I’m in the process of updating an old garage with new roofing, siding etc.
The garage is 15ft wide by 20ft long with a 6:12 pitch roof.
I live in Vancouver BC so snow load or wind uplift aren’t huge concerns.
It was built a long time ago and is stick framed with rough cut 2×4 rafters (actual dimensions 1.75×3.75”) 24OC.
There is no ridge board. The rafters butt up against one another at the ridge. there is no blocking in between rafter pairs.
2×8 joists/rafter ties are used on every other rafter (originally a total of 4 in the garage but a previous owner cut 2 of them out to make room for a garage door)
New roofing will be asphalt shingles and I plan to insulate the rafter cavities to create a cathedral roof.
I plan to increase the depth of the rafters with either sistered lumber or furring on the underside in order to provide enough cavity space for an air gap and standard 2×4 insulation.
I plan to replace the missing joists with raised rafter ties to provide more clearance for a vehicle. Rafter ties will be positioned in the bottom third of the roof height and I plan to use 2×8 so I can store things on top
Here are my specific questions:
Is there any value sistering new 2×6 lumber to the rafters? Will it actually provide any significant structural improvement given that the bird’s mouth will be cut to match the existing 2×4. The bottom chord of the 2×6 would sit below the top plate. With the current cost of lumber I don’t want to sister every rafter if I don’t have to. Additionally the smaller cavity will require more cutting of the insulation
What fastener schedule/qty/pattern should I use for attaching the raised rafter ties to rafters? Thinking of using GRK RSS structural screws but because of the size of the 2×4 rafters I think I’d be limited to 3 screws on each end.
Is it advisable to try to add a ridge board? It would be a lot of work to cut a slot for a new ridge board and I’d be concerned about making something worse when it has stood up for many many years.
Should I add some type of blocking between the rafters?
thanks I’m advance for any advice
Jon
Replies
I would forget the 2x6 sisters. Use the 2x6’s as collar ties on all rafters attached with plywood gusset’s. Then use the furring strips to help with the insulation.
Good luck,
Brad
Hi Brad
Thanks for your feedback.
It is best to build the the garage walls on the sheathed deck before the wall framing for the main house. here are some tips keep in mind while installing the garage door :-
1.Garage Door Framing and Headroom
Garage doors with torsion springs require 12"s of clearance between top of door opening and ceiling.Doors with regular springs require 10"s of clearance.Both can be reduced to 4 1/2"s with a special kit which will be an added cost. Keep in mind the clearance needed for the door opener, special ones can be needed to purchased.
2.Door Width and Header Size
We've all seen 16' 2 x 12 garage door headers sag, even in a gable wall let alone in a load bearing situation. With the use of micro lam beams, getting the proper size header should be easy. Check with your local building department.
3.Rough Opening for Garage Door
The opening size for roll up and garage doors are different than other types of residential doors. They close up against the rough opening, any other type fits inside the opening. You must decide what type of jamb material is to be used before determining the rough opening for a garage door.
In my area, for 2 x 4 walls with 1/2" sheathing, a 2 x 6 is used for both side and head jambs. This allows SPF #2 lumber to be used and then wrapped with aluminum for a maintenance free finish. When held flush to the inside of the garage wall there will be 1 1/2" sticking past the door opening to butt the exterior finish against.
By the time the jamb is applied the garage door framing size should be the same as as the door. If the door is called a 16' x 7' then the opening should be that size.All garage door headers should have double trimmers (cripples) due to their length and amount of weight bearing on them. Adding up the four trimmers and both side jambs plus the width of the door will give you the header length. For a 16' door the length would be 16' 9"s. The header height using this method is 1 1/2"s higher than the door, from the concrete floor.
4.Garage Door Framing Tips
Install side and head jambs after the wall is in place. Leave side jambs 1/4" off concrete floor to prevent moisture wicking and rot.If using 1 x material for side and head jambs, reduce header length and height accordingly.Never use treated lumber for door jambs, it will rust the steel and eats holes in aluminum.
Did you by any chance read the original post?
"Is there any value sistering new 2×6 lumber to the rafters?"
Yes, it will make the structure stronger. Is it worth it? Well....
"Will it actually provide any significant structural improvement given that the bird’s mouth will be cut to match the existing 2×4. The bottom chord of the 2×6 would sit below the top plate."
It would still improve the structure. Does it need it? Is it worth it? Well...
"What fastener schedule/qty/pattern should I use for attaching the raised rafter ties to rafters? Thinking of using GRK RSS structural screws but because of the size of the 2×4 rafters I think I’d be limited to 3 screws on each end."
Sure.
"Is it advisable to try to add a ridge board?"
Do you need one? How is the roof holding up in regards to racking? Plywood roof sheathing holding it all together well?
"Should I add some type of blocking between the rafters?"
It won't hurt, but with such small rafters and such a small structure there isn't a ton of risk for the rafters to roll.
The question you didn't ask is "How is this going to be ventilated?" which leads to why do you want to insulate it? Is this just storage? Or do you have grander plans to drywall it and use it for something more? Hanging drywall off the rafters might get a little wonky with 2x4s on 24" centers. That doesn't really leave much space for insulation anyway.