Composite beams/joists

[rosinante]

Hi Does any one on the forum have any technical knowledge /experience of what size the joists would need to be to span a nominal 6M supporting an insulated panel flat roof with a specified weight of 11kgs per M2,I realise that I could use humungus timbers but would like to lighten the appearence and cost by making them myself,I have seen somewhere I beam type shapes but need to know the sizes I would need,I dont fancy steel beams as I am concerned with condensation, the building in question is of course my workshop,all advice welcome

Cheers Chris

 

[Jelly]

Look at finnforest's and oakworth's websites for a rough idea, however I suspect that

• It's probably not as easy as just making a groove in some timber and gluing some OSB into place
• The Building Control people will be skeptical at best because they're not a well established construction technique and would not have been sourced from an established supplier that has completed significant load-testing and obtained a certificate of approval*

Good Luck and I hope you can crack it.

*It is unheard of for people to make their own laminated timber beams... Presumably for this reason> In fact as far as I know there's only a handful of manufacturers in the world and just Finnforest & Howarth Timber Engineering manufacturing them in the UK.

 

[carlb40]

I might be mistaken, but i think the beams you mean need to specced by an engineer for the weight load etc. Not sure you can just get them off the shelf. I think the roofing ones are graded differently to the ones designed for floors.

 

[zb1]

The flat roof span table I can put my hands on has a 6.07m clear span up to 10 degrees with C16 75x220 @ 400 centres with a load of upto 0.5kN/m2 which is upto 50kg/m2 I think (would seek confirmation on that). On the subject of load, does your 11kg/m2 include imposed snow and wind loads as well as self weight?

 

[twothumbs]

This method of creating light weight beams has been around for decades (at least the 60's I think) and used to be with ply webs and this was before stress grading, etc. I dont think there is anything special except 'good' glue. (dont know what that would be though). Long time ago span tables and lots of info was readily avaialble but not sure where to look fot it now. If I remember anything heplpful I will add to the thread. You could contact one of the suppliers and ask for a size for your span and they will give a load of live + dead and so be a guide for you. Best wishes.

 

[Jelly]

This method of creating light weight beams has been around for decades (at least the 60's I think) and used to be with ply webs and this was before stress grading, etc. I dont think there is anything special except 'good' glue. (dont know what that would be though).

Accounting for the differential in moisture movement between dissimilar materials is the obvious one to me? I know Boise Cascade use laminated timber rather than solid to create a product where all components expand and contract uniformly together... The right glue is probably one that can cope with that issue.