Timber roof design

[oneintheeye]

Hi, I designing the roof rafters for an extension I'm doing to my house. It only 5m span with a central wall to split the span. Its a slope on the roof <30 degrees.

Ive gone through the code and designed the beams/rafters. There are current rafters spaced at 350mm which I'm going to stick with spacing wise. I get 6" deep by 2" wide on quite a low strength grade. Does this seem reasonable. Effective max span is 3.25m.

Thanks, I usually deal in concrete and steel.

 

[diarmud]

Need to know where you are (country, state, region) for the snow/wind load.

Also, if you are supporting the roof at the center with a "central wall", is the "central wall" supported below it (ie basement, crawl space, slab) - have you accounted for transferring loads through the wall to the foundation?

Let us know,

Regards,
Dermott

 

[oneintheeye]

uk, the central wall is existing although will be built up so happy with loading on this element. Its supporting the roof currently anyway although it will take more load. Snow load here is 0.75kn/m2

 

[diarmud]

Note,

Check your local building code. They are usually prescriptive and in this case may dictate what you have to use as minimum. You need to run this past a licensed engineer with experience in residential construction.

Don't just rely on me, I may have misunderstood your explanation, but, it would appear you are okay but check the deflection, it may be too much and you may want to go to a deeper member.

Good luck,
Dermott

 

[apsix]

For that span your joist size will nearly do as floor joists. TRADA publish span tables and they can also be found in the old Approved Document Part A.

http://www.canterbury.gov.uk/assets/buildingcontrol/approved-document-a-1985.pdf