Gable rafter span, what is a rafters 'span' exactly?

snoz · Sep 03, 2022 8:17 am

In a gabled roof, what is the definition of a span of a pitched rafter?

In the image below, is the span A? or is it half of C?

Bonus question, if the member marked 'b' is unsupported at each end, is that called a ridge board or a ridge beam?

I can read the span tables, I just cant get 100% answers on the above two questions. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

StructuralBIMGuy · Sep 03, 2022 9:18 am

Hey nice 3D Bim Model
Unfortunately, SketchUp or any other 3D Software package, AS1684 span tables, Copying the neighbours certified As-built Structure as your alternate solution, may not get your drawings certified and approved.
I suggest you talk to your Council building surveyors for min requirements
Here, for approvals you will need Structural Engineers (signature with professional qualifications and 5 years min experience at least) on footing details/notes, wind classifications, structural frames drawings/3Dmodels, data etc

Alternatively, Patio one stop fab shops have certified over designed sizes that fits all engineering type members.
They are very popular in my area but at $1K/m2 it's a bit of a price gouge, but hey that's where it's at? or alternatively flat patio tube (Roof and 4 tube poster) at $500m2 with the salesman complaining I'm barely making 30%? (humour)

snoz · Sep 04, 2022 8:26 pm

Thanks structuralbIMGuy - i have completed the drawing and submitting them to council already and they have been approved. So I am 99% home and hosed. Can you tell me if the span is read at side 'A'?

Because if that is correct, a rafter 140x45tp should work here at 900mm centers to reach up to 4,200mm on the 'A' in medium wind load to cover 'C' at 6,000mm.

Beams are no issues that is clear, its the gable/angle that is confusing me as there are two rafters on each side, so I am mixed up if they are doubling, or treated as independent for calculation reasons.

Bimbob · Sep 04, 2022 8:30 pm

Aaaaaand the answer is……….A

StructuralBIMGuy · Sep 04, 2022 11:04 pm

Have you looked at As1684 _2.6.4 Roof Load width Images+ supplements (span table 29 and 30)?
Type of construction (a) No ridge struts Roof Load width (rlw) =X+OH, or (b) Ridge Struts rlw =x/2 +OH
Wind loading classification there is no medium? N1-N4?
Your model shows NO RIDGE at all?
What you've modelled is a TRUSS?
Connections?
Sorry looks good, but it's not complaint, cross check the codes or is it an alternate solution?
Hint
Please reference what you're using and why you are using them to the council BSs, unfortunately sounds like it's (Table) out dated? eg, Backyard deck Bim + cookbook recipe
Start with the referencing the proper codes , labelled details and generally include a simplified table generic diagram for the council BSs... Council guys do make mistakes
Include 2D details, 3D Bim Model, Hold downs, load path diagrams, data/proofs, etc.
Curious if you are 99% there why aren't they helping you with the other 1%?
BTW, if you are putting up Bim/diagrams/photos put up as much info ie dims, sizes, references ,etc as you can, the better the info..the better the reply plus it helps others.
hth

snoz · Sep 06, 2022 11:50 am

Thanks @Bimbod that was pretty much all I need as I can work out the rest now.

@StructualBIMguy they will help but I just wanted to check I wasn’t being a complete idiot first. This is more of an illustration I used sketchup to design which is what this is from but I used layout to send in the plans. The plans are compliant and show the central member except it’s named a ridge beam which is the core error.

Cheers lads

StructuralBIMGuy · Sep 06, 2022 6:18 pm

snoz

The plans are compliant and show the central member except it’s named a ridge beam which is the core error.

Here's one way of supporting the ridge beam and decreasing it size?
Try sizing the timbers using the correct tables -> Continuous beam ?
Finally, better presentations earn brownie points in Council
hth

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