I had planned to build my roof from 300mm Finn I joists, hip roof, no purlins, two mtr ridge, osb lined inside for racking and vapour barrier, full fill with 300 mm batt insulation, felt, battens, slates etc. I have been told this is incalculable for the roof by the I beam suppliers! They are talking ( to my builder who I trust) about a raised tie truss roof battened out to 300 for insulation and using a glulam ridge beam or even needing steels in some way.

With regard the glulam ridge beam ( which I cannot see the need for) I have had this discussion before, with a pitched roof the ridge beam simply ties the two sets of rafters together and carries no weight as such. If ( like a loft conversion) one pitch was removed then yes a beam, steel or glulam is needed to carry the weight of the slope retained but with equal and opposite slopes surely this is not required.

Surely 300mm I joists over a 5 mtr span at an angle of 35 degrees is not rocket science to calculate ( I
wish I had become a SE or at least learnt the theory before I started this project). We are also getting into the realms of paying for an S.E. to make these calculations which obviously I would like to avoid.

So, am I barking up the wrong tree?, am I trying to be too simplistic?, I cannot for the life of me see the requirement for steels.

HELP

You could find that the I beam suppliers are unhappy about the use of their product outside of its normal use. They won't want to approve any nonstandard application (liability insurance etc.).

You might find that in the end you will need an SE (with professional liability insurance) to calculate and assure the design. Such things happen when you step out of the 'deemed to satisfy' or the standard loading tables areas of building design

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Posted By: joe90I wish I had become a SE or at least learnt the theory before I started this project

Never too late

http://web.mit.edu/4.441/1_lectures/1_lecture5/1_lecture5.html

Tychwarel, thanks for that, very informative and yours looks more complicated than ours and needed a ridge beam ( but I understand why from your pics) I would like to know of anyone who has had a design done for a similar I beam roof as I am convincinced there is a fairly easy solution to this problem.

Nick, yes never too late and I would be interested to learn just wish I had done it already to get me out of this mess.

I wish there were architects that understood structural engineering and “buildability”, it seems that architects don’t care how hard it is to build, or what size beams will be needed for their nice shapes, structural engineering don’t think about how beams will be moved etc, and builders don’t understand the “why” of most of what they do….

Posted By: ringiI wish there were architects that understood structural engineering and “buildability”, it seems that architects don’t care how hard it is to build, or what size beams will be needed for their nice shapes

Architects have the dreams, the SEs job is make sure the dreams don't fall down..........Daughter-in-law an architect, Son-in-law a SE.

Joe, sounds like you have basically a monocoque shape (if you can assume it's 'closed' at floor level) which should be calculable as such by a SE - the sheathing doing most of the work - but then would you need I-beams when all you really need is to connect the sheet edges together and make sure they don't sag too much between those edges?

Anywhere near Devon Joe? My favourite SE is brilliant, quick to understand, does minimal paperwork knowing that I like to do the drawing-up, earns every penny.

Joe90 can smell the pasties where he is. On the wrong side of the border though as far as the planning department is concerned.

Careful Tom, you will be putting the jam and cream on the wrong way on your scone if you are not careful.
That is then a very slippery slope.
If anyone is interested, and I doubt if they are. It is pronounced scone (with a capital O) before it is eaten, and scone (as in gone) once eaten.

Hi Joe 90,

We have an I beam roof (walls and floor) with 300mm insulation. Look for Little Dockens build part 2 (?) video on Youtube, Might be of help

Another design suggestion for you Joe, use a "parallel chord scissor truss". We used such a thing in part of our new build to gain an attic room, 400mm deep space for insulation etc. and the truss companies can do the stress calc for roofs. I had to ring around a bit before I found a couple of truss companies prepared to try the calcs (not totally standard so had to do more than press one button), but they both said that the result was surprizingly good.

Posted By: joe90Sorry guys but this is bugging me so I needed to do a drawing, now from my very simplistic drawing I fail to understand why "W" needs support from below
1/. flex is the bend within the rafter and it depends on the weight and depth of the timber/ijoist
2/. Spread is the tension in the ceiling joists created by the weight of the whole roof ( I admit this could be less if a ridge beam were installed)
3/. W1 is the equal weight on both wall plates of the whole roof ( unless a ridge beam is installed)
4/. W2 is the equal and opposite weight both sides of the roof create by resting on one another ( think of house of cards)

The various pressures are dependant on angle a.

therefore if all the above is adequate there is no need for W to be supported I.e. No ridge beam.

Oops unable to load diagram it says it's too big! ( I think this is an iPad problem)

Yes ST, problem is I drew it and took a photo, I don't know how to make the photo smaller in data terms.