Flat roof midspan support question

[razorman]

Hi guys im after some advice regards a small extension flat roof build I am currently diying.

The extension is a 2.5m x 2.6/3.7 addition to an existing flat roof extension wnich will give us a much larger single storey extension to rear of house.

Its an odd shape due to an underground lpublic sewer and the outer wall reflects the angle and cleareance tolerances required by water board without a build over.
So basically one flank is around 2.5m long while the other is around 3.7m long. The proposed mid span bearer from house to rear of extension would be 3.3m in length.

It was originally intended to change existing roof set up from170x50 joists span across flanks and roof falls to its short flank, to turn the fall around 90 degree so it would fall to the rear and build new addition extension roof to the same with new 150×47 joists.

Another idea came up to keep existing 170x50 joist work in place and add new extension roof to continue as the same set up. Obviously to do this there would need to be a mid span bearing for each flanks joists.

My question is could this mid span bearer be made up of the same size timbers as the joists ie 170x50 x3 bolted together and the flanking joists connected to this bearer via timber to timber hangers?
To me this would save a lot of work than changing around the existing set up. Ive been trying to find info on how to size up this mid span bearer to effectivley support roof but have so far not found much info.
The roof construct will be 130mm 7nsulation board topped with 18mm osb3 tongue+groove with a grp waterproof membrane.
Any advise that could get the roof going would be well appreciated.
cheers.

 

[razorman]

Sorry just realised an error in my initial post.
the joist spans from each flank to the proposed mid span bearer would be around the 2.5m mark.
Each joist and midway span will sit on 100mm internal blockand builtin with around 200mm brick work on top.

 

[tony1851]

Can't follow this - can you post a sketch plan?

 

[razorman]

Yep sounds like English to me but Russian to everyone else!

I will try to upload a drawing or two later but to try and make it easier if you imagine a rectangular area dissect it in half between the long lines that would be my midspan line.
This line would be 3.3m long. The 170x50 joists would each run from this midspan line to each outer edge of the rectangular short lines. These joists would each be 2.5m.
my question is do you think the midspan bearing joist would be able to cope with the loading by bolting 3x170x50x3.3 m joists together to form the bearer?
Bearing in mind the joists and bearer will all sit on the internal cavity 100mm block or brickwork walls and built in with the same up to 1 course block and brick above.
Sorry if this is no better.

 

[munchingB]

so is the total span of the room 5m?

you want a bearer running down the middle with hangers supporting the 2.5 joists?

and your asking if 2 170x50 timbers would work as a bearer?

if the room is only 3.3m long why cant you run joists along that length?

also if your room is 5mx3.3m how is the 3.3m wall the 'long lines'?

 

[tony1851]

Yep sounds like English to me but Russian to everyone else!

I will try to upload a drawing or two later but to try and make it easier if you imagine a rectangular area dissect it in half between the long lines that would be my midspan line.
This line would be 3.3m long. The 170x50 joists would each run from this midspan line to each outer edge of the rectangular short lines. These joists would each be 2.5m.
my question is do you think the midspan bearing joist would be able to cope with the loading by bolting 3x170x50x3.3 m joists together to form the bearer?
Bearing in mind the joists and bearer will all sit on the internal cavity 100mm block or brickwork walls and built in with the same up to 1 course block and brick above.
Sorry if this is no better.

So the 3,3m line is in the middle, right?
Then it will be supporting 3.3 x 2.5 sq m of roof, right?

If your dead load is 0.7 kN and live load 0.75, total max. load will be around 12Kn-ish.

using three pieces of 2x7, you willl be OK for bending stress, but the deflection will still exceed the maximum recommended for a plaster ceiling to prevent cracking.

Why not use a deeper main beam (keeping the top level with the tops of your joists) and batten out the ceiling from below to bring it flush.

Or as above, just span the 3.3m?

 

[razorman]

so is the total span of the room 5m?

you want a bearer running down the middle with hangers supporting the 2.5 joists?

and your asking if 2 170x50 timbers would work as a bearer?

if the room is only 3.3m long why cant you run joists along that length?

also if your room is 5mx3.3m how is the 3.3m wall the 'long lines'?

Yes the total span of room is 5m.
The reason I was thinking of doing the midspan bearer is because half the roof (the original half of room) is already spanned the 2.5 m with 170x50 joists.
I originally did plan to run the joists the 3.7 m span but this is proving more complicated due to an existing 2m house window which will become a 2m entryway missing its external lintel. I intended to put implace a heavy duty box lintel but the height around 215mm would mean that the joists would have to sit on it directly. Not sure if this would be correct?

So the thought occured to just continue with an exact copy of the existing joist configuration with a central bearer.

 

[razorman]

Sorry Tony having probs getting any drawings lnto the site via my tablet.

Thanks for these calcs. What sort of size beam would work more effectivley for eliminating the deflection? Im guessing deflection would be bounce in timbers?
Would adding more between joist strutting also help?
If I can find a suitable calc im hoping it woukd make this roof build effectivley nearly halfway finished without me having actually started