My roof covers 2,000 square feet with three louvered, screened gable
vents amounting to 9 square feet or 4.5 square feet of free area.
I'd like to add ridge vents. With a pitch of 6 in 12, I want to be able
to throw a rope across the ridge for safety if I work up there in the
Will a rope crush a ridge vent? The metal ridge vents in my
neighborhood look bent.
I see three kinds for sale:
Air Vent Multi-Pitch Filter (aluminum)
Air Vent Shinge Vent II
Dow Corning Vent-Sure Rigid Roll
So why do you want to add ridge vents? for ventilation? if so, why
don't you use powered thermostatic controlled vent fans inside the
gable vents, that's just what I did, I can't believe the temperature
drop in my attic of 3000 sq ft, from 160+ to now, around 100,
will save on the shingles, keep the insulation from absorbing heat,
moisture control, and save on my compressors run time also put foil
over my insulation leaving some space between for moisture to escape.
But, if it's for another reason, please excuse my interuption, didn't
mean to but in.
Do you run your thermostatic fan in winter? How much power does it use?
Where do you measure the temperature?
I'm about to add ridge vents because they are the most cost-effective
method. They vent the hottest air all along the roof, and that air
tends to sweeps the underside of the decking.
I have a complex hip roof with no soffit vents. I have a north gable
and a west gable. I could put an exhaust fan in the west gable, which
has a single louvered vent. If I ran it, air would enter through the
vents in the north gable. Most of the attic would be dead air. So
ridge vents will be worthwhile if I get an exhaust fan one day. Before
I bought a fan, I'd install soffit vents.
You might not need the fan. How many cfm would it be? A few large soffit
and gable vents can be as effective as continuous vents, with less labor.
You might keep your attic 100 F on an 80 F day with a 1000 cfm fan, moving
about 1000x20 = 20K Btu/h out of the attic, OR use soffit and gable vents
with area A and an 8' height difference, where 20K = 16.6Asqrt(8)20^1.5,
ie A = 4.8 ft^2, with no fan.
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My question was whether it's less expensive to install (say) 2 2'x2' gable
vents than a 2"x48' ridge vent, on an existing house, with soffit vents
in either case. Also, it seems to me that 2 2'x2' soffit vents may be more
cost-effective than continuous soffit vents, with equivalent performance.
If the soffit vents are in the center, air will spread out under the roof.
If not, convection currents through the large free "vent areas" inside
the attic will tend to equalize the air temperature.
With A = 8 ft^2 (the 2 2'x2' vents), 20K = 16.6x8sqrt(8)dT^1.5, so
dT = 14 F, and the air coming out of the gable vents would be about
80+14 = 94 F. With soffit vents near the gables, the air in the middle
of the attic might be dT warmer than the air near the gables, where
10K = 16.6x48ft^2sqrt(8)dT^1.5, so dT = 2.7 F, for a 4/12 roof.
My 100' greenhouses ventilate well with roll-up sides and gable vents.
On an existing house, you remove the cap shingles, saw a slot along the
ridge, and nail on the vent, which costs about $2 a foot.
I understand one reason for venting is to make the roof last longer, by
reducing heat gain from the sun and removing moisture from tiny leaks.
Ridge vents look ideal to me because air would tend to move under the
A heating system may have one big air inlet because as you say, that's
the most cost-effective way to move a given amount of air. It will have
a lot of small outlets because one big outlet wouldn't distribute the
heated air properly. Similarly, it seems to me that several small
soffit vents would be the way to make sure ventilation was distributed
throughout an attic.
Roll-up sides and gable vents sound like a good way to regulate the air
temperature at plant level. If the purpose were to use a small amount
of ventilation to keep condensation from forming, I'd go with a ridge vent.
Having recently installed such a gable-vent fan (15" diam, ~1500cfm)
for an attic of over 1000 sq.ft., with eave vents of a bit over 2
sq.ft. at each end ...
Thermostat that came cabled to fan has ~15 deg. F differential, makes
on rise at setting from ~60 to ~130 deg F IIRC. Mfg suggestion: set to
105 deg F. (Thus switches off at 90 deg. F.)
Set to 105 deg. F, I wouldn't wait around for it to switch on in
Meanwhile, I've a temp/humidity logger sitting in the attic, to see
what's happening over the course of a week. Fan motor is rated to draw
4 a., but it sure doesn't output 1/2 hp. Real_soon_now, a kill-a-watt
will have to go into the circuit to collect some data.
Biggest challenge in installation was in making a transition from 1/2"
ply to connect fan to existing eave vent. Couple hours' cobbling about
got it done. Otherwise, whole upper section of attic end-wall would
have required r&r. Maybe some additional soffit vents later.
When fan runs, a little smoke lets me see gentle airflow across attic.
Not "dead air." I'd ensure that air inlet was as far away as possible
from exhaust fan, for single inlet.
Opening door into attic enables using fan as effective whole-house
vent- maybe a timer-switch for that soon too. For those sunny fall
ridge vents work from stackeffect, just lke a chimney and use no
electricity. They have no motors to go bad and basically no
maintenance. Electric fans pull the attic into a negative pressure
with respect to the house. If there are any openings in the ceiling,
conditioned air is drawn out of the house to help cool the attic. Then
air is drawn into thw house tfrom outside to replace the air that was
drawn into the attic.
Big holes in the ceiling are can lights, chandeliers (there is a
opening around the electrical junction box above the escutcheon),
ceiling fans, Kitchen cabinet bulkheads, Bulkheads in bathrooms, around
wires coming up through walls, around pipes coming up through walls,
bathroom exhaust fans, leaks areoun ceiling grille boots, etc.
I would go with the ridge vents, less problems in the long run, fewer
side effects and no electricity costs.
Yes, you need more ventilation. And it wouldn't suck to add some at the
eaves, also. 6/12? That's a pitch that's starting to get exciting. If
you toss a rope over the ridge, and find that you _need_ it, it'll more
than likely damage the caps and the vent. I can't say which is best,
as I haven't seen these varieties. HTH. Tom
For every square foot of ridge ventillation, I can add 2 square feet in
the soffits (actually 4 square feet of screened vents). It probably
won't bring me up to the 13 square feet that's recommended, but it's
bound to be an improvement.
I weigh 230. At that pitch (26.5 degrees) I would have 206 pounds
perpendicular to the surface and 103 pounds of sheer. With rubber shoes
on clean pavement, the coefficient of friction could be about 1. On
that roof I'd need 0.5. (103/206 or 6/12).
The rope would be in case I step where I don't have quite that much
friction for a moment. If the rope supplied 30 pounds, friction would
have to supply only 73, for a coefficient of 0.35. A little extra pull
can make one more confident working near an eave on a weathered or dirty roof.
A rope can also guarantee I won't have trouble with balance near the
edge. A pull of 30 pounds on a rope running over the ridge would mean
27 pounds on the ridge. I wonder what 27 pounds would damage.
I roofed for 20 years, and 6/12 is one of those "gotcha" pitches. You
get complacent and comfortable, then you're sliding away on loose
granules. When I said _need_it, I meant all your 230 lbs..With some
snatch force! If possible, some roof jacks and 2x6's at the eaves are a
nice way to go. Tom
I've got jacks. Is it important to nail them over rafters? Are the
nails put under shingle tabs? What size nails?
The nighttime temperature has been 75. Next week it may go down to 62.
That suggests a low enough dew point for me to endure working on the
roof during the day. For now I want to do a section five squares,
bounded by two ridges and two hips.
When I get a chance to reshingle the other side of those ridges, I'll
have to pull the nails in the Dow Corning Rigid Roll ridge vent and I
suppose renail in the same holes. Will the decking hold the nails
securely the second time?
Maybe I should nail the first time, then use screws in the nail holes.
I suppose panhead screws would be best. Years ago, I used panhead
screws with rubber washers for galvanized roofing. Should rubber
washers be used against shingles?
It might be a gotcha pitch for a roofer, but for
the rest of us it is scary as hell. No way am I
getting on a 45 degree slant, without having my
feet on something solid, that won't slip. Heck,
anything more than a 1/5 pitch starts making me worry.
The fact that he wrote 6/12, instead of 1/2 is a hint: 6/12 in roofing
terms is not the same as 1/2 slope = 0.5 slope = 45 degree slant = 100%
grade. It is short for 6 in 12, like, 6 inches drop per foot. Steep,
but not crazy. 4/12 is your classic american suburban house. 12/12 is a
45 degree slant. Your "1/5 pitch" would I guess mean 2.4 in 12, which
is nothing much, since 2 in 12 is considered a flat roof in these
parts, requiring membranes or tar and such, rather than shingles.
I find slope, pitch, and grade a little confusing. In math, slope is
rise over run, which is the same as roofing pitch and ground grade.
If a 0.5 slope is 45 degrees, I suppose slope means a fraction of 90
degrees from horizontal. I haven't come across that before.
Years ago a tree turned out to be significantly taller than I'd
estimated. When I cut it down, it hit a fence. That was embarrassing.
To measure tree heights, I made a clinometer from a short board, a small
piece of plywood, a piece of graph paper, and a weighted cord. You
raise the board to sight the slope, wait for the weight to stop
swinging, pinch the cord against the the plywood, and read the rise and
run on the graph paper.
That has shown me why one section of my roof is less treacherous to walk
on than the rest. The pitches look the same, but the latter is 6/12
while the former is 5/12.
Across the street, my neighbor has 10/12. He weighs 350. While
reshingling, he went up to work with no rope or jacks. He caught the
chimney as he hurtled toward the eave.
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