Modified Rafter Design

[jim57]

I have a roof system in which the total span is 17' and the rafters are 2 x 6s. The ceiling rafters were removed and the 2 x 6 rafters are now supported with 2 x6 attached at the eave at one end and at the rafter (approx top 1/3) on the opposite end. Does this mean that I still have a collar tie at the former ceiling level? This was installed as a modified cathedral ceiling.

Thanks

 

[msquared48]

Do I understand from your sketch that the lower members of the scissors truss were removed, leaving only the top chord rafter?

If so, this is a big nono as 2X6's will not span 17 feet. There is also the matter of enough depth for insulation... and venting.

If I understand the problem correctly here, you relly need to engage the help of a local structural engineer here... and REAL, REAL soon. Temporary shoring at the least would be very appropriate.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jim57]

No this is the final design. The horizontal ceiling rafters were removed

 

[jim57]

not the diagonals

 

[msquared48]

Then what you have created is a scissors truss. If you are not a structural engineer, I still strongly suggest you contact a local structural engineer to verify your design, especially the connections.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[woodman88]

From your sketch what you have is either a modifed raised collar tie system or a scissor truss. I doubt that it meets the building code (IRC/IBC) requirements. As stated above, you should get a local engineer to correct the design.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[MiketheEngineer]

This might just possibly maybe could be made to work. BUT I have my doubts. It would depend heavily on loadings required, lumber grades, roof pitches, connections, etc.

As noted - get some experienced help.

 

[4thorns]

Are you saying that the span from wall to wall is 17'(8'-6" rafter span) or the span of each rafter is 17'? If the rafters only span 8'-6" and the load conditions aren't too bad then the rafters themselves may not be your biggest problem.

 

[MiketheEngineer]

My point - I agree w/4T !!

It may just be possible?? Need all info - or better yet the guy in your area who is familiar with this type of construction will need it.

 

[csd72]

ditto

A little bit of knowledge can be a dangerous thing.

 

[csellc1]

It looks like a scissor truss. Scissor trusses still have a horizontal thrust that needs to be resolved. If it is not properly resolved, the exterior wall or walls will be pushed out and start to lean. look at the joint between the sloped 2x6 and top of the wall. You may find a hairline crack in the joint. If you do it could signify that the wall has started to lean out.

You should should hire a local structural engineer to review the situation and prepare any repair sketches that may be required.

 

[Ron]

Going back to Mike McCann's post....the connections are key here. It might be workable, but there can be a fair amount of tension to carry in those lower chords. Get a structural engineer involved.