How to extend a 2"x6" rafter

[armvz]

I would like to know how to extend a 2"x6" southern pine wood rafter so that it keeps the same structural properties as if it was a continous member. Will a 4' overlap an two 1/2" bolts suffice? How is this calculated?

 

[SlideRuleEra]

Keeping the same structural properties may not be possible. For example, with a "perfect" splice both moment of inertia and section modulus for the overlap portion will be double that of the rest of the 2x6.
With that said you may be able to make a splice that will work for many applications but the following should be considered:
1. What is the load and how is it applied?
2. How long is the spliced 2x6?
3. Does the 2x6 have lateral support?
4. Where is the splice? Middle of span? Near the end?

For spans where a 2x6 would be satisfactory the 4 foot overlap will probably be adequate. Chances are you will need more connectors than 2 bolts (of any size). In general try to spread a lot (maybe a couple of dozen or more) of connectors (nail, screws, bolts, glue) over the entire overlap area. Most failures result from "bad" connections not inadequate members.

 

[armvz]

1. The extension is for a small covered terrace. The main load will be the wind load.
2. The added piece is 8'long.
3. Lateral support is provided by the roof deck.
4. The splice occurs at the end of the main rafter, sticking out of the outside wall.

 

[RDK]

If the main rafter is only sticking out a short distance i.e. 1 or 2 ft. Then this is not an extension to the rafter but a separate rafter.

You could simply support it on the existing wall and install it without a splice. The two rafters would not have to line up but that would be simpler for installation.




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com