Dimension of roof trusses

[Niklas_d]

Hello!
I am planning to install a 10mm insulated roof above the terrace with a span of 4.8m. I live at the same latitude as Gothenburg.
My question concerns the dimension of the rafters. Do I need glulam or is it sufficient with a glulam beam at the front as a load-bearing beam and 45x220mm as rafters?
How is the load-bearing capacity affected if you screw together two 45x220 mm pieces, does it improve the load-bearing capacity or will it flex more due to the weight?
Grateful for an answer!
Thank you in advance //Niklas

 

[hakaner]

Hello!
Gothenburg is in snow zone 1.5, BUT check which snow zone applies exactly where you live! There is an area with snow zone 3.0 at the same latitude as Gothenburg!
http://www.boverket.se/Global/Bygga_o_forvalta/Dokument/Boverkets%20konstruktionsregler/BKR_2010-2_BKR_13.pdf

Depending on exactly where you live, it is on the borderline
to manage a 4.8m span with construction timber.
http://www.traguiden.se/TGtemplates/PageTwoColumn.aspx?id=1465

Example of approved construction, Construction timber:
Snow zone: 1.5 (Gothenburg)
Span: 5.0m
Dimension: 45x220mm
Quality: C24
c-c measurement: 600mm
Roof slope: Maximum 10 degrees
Execution: The beams are braced against tipping

Example of approved construction, Glulam:
Snow zone: 2.0
Span: 6.0m
Dimension: 90x360mm
c-c measurement: 2400mm
Roof slope: Maximum 10 degrees
Execution: The beams are braced against tipping
Roof: Self-supporting between roof beams, c-c 2400mm

 

[hakaner]

And:
Yes, if you screw together two 45x220mm, the load-bearing capacity increases to double.
BUT, the deflection doesn't decrease to half & often it's deflection that determines the dimensions...
(it makes minimal difference if you screw them together or not)

For tables for glue-laminated timber, check the Design Guidelines:
http://www.byggmagroup.se/dt_article.aspx?m=2479

But my advice would probably be this:
* Check the prices of glue-laminated timber versus construction timber, my guess is that there isn't a large price difference for a patio roof (calculated in kronor, not in percentage)
* Also check what c-c spacing your 10mm insulated roof can handle? (in your snow zone)
It might look nicer with glue-laminated beams and larger c-c spacing?

 

[anaitis]

And:

BUT, the deflection does not decrease to half & most often it is
the deflection that determines the dimensions...
(it makes minimal difference whether you screw them together or not)

Almost right
With two beams side by side, the following happens:
The moment of inertia doubles.
The stress is halved.
The deflection is halved.
No difference if they are screwed together.

You were probably thinking of changes in span length.