Thoughts on a beam

[TeachMeObiWan]

Hi,
I'd appreciate anyone's thoughts or suggestions or recomendations.

I have a crappy old conservatory that needs something doing. Also it's too hot, so my thinking is to convert it into a garden / sun room. It's currently a lean-to with the left side wall being the garage wall, so the front and the right wall will need replacing.

My thinking is to rip out what's there, and build a simple stud frame, with a big window, some doors and a lantern on the flat roof. Joists would hang off the wall plate at the back onto the beam across the front wall. I'm think of building the whole lot out of C24 170x47. I'd be going for 400mm centres for the roof joists with double joists / trimmers where the lantern is going. Same C24 170x47 for the walls, spacings vary because of doors & windows but nothing exceeding 600mm, most a lot less. The front is 7.2m wide and the side wall is about 3.4m, say 3.6m for the joists. The front wall will have French doors of 1.8m and a window of 3.5m.

So now I get to my actual question, I'm thinking of creating a beam to span the 7.2m which will act as the load bearer for the joists and taking the place of any headers above the door and window because of height issues. It will sit on top of the stud wall top plate, which will effectively be the header timber for the door and window. I've done a few rough load calculations & snow loading etc, and it seems what I'm thinking is easily strong enough, but I wanted to get a more knowledgable eye to just look and give it a gut feel incase I'm totally out in left field.

The beam I thought could be a sandwich, bolted/nailed or both, of three C24 170x47 timbers, probably 4.8m, with staggered joints. To be honest I could almost fit four (188mm) on top of the 170mm to plate if I didn't t think it was overkill. By the way, all of the full height wall studs would be doubled because they fall either at each end or at the openings for the door and window, it's only the cripples under window that would be single.

Thats's my plan, unless anyone yells "STOP", roll on summer
Thanks for your time.

PS. 18mm OSB3 T&G roof with EPDM, 9mm OSB3 sheathing that will be clad

 

[mikeey84]

I'm no expert, but could you use a filtch beam?

I would say deflection is probably your biggest issue at 7.2m, but again, I'm no expert. There are a few SEs on the building forum, so probably worth asking there

 

[Deleted member 174758]

Same thoughts here - flitch beam would be smaller (at least in cross section and probably cheaper). Better to ask some of the structural guys on the "Building" or "Building Regulations and Planning Permission" forum - or ask the Mods to move it there for you

 

[TeachMeObiWan]

Thanks for the responses. I had thought about a fitch beam, but because I'll have the lumber, and the space to fit the beam, the compound beam seemed a simpler option. Although the weight is an issue I hadn't considered. As for deflection I was thinking the thickness would probably cope with any issues there, I've seen smaller beams over similar if not longer spans.
I'll drop a question over on the building forum though, good idea.
Thanks again

 

[TeachMeObiWan]

Ooops, it looks like the moderator pixies have already done my work for me and moved my thread across to "Building"
Thanks.

 

[Notch7]

It rather depends on how big the lantern is.

The biggest point load will be where the ends of the doubled lantern beams land on the front ring beam.

If you have a central lantern say 3.0m x 1.8m then that leaves you with a flat roof perimeter of say 800mm front and back and 2.1m at the ends.

One way to do it is, make the lantern beams say 10 x 2 but cut down leaving the full 10” at the lantern up stands. Either side, cut down so the beams don’t stick up too much above the firrings.

Doing that, you can make all your flat roof perimeter roof joists out of say 4 x 2. Instead of using loads of joist hangers, you can fix 2 x 2 around and notch out the joist ends.

Using 4 x 2 for the flat roof infills allows you to have a proper warm roof, with just a cut out for the front to back structural lantern beams.


In regards to the ring beam at the front, you could let in some angle iron to support where the lantern beams land - which could well be above the French doors.


I assume water will discharge into an external gutter?

If so, your firrings will be rather complex. The highest point will be mid point behind the lantern, falling either way but also having a fall forwards to line up with the fall of the end flat roof perimeters.

Don’t forget, firrings go in first before warm roof insulation.


The design won’t have any consideration for lateral stability but you will have the house and garage acting as a buttress to resist wind load.

 

[tony1851]

I would say deflection is probably your biggest issue at 7.2m,

No 'probably' about it! .

Deflection is the factor which kills timber floor- and roof joists, and rafters.

(I can't follow the OP's idea - a quick back-of-envelope sketch would help get more suggestions)

 

[mikeey84]

No 'probably' about it! .

Deflection is the factor which kills timber floor- and roof joists, and rafters.

(I can't follow the OP's idea - a quick back-of-envelope sketch would help get more suggestions)

I've obviously learnt something listening to you lot bicker on here

 

[Notch7]

No 'probably' about it! .

Deflection is the factor which kills timber floor- and roof joists, and rafters.

(I can't follow the OP's idea - a quick back-of-envelope sketch would help get more suggestions)

The 7.2m is sitting above the doors and windows at the front, like a ring beam / headplate beam on a conservatory.

his concern is the point loading joists landing above French door openings etc

 

[^woody^]

Deflection is failure for finishes, not really a failure of the beam.

 

[Dereekoo]

STOP. If I understand right and the beam is to span 7.2 m unsupported along its length even assuming you had a solid 200 x 200 beam s with the 1.8m width of your front roof load (taken as 1.25kn/m2) acting on it the deflection will be 5 times allowable and the strength would be half of whats required

 

[Notch7]

I'm thinking of creating a beam to span the 7.2m

this phrasing is causing confusion - it’s not spanning that distance, the beam sits above the framework, doors and windows across the front.

 

[Dereekoo]

this phrasing is causing confusion - it’s not spanning that distance, the beam sits above the framework, doors and windows across the front.

But as is the usual case the OP knows exactly what he means and intends

 

[Deleted member 174758]

...I it looks like the moderator pixies have already done my work for me and moved my thread across to "Building"

Yes. I asked them last night...

 

[cdbe]

this phrasing is causing confusion - it’s not spanning that distance, the beam sits above the framework, doors and windows across the front.

In which case it only needs sizing based on the longest span - the 3.5m window.