Soffit versus gable vents – best solution for small cabin?
PercyJ
| Posted in General Discussion on
We have a small cabin (a 32 x 22′ box). It has a simple roof (4/12) with gables on two ends. We replaced the shingles and had a ridge vent installed. At this point, it’s been fully gutted. The ceilings will be 9′ high inside so the attic space isn’t very big. It is mostly a seasonal place – although the attic and walls will be insulated.
Can we just use two gable vents or is soffit venting required?
We think the upside of just doing two gable vents is there are fewer spots to monitor for creatures getting in and hopefully reduced install cost. We have a limited budget and we are trying to do the right thing, but keep costs down.
Any insight would be much appreciated. Thanks!
Replies
https://www.finehomebuilding.com/project-guides/roofing/adequate-roof-venting
You really want vents in the soffit to enable the whole roof to be vented.
When you say the ceilings are 9 ft high, are the walls that high, or are you extending the ceiling along the roof line above the wall to get the extra foot? If the latter, it is even more important to have adequate ventilation of the whole roof deck.
Thanks for the insight. The walls are 7' 9" and we are extending the ceiling along the roof line above the wall.
How many soffit vents are typically put in a 22 x 32 structure? If we are going with 4x12" - would we do 4 in each side?
Thanks again!
Just have to do the math
one thing you do need is room below the roof deck for air for the whole roof, which means some kind of spacer to keep your insulation away from the roof deck in the part where you have the ceiling running with the roof profile. depending on how much room you have reserved for insulation when you put the new 2x4 along the roof rafters, you might want a bit more to have room for the ventilation space and insulation above the slanted part of your new ceiling.
lets look at the numbers.
22 x 32 = 704 square feet
if you air seal the structure (careful vapor barrier inside all the walls/ceiling over insulation and under the wall covering) rule of thumb is 1: 300 vent area to structure footprint (without the air sealing, the rule would be 1:150)
704 / 300 = 2.35 square feet of vent space, split between ridge and soffit
2.35 x 144 = 338 square inches total
half is 169 square inches.
But you probably want to match the effective area of the soffit to match that of the ridge, to avoid a partial vacuum in the attic. (if you do a perfect job of sealing your ceiling, this is not so much a worry, but still a good idea)
ridge vents have a rated net vent area. someplace between 17 and 20 square inches per foot. if you can look up the actual type of ridge vent installed, you can find the rating.
if the ridge vent is 32 feet long, it has 500-600 square inches of effective vent area. (if they cut the right size slot under the roof vent-- a narrower slot would have less vent area)
Since you have the enclosed under rafter space, it is good to consider how the soffit is set up, and how much room there is between rafter bays outside the wall. you have to look at the open space above the soffit to see what this is. this will depend on the overhang and roof rafter size.
you want to look at the NFA (Net free Area) for the specific vents you consider.
Off topic:
Looks like the original ceiling joists are replaced by new joists toe nailed to the ledger on the partition wall, and no continuous connection between the two sides of the building
Where are the ties that stops the roof loads from pushing the walls out?
Exactly right, the new ceiling joists will not function as needed collar ties between the two sides of the building because these joists are not continuous; they are free to pull apart in the middle if and when the snow load causes the ridge to be pushed down, spreading the walls. Even if these joists were continuous, the fact that they are 2 x 4s will have limited the nailing area for attaching them to the roof rafters. Depending on the snow load, the same force that wants to spread the building apart could be sufficient to exceed the shear value of the nails attaching the joists to the rafters. While adding bracing from the middle up to the ridge would seem to help, this also transfers a roof load down through the wall to the floor, which was likely not designed to carry a roof load. I realize that this is just a cabin, and you're on a limited budget, but before you worry too much about insulation, make sure you have the engineering right so after a winter with a big snow load, the cabin will still be standing in Spring.
Thanks for the insight. This really hasn't been a lucky year for us. The area with the cabin will be getting snowed in soon (20-30cm of snow coming on Saturday). We are not planning to do any further work until the spring. We also don't intend to remove the snow on the roof - this year was supposed to be a test to ensure it holds together properly before we finish it.
I think the ceiling joists are 2x6s. The original collar ties are all still in place. The cabin has a pier and beam foundation. The cabin is 22 x 32'. The walls are 7' 9" high and it's about 14' up to the peak.
We have sent a message to the contractor who did the framing - identifying the concern. He has a very good reputation in the area and we checked references before hiring him.
What should we do next? The local lumber yard has structural designers - should we ask them to review it? The contractor/building community in the area is tight-knit and they all talk so this is a bit tricky to navigate.
Thanks for all the assistance.
Thanks for the numbers - I'll look into this further.
A contractor did the framing work. There are joists on both sides tied into the middle. Additional bracing (from middle up to roof peak) was added after this photo was taken.
We do get a considerable amount of snow. We have not finished the cabin any further and will review the situation in the spring.
Specifically, what should we be looking for?
Thanks
Ceiling joists typically are used to tie the rafters and prevent spreading. To do so, they must be a single piece, or be spliced over a supporting structure. For the joists that are butted to the ledger, steel straps, attached top and bottom of the joists, could be used to transfer the load across.
The number of nails required to attach the joist to the rafter depends on roof pitch and the loads on the roof. The local building code will have the nailing schedule.
You didn't say where the location is, but with a heavy snow load, I would expect 8 or more 16d ( 3½") nails at each end and at the splice. If the ceiling joist is not at the top of the wall, additional nails may be required due to the lever effect of attaching higher on the rafter.
An alternate way of supporting the roof is to support the ridge. Converting a ridge board, which is there now, to a ridge beam requires a vertical support every 4 ft. That vertical load has to be transferred to properly sized footings, so all the supporting structure needs to be checked for suitable size and fasteners. This may be what your contractor did when adding the vertical supports.
The existing rafter ties, which appear to be attached to the mid span, are there to minimize rafter bending. They are not effective in preventing spreading.
A link explaining how all of this works. Rafter ties are ¼ way down the page.
https://inspectapedia.com/structure/Roof_Framing.php
Thanks for all the help with this. We have spoken to the contractor and he did put 18" steel plates on the tops of the boards (12 total) with apparently "lots of nails". You can see them in the attached photo.
We are still very perplexed as to why he did the ceiling joists/rafter ties this way rather than overlapping them and running them over the top of the wall. The wall height was changed after the wall was put up (see extra framing above) but that still doesn't explain it. The span on one side is 11' and the other is 9' - that's without accounting for the sloped bit (so even less). Have you ever seen this approach taken?!
We are in Ontario, Canada - I'm not sure how to look up the building code for this type of framing to ensure it's to code.
What should we do next? Does this need to be ripped out and redone?
We don't have plans for more work until the spring so any structural issues/implications will be obvious. The area (just south of Algonquin park) gets lots of snow.
Thanks again.
This is a good solution after the change.
the metal ties that join the ends of the lower ceiling joists (with the lots of nails) should do the trick to keep them together, and keep the roof (and snow above) from moving the outside walls away from each other.
When you changed the wall height mid-job, it made things a bit more difficult. In order to make the new joists overlap, they have to sit on something. The method they used provides a clean joint at the wall.
Each ceiling joist has to be connected , so more of those steel plates need to be installed.
Check that the required number of nails have been installed, and the wood has not split.
Large number of nails in a small area can sometimes be a problem. May be an issue at the splice, as the nails will be close to inline.
The nailing schedule is defined in the Ontario Building Code, available online at the Ontario government Laws site: https://www.ontario.ca/laws/regulation/120332
Part 9 is the prescriptive code for small building. The requirements for rafter ties is paragraph 9.23.13.8. I've attached an extract of that paragraph.
For the Bancroft area, snow load is 2.59kPa.
For 4/12 (1/3) roof pitch, snow load 2 kPa or more, 3" (75mm) nails
For rafters16" OC ( 406mm) 6 nails required
For 24" OC (610 mm) 9 nails required.
For the ceiling joist splice, an additional nail needed.
The nailing schedule is for "the lower ends of the rafters are adequately tied". With the raised ceiling, the number of fasteners should be increased by the ratio of "span at top of wall / span at bottom of ceiling joists' to account for the increased leverage.
The nailing schedule is for the common hand driven nails. If a nail gun was used, the number of nails required is increased by 1/3 due to the smaller diameter of the nails.