but haven’t engaged builders yet due to the lockdown. My question relates to how the steel rsj’s will sit in the roof space. Our structural engineer has stated that normally the builder would remove a section of the timber wall plate on the internal cavity wall and sit the steel on a bearer plate on top of the internal cavity wall block work. (We have a detached bungalow, so no gable ends to insert steel into, to then hang floor hoist from) I’m ok up this point, but this is where my understanding stops! I’ve attached a drawing on this post (this is not our drawing, but just one I found online which I’ve sketched over to try and demonstrate my question). My question is, if the wall plate is cut away, the steel will cut across the ceiling timbers which will run in the opposite direction to the steel, so are you meant to cut these timbers out where the steel goes, and then re-fix back into the steel web somehow to give the ceiling its strength back?  The only way I can see to do this is to sit the steels well above the wall plate height somehow, but there just isn’t the height due to the slope of the roof, so even with cutting the steels at an angle at the ends to accommodate the slope of the roof, I don’t think this will work. Don’t really want a steel running through the ceiling below, as it will drop the floor below ceiling height. Hope this all makes sense! Any help with answering this would be very gratefully received - I’ve been searching around on the internet trying to find a diagram of how this works, but no luck so far.

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@CJE it may be easier to redact any personal data from the plans and then upload them as it’s difficult to comment without. 
 

Also need to know if the steels are supporting the floor or the roof - there are lots of options if it is just the floor, and steel isn’t always needed.
 

Depending on the plans, consider something like Telebeams as they are ideal for simple conversions 
 

 

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Apologies for the confusion, the drawing was just one adapted to illustrate the issue. Please ignore the ridge beam, this is being dealt with by sitting the ridge beam on 2 steels pillars supported by other small steels. Essentially there are two large steels running parallel, one at the back of the bungalow, and one near the front. In a nutshell, I can’t understand how the steel is supposed to sit on the existing structure of the house, given the absence of a gable end to insert the steel into - property is a bungalow with a pyramid-ish roof (small ridge beam). As far as I can understand, I can only see one option, in that to remove a section of wall plate, and sit it on a bearer plate on top of the internal block work wall. The obvious problem with this is that then all of the existing ceiling joists are in the way, as the steel cuts right across them for the full width of the house. Do people usually cut out a section of these ceiling joists to create a gap to sit the steel into, and somehow tie them back into the steel to reinstate the strength? Given the size of the steel, to have it on the underside of the ceiling would cut into the ceiling height of the ground floor by so much, it wouldn’t be practical. Just want to have a decent understanding before work actually starts, so hope you can help with this query.

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It would help to see the plans you have had drawn up so we can tell why you need the steels. 

 

In some cases they can go above the existing ceiling joists and be hidden in stud walls. If you are having dormer windows rather than Velux you cant do that because there wouldn't be a stud wall there.

 

2 hours ago, CJE said:

Do people usually cut out a section of these ceiling joists to create a gap to sit the steel into, and somehow tie them back into the steel to reinstate the strength?

 

Depending on how high the steels have to be that might be the correct option. I did a loft conversion on a bungalow with my dad back in the 1960s but not since. It went something like..

 

Fill in sides of I beams with timber (typically shot nailed or bolted to the web).

Support ceiling and cut existing joists.

lower I beam into place.  

Fix sister/floor joists alongside the existing ceiling joists and fix to I beam infill using joist hangers.  

Bolt existing ceiling joists to the new sister joists. 

 

Ideally the bottom of the I beam and bottom of the new joists should be about 1" above the top of the plasterboard so any flex in them does not transfer to the ceiling. That reduces plaster cracks in rooms below. For the same reason only the ends of the ceiling joists are bolted to the new deeper floor joists. However that assumes the new joists are sized to carry the floor load on their own, leaving the existing ceiling joists to just carry the ceiling load. If they are meant to share the load then more bolts are required and there is greater risk of cracking. It depends if you have the depth for the new joists needed.

 

If you need to you can use long tailed joist hangers and wrap the tails over the top of the steels and nail to the timber infill on the other side as well..  

 

Edit: A lot of care is required. For example in some houses the ceiling joists triangulate the roof stopping the roof pushing the walls out. So cutting all of them at once is not a good idea.

 

 

 

 

 

 

Edited by Temp
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