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solid v posi joists guidance needed please


redtop

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SE proposed 253mm (or similar) posi joists for all floors (ground floor is raised off ground on posts).   Roof is flat, green roof.  frame builder (chippy, stick build) reckons solid timber joists would be better, more solid, less bounce for ground floor and will take a higher load for roof.  Going up a size in posi joists to get higher roof loading (3xx mm, to get to the 90KG per m sq) pushes price up a lot. We are near Sea in cornwall so mild but wet and windy

 

we will be running MVHR and do have quite a few steels at ground, and first floors but again builder reckons he can orientate joists soo we wont need to cut big holes for the pipes

 

what do you reckon guys???

 

 

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It’s many years since I’ve seen solid joists used on site Always posi or I joists 

There’s little difference between posi and solid though I would use solid on the ground floor and posi in the first floor 

 

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We used JJI I joists on the ground floor and posi joists on the first floor.  I can'r remember why the SE recommended different like that, other then the obvious, hardly any pipes in the ground floor, but plenty in the first floor which is where posi joists really shine.

 

Both have done the job well.  In fact I would say the split is good.  The JJI's downstairs have a natural ledge to lay strips of OSB to form a hard shelf to support whatever insulation you choose.

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7 hours ago, ProDave said:

We used JJI I joists on the ground floor and posi joists on the first floor.  I can'r remember why the SE recommended different like that, other then the obvious, hardly any pipes in the ground floor, but plenty in the first floor which is where posi joists really shine.

Maybe to remove metal from a cold ventilated space? Even though they’re not in ‘direct contact’ maybe it was a wise precaution. 

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22 minutes ago, Big Jimbo said:

I am a huge fan of either the metal web, or jji joists. Less bounce.... Is he taking the p1$$. Will be far better than solid timber without question. The timber you can buy these days is shite.

 

+1, solid timber also is not very accurate always a little bow somewhere, on my build jji were specked and used but I asked fir said joists (calculated at 600mm centres) to be installed at 400mm to make even more solid. I was able to cut  holes in the webs for electrics and MVHR without a problem (they come with a cutting guide).

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he owns a sawmill so I think was going to cut the joists himself from douglass fir trunks.  Seeing him tomorrow to discuss.  Apparently the extra cost for taking the post joists in the roof to the 3xx mm size costs an extra £5k!

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