Rafter Ties
Rafter Ties
(OP)
Attached is a picture that the homeowner had in mind.
Question 1:
My question is if a ridge beam is not installed then the rafter ties need to be installed near the bottom of the rafter and not in the top portion right?
Question 2:
Another option I was thinking of was installing a 2x4 from the rafter back to the 2x4 stud wall? If this was done could a regular ridge board be installed with collar ties in the top portion?
Thanks for the help.
Question 1:
My question is if a ridge beam is not installed then the rafter ties need to be installed near the bottom of the rafter and not in the top portion right?
Question 2:
Another option I was thinking of was installing a 2x4 from the rafter back to the 2x4 stud wall? If this was done could a regular ridge board be installed with collar ties in the top portion?
Thanks for the help.






RE: Rafter Ties
#2. I would need to see a sketch of what you are thinking to comment further.
Mike McCann
MMC Engineering
RE: Rafter Ties
RE: Rafter Ties
Mike McCann
MMC Engineering
RE: Rafter Ties
BA
RE: Rafter Ties
Mike McCann
MMC Engineering
RE: Rafter Ties
After reading the replies it sounds like I should leave the rafter tie (B-C member), install diagonal 2x4's fastening them to the rafter and floor joists @ 4' o.c., increase the rafter size to 2x10's, and install a tension tie where the floor joists meet.
What is typically used for a tension tie?
Thanks.
RE: Rafter Ties
RE: Rafter Ties
BA
RE: Rafter Ties
Don't forget that the roof joists would need to be designed as beam-columns. The calculation gets pretty hairy. Another reason I don't like rafter roofs.
RE: Rafter Ties
Not true. Ordinarily, the lower the collar tie, the lower the tension. But in this case, we have two ties, B-C and F-G-H (see attached).
Tie B-C is actually a redundant member which holds the roof together during wind uplift. But under snow load, B-C is in compression, not tension. If snow is equal on each side of the ridge, the walls at points D and E carry all of the gravity load. Members D-F and E-H are tension members and the wood stud walls at F and H will be in compression. Member F-G-H is a tension member and requires a metal strap tie or a plywood gusset each side to carry the calculated tension force.
BA
RE: Rafter Ties
Mike McCann
MMC Engineering
RE: Rafter Ties
RE: Rafter Ties
You need to develop the tension through side grain nailing that imparts a shear to the nails, ie, side plates or knife plates with through bolts, or metal straps.
Rather than connecting to the side of the joists, it might be possible to connect to the top of the joists with strapping to develop the tension force. Should be less visible there.
Mike McCann
MMC Engineering
RE: Rafter Ties
@jim, What beam? I thought they were ceiling joists and would be covered by ceiling. Are you using beams at 4' o/c? If so, your elevation should say so. I agree with Mike that you need a more positive connection than one or two spikes per lap. A strap tie on top of the beams would be okay, but why are you asking us? Isn't this pretty elementary stuff? What is the magnitude of the tension you are carrying?
BA
RE: Rafter Ties
I guess I am confused here too. Ususlly you attach a joist to a beam and a beam to a column, not a joist to a column. Is this detail at the splice of the lower chord which consists of floor joists?
Mike McCann
MMC Engineering
RE: Rafter Ties
Can a strap tie could be installed on top of the beams along with the spikes? This would make it so it isn't visible.
RE: Rafter Ties
If you are going to use beams at 10' centers, why don't you provide posts above the ceiling at 10' centers and a ridge beam? Then you don't have to worry about tie forces.
Another option is to span the beams parallel to the ridge, then use joists perpendicular to the ridge with ties and diagonals spaced at 4' centers as originally intended.
A third option is to space diagonal members D-F and E-H at 10' instead of 4', but the attachment would be visible on the sides of the 8x8 beam, so the client isn't going to like it. Also 10' is probably too far apart for the roof to work as intended. So this is not really an option.
BA
RE: Rafter Ties
RE: Rafter Ties
RE: Rafter Ties
It would be more sensible to use 2x8 joists on top of the beams to avoid notching or alternatively, to use joist hangers and flush framing.
I doubt whether an 8x8 beam is adequate and, in any event it is not an economic size for a beam. The lap connection over the central column will tend to promote cracking in one of the beams and the notched connection to the exterior column is a poor detail.
The 2x4 diagonal brace fastened to rafters @ 4'-0" centers makes no sense at all with the framing system proposed. If you don't see this jimtheengineer10, I think you should be working under the supervision of a more experienced engineer.
BA
RE: Rafter Ties
I was skeptical of the plans and that is why I am asking the questions before I take on the project. The owner says he has built something just like this before and that it is still standing. Due to this he thinks he should be able to build the exact same thing even if engineering calcs show otherwise.
I think I will talk to the owner and recommend the following:
- install an lvl ridge beam and a couple posts on the second floor
- use 2x8 joists to sit on top of the beams instead of the 4x6 joists
- use hangers to connect the 8x8 beams to the posts
Would you recommend using hangers on each side of the post to support the two beams instead of the lap connection or is there a better option?
Thanks for all the help.
RE: Rafter Ties
My preference would be to butt the beams together at the middle of the column. A two span continuous beam is another option, but it may be difficult to buy one that long. In either case, I would use a suitable post to beam attachment as made by Simpson Strong Tie or similar.
The attachment to the exterior column should be a suitable hanger if the column is continuous as shown, but it might be easier to cut the column off at the underside of beam and let the beam run through. The upper length of column can easily be braced back to the beam.
BA
RE: Rafter Ties
I have seen hammer trusses employed, but that's an entirely different animal.
I am not saying that is cannot be done, but it is definitely not the norm.
Mike McCann
MMC Engineering
RE: Rafter Ties
@jim: Why is the owner using 8x8 posts on the North and South walls? It would make more sense to span the second floor joists perpendicular to the ridge. They would bear on two exterior walls and a beam in the middle with a ten foot span between columns.
The roof rafters could be supported on stub walls bearing on second floor joists and rim boards. Ties at 4' centers could be provided as originally described in your earlier post.
BA
RE: Rafter Ties