toddmanqa
Specifier/Regulator
- Oct 13, 2011
- 2
I'm raising my ceiling joists in a second-story master bedroom and wanted to get some advice on bolt distances/location.
The roof is 8:12 (~34 degrees) with a roof span of 24 feet (L=12), and the attic uses rafters vs trusses (similar to the example picture in Figure 1, but there are two stories—not one).
There is a ridge board, but not a ridge beam. The rafters are Douglas Fir (DF) and nominal 2x8, 16" OC. The current joists are nominal 2x6, 16" OC DF and I will be replacing them with nominal 2x8, 16" OC DF #2. The height (H) of the ridge above the plate is 96 inches. The ceiling joists are already raised 15" above the top plate, and I want to raise them an additional 11 inches, so they would be 33 inches above the rafter plate, so the distance from the ridge to the new joist location would be (h) 96 – 33 = 63. Based on my reading, this would be allowable (1/3 distance).
I am in Albany, NY, and I used a total roof design load of 60 (40 dead + 20 live) (I have a slate roof too).
I calculated the W load on the rafters as:
L x rafter spacing x roof design load
W = 12' x 16"/12" x 60 psf = 960 lbs.
Tie-Force tension at the raised plate as:
T = tie force (W/2 x H/h x run/rise)
where H = height above ridge plate
and h = distance between tie and ridge (h <= 2/3H)
T = (960 lbs/2) x (96/63) x (12/8) = 1097 lbs
In regards to nailing/bolts, I believe I need either 11 nails or three 1/2" zinc-plated hex bolts (not lag bolts)( I figured out the capacity of the bolts at 398# based on the angle in the DF in Figure 2).
Similar to the example shown in Figure 3, the two sets of rafters will meet at a doubled-up nominal 2x8 DF #2 flush-beam which I will connect to the ceiling joists with a Simpson Strong Tie LUSA28Z Shear Joist Hangers w/10x10d nails, and a Simpson Strong Tie LSTA36 connector with 24x10d nails (Allowable Tension Load of 1640 in DF) that will connect the two joists over the flush beam. The flush beam ends rest upon an interior load-bearing wall and the exterior load bearing wall.
I have a few questions:
1. Do my calculations look right?
2. Will I be putting too much force on the rafters at their mid point?
3. I believe I need to be diameter (d) x 7 away from the end of the ceiling joist and d x 4 away from any edge, but I couldn’t find any example confirming that.
4. I know that there is a reduction in holding power for the bolts due to their angle in the rafter--is there a similar reduction for nails?
5. I have created a bolt pattern in the picture shown in Figure 4 based on my understanding of not placing bolts in the same grain line and away from the edges/end of board. The bolts will be more than 4" away from each other centers. Does this look right?
6. I've over-engineered the connecting strap over the flush beam (1640 vs needed 1097), but I was also curious--do the joist hangers add any lateral strength from the double-shear nails, or do the joists hangers only provide vertical strength?
Thanks for taking the time to read this.
Link to example/figures:
The roof is 8:12 (~34 degrees) with a roof span of 24 feet (L=12), and the attic uses rafters vs trusses (similar to the example picture in Figure 1, but there are two stories—not one).
There is a ridge board, but not a ridge beam. The rafters are Douglas Fir (DF) and nominal 2x8, 16" OC. The current joists are nominal 2x6, 16" OC DF and I will be replacing them with nominal 2x8, 16" OC DF #2. The height (H) of the ridge above the plate is 96 inches. The ceiling joists are already raised 15" above the top plate, and I want to raise them an additional 11 inches, so they would be 33 inches above the rafter plate, so the distance from the ridge to the new joist location would be (h) 96 – 33 = 63. Based on my reading, this would be allowable (1/3 distance).
I am in Albany, NY, and I used a total roof design load of 60 (40 dead + 20 live) (I have a slate roof too).
I calculated the W load on the rafters as:
L x rafter spacing x roof design load
W = 12' x 16"/12" x 60 psf = 960 lbs.
Tie-Force tension at the raised plate as:
T = tie force (W/2 x H/h x run/rise)
where H = height above ridge plate
and h = distance between tie and ridge (h <= 2/3H)
T = (960 lbs/2) x (96/63) x (12/8) = 1097 lbs
In regards to nailing/bolts, I believe I need either 11 nails or three 1/2" zinc-plated hex bolts (not lag bolts)( I figured out the capacity of the bolts at 398# based on the angle in the DF in Figure 2).
Similar to the example shown in Figure 3, the two sets of rafters will meet at a doubled-up nominal 2x8 DF #2 flush-beam which I will connect to the ceiling joists with a Simpson Strong Tie LUSA28Z Shear Joist Hangers w/10x10d nails, and a Simpson Strong Tie LSTA36 connector with 24x10d nails (Allowable Tension Load of 1640 in DF) that will connect the two joists over the flush beam. The flush beam ends rest upon an interior load-bearing wall and the exterior load bearing wall.
I have a few questions:
1. Do my calculations look right?
2. Will I be putting too much force on the rafters at their mid point?
3. I believe I need to be diameter (d) x 7 away from the end of the ceiling joist and d x 4 away from any edge, but I couldn’t find any example confirming that.
4. I know that there is a reduction in holding power for the bolts due to their angle in the rafter--is there a similar reduction for nails?
5. I have created a bolt pattern in the picture shown in Figure 4 based on my understanding of not placing bolts in the same grain line and away from the edges/end of board. The bolts will be more than 4" away from each other centers. Does this look right?
6. I've over-engineered the connecting strap over the flush beam (1640 vs needed 1097), but I was also curious--do the joist hangers add any lateral strength from the double-shear nails, or do the joists hangers only provide vertical strength?
Thanks for taking the time to read this.
Link to example/figures:
