How to salvage this roof construction?

[JT101]

Hi guys

I've been asked by a friend to finish off their shed/workshop with insulation and plywood internally, but I've some serious concerns about the roof structure.

As you can see from the image, the original builder used 47 x 97mm timbers at 600mm centres across a 2900mm span on joist hangers.

So, we all know that doesn't meet standards for the spans. The maximum at 600mm centres is 1.72m from my understanding assuming C16 timber used (I don't know what was actually used so I'm assuming worst case)

They also originally built it too high for regulations, so whereas before the joists sat on the stud walls front and rear, they had to drop down the front onto joist hangers on the inside of the front wall.

However..............they have put three 47 x 97mm cross braces beneath the roof joists also on mini joist hangers at 4100mm spans and 950mm centres!

Not sure how to try and understand the loading on this one. Obviously it's helping a bit but not splitting the load because it's unsupported from below.

Just wondering is there any remedial work can be done to strengthen up the roof. Once I add insulation, plywood, and you factor in potential snow loading, my fear is it will collapse.

I'm not sure the cross braces are doing much.

Thanks

 

[JT101]

Been thinking about a few solutions, obviously the quickest and simplest solution being the ideal.

1) Replace all 4in joists at 600mm centres for 6in joists at 40mm centres. A lot of work assuming the OSB roof is screwed to the joists. Plus they are sitting on top of the wall at the back, NOT sitting in joist hangers as per my sketch. So I would have to notch it in at the back, and hang it at the front. Access to putting in strapped hangers may also be an issue.

2) If I slot in more 4 x 2in joists between the existing joists, does that not effectively halve the span? So now they're only supporting 1450mm instead of 2900mm, which would put them in spec.

3) Run metal rods through the centres of the joists, threaded at each end and bolted tight onto outer most joists.

Clearly I need to add noggins to stiffen the structure too, and probably some restraint straps either side.

 

[catlad]

It will take longer to mess about with the old roof than it will to replace it, save the original timber for something else would be my advice.

 

[garyo]

What about replacing the cross braces on the 4100mm span with a steel equivalent?