Low slope roof ventilation question

Posted by Ron on July 29, 2006, 10:41 am

Hi,

I am re-doing a 20 ft by 10 ft. low slope roof on an enclosed porch that
is attached to the side brick wall of the house.

I removed the old roll roofing and plywood. The 2 x 8 inch roof joists
are 16 inches apart and run up to the brick wall from the roof edge.
The plywood was originally nailed to the joists essentially creating a
near airtight box with no ventilation.

The ends of the joists at the soffit are blocked. At the brick wall the
space between the joists is empty. There is a vapour barrier and mineral
wool insulation in the right place.

I will put down new 5/8 plywood and self-stick roll roofing.

Since there is no existing ventilation I have been trying to come up
with a strategy to get a ventilation flow from the soffit to the
higher portion of the roof.

I am considering drilling holes in the end blocking boards at the soffit
bay and holes in the top end of the joists in a line where they sit on a
"sill" like beam and vent that line of holes to the soffit at the front
and back.

I hope my description is sufficently clear.

I would appreciate any suggestions.

Thanks.

Posted by marson on July 29, 2006, 10:01 pm

Ron wrote:

> Hi,
> I am re-doing a 20 ft by 10 ft. low slope roof on an enclosed porch that
> is attached to the side brick wall of the house.
> I removed the old roll roofing and plywood. The 2 x 8 inch roof joists
> are 16 inches apart and run up to the brick wall from the roof edge.
> The plywood was originally nailed to the joists essentially creating a
> near airtight box with no ventilation.
> The ends of the joists at the soffit are blocked. At the brick wall the
> space between the joists is empty. There is a vapour barrier and mineral
> wool insulation in the right place.
> I will put down new 5/8 plywood and self-stick roll roofing.
> Since there is no existing ventilation I have been trying to come up
> with a strategy to get a ventilation flow from the soffit to the
> higher portion of the roof.
> I am considering drilling holes in the end blocking boards at the soffit
> bay and holes in the top end of the joists in a line where they sit on a
> "sill" like beam and vent that line of holes to the soffit at the front
> and back.
> I hope my description is sufficently clear.
> I would appreciate any suggestions.
> Thanks.

has it been working without ventilation? if it has, i would follow the
motto "if it ain't broke...."

consider skipping ventilation altogether. as long as moist air doesn't
reach this space you will have no problems. insulate carefully, and
then use a carefully detailed vapor barrier and you should be fine.

if you want ventilation, another option would be to use a peice of
cobra ridge vent up against the brick wall, covered with shingles or
metal flashing. cut the plywood back 2" from the wall. it might
require you to install a new flashing against the brick wall.

Posted by No on August 9, 2006, 1:56 pm

Ron wrote:

> Hi,
>
> I am re-doing a 20 ft by 10 ft. low slope roof on an enclosed porch that
> is attached to the side brick wall of the house.
>
> I removed the old roll roofing and plywood. The 2 x 8 inch roof joists
> are 16 inches apart and run up to the brick wall from the roof edge.
> The plywood was originally nailed to the joists essentially creating a
> near airtight box with no ventilation.
>
> The ends of the joists at the soffit are blocked. At the brick wall the
> space between the joists is empty. There is a vapour barrier and mineral
> wool insulation in the right place.
>
> I will put down new 5/8 plywood and self-stick roll roofing.
>
> Since there is no existing ventilation I have been trying to come up
> with a strategy to get a ventilation flow from the soffit to the
> higher portion of the roof.
>
> I am considering drilling holes in the end blocking boards at the soffit
> bay and holes in the top end of the joists in a line where they sit on a
> "sill" like beam and vent that line of holes to the soffit at the front
> and back.
>
> I hope my description is sufficently clear.
>
> I would appreciate any suggestions.
>
> Thanks.

Is the enclosed porch heater and cooled? if not don't worry about
ventilation, its not needed.

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