Take not of Maurice if load bearing and wrong one used you'll have problems a structural engineer will tell you and give correct size of rsj and I think in any case you should contact building control at local authority office planning section first. Many upstairs ended up downstairs when people ignore warnings.
Bob
January 2011
Suggest you speak to a structural engineer to check out loadings.
He will also tell you what involved in installing and pad stones etc required
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Try fixfault.com to fix general d.i.y.