Venting a shed roof?

[Drew]

Hi all,

My 18'x24' straw bale lodge is going to have a shed roof.  I know how to vent a peaked roof, but not this.

I am also concerned about water getting into the bale wall.  How should I go about doing this?

Thanks,
Drew

[glenn kangiser]

Are you going to have any space to vent or is the ceiling the bottom of the roof?

...and what are your concerns about the straw bales?  If your roof extends over the tops of them, that is 90% of the battle.  Rain does not bother the sides of straw bales much so your earth plaster should pretty well take care of any moisture problems on the sides.  Be sure you don't get leaks on the top though.  That will make them rot out right away. 

How much overhang are you planning outside of the strawbales?

[Drew]

Thanks for the reply, Glenn.

1.  The ceiling is the bottom of the roof.  There is no loft.  The roof will be 2"x8"s.  I could run a channel through the top bale if necessary, or between the rafters...

2.  My concern is likely based on lack of experience.  A hole from the outside of the house to the inside provides an opportunity for water, yet there must be ways of preventing that.  Aside from the venting, I feel my plastering plan is quite complete and won't allow for any dripping into the wall.

3.  There will be 24" of overhang outside the straw bales.

[glenn kangiser]

I forget if you mentioned or not - Metal roof?- purlins? sheathing? - felt - roll roofing ?

[Drew]

I hadn't mentioned, Glenn.  I've been figuring on asphalt shingles, but would consider corrugated metal if I could get an idea of the cost of using it.  I use Home Depot as an estimating tool then buy the stuff from a local building supply store.  HomeDepot.com never heard of it.

If there's a roofing approach is easier and more appropriate for straw bale, I'm all ears.  d^_^b

[glenn kangiser]

Here are some examples -

http://www.metalroofingwholesalers.com/metal_panel_examples/index.htm

The top one in Fowler, CA I think is about $17 to $20 for factory second - 3'x10' sheet - they can lap.  I actually think this is just first class material they run in 10 foot sheets -- I've never seen anything that made me think they were seconds. 

You would need purlins every 4 feet minimum over your joists - 30 lb or #30 felt vapor barrier under the sheeting on top of the purlins. Corner trim - drip edge flashing at top to keep rain from running down the underside. Rake trim (steel bldg term ) over the sloped ends of the roof above the endwall.  There are self drilling screws available with neoprene seals - you would use the ones for wood that are similar to a wood screw with a 5/16 hex head.

It would pretty well self vent through the ribs.  In addition I assume you will use 6" insulation so there would be a gap there and if you use purlins (running across the joists to fasten the sheeting to) you could make it so they vented at each end through the 1 5/8 gap above the joists if using 2x4 purlins.  You could screen it if you were worried about critters.  It could vent under your rake trim or overhang on the endwalls ( this would be ventilation under the vapor barrier but above your ceiling and insulation.  The ribs above the felt could vent between the felt and bottom of the metal roofing.

[Drew]

Thanks Glenn.  I like this approach.  Much appreciated!

[Drew]

Okay, I'm going through this carefully so I understand you properly.

Our givens -
Roof pitch: 2:12
Building size: 18'x24'
Roof size: 22'x28' (i.e., 2' overhang on each side, not counting the slope).

You would need purlins every 4 feet minimum over your joists - 30 lb or #30 felt vapor barrier under the sheeting on top of the purlins.

The sheeting here refers to the metal roof material, not OSB sheathing, right?  Besides the felt vapor barier, there is nothing else between the purlins and the metal roof material?

Corner trim - drip edge flashing at top to keep rain from running down the underside.

Is the "top" the highest edge of the roof, or are you referring to the top of the roofing material?  I am imagining a long (28' in our case) strip of flashing folded at a 90 degree angle that would fit over the roofing material, the outermost purlin, and at least part of the rafters.  Is this right?

Rake trim (steel bldg term ) over the sloped ends of the roof above the endwall.

Not sure how this is specifically different than the corner trim above - Probably due to my lack of absolute clarity on the meanings of rake trim and endwall.  (Google gets me close, but not close enough.)

It would pretty well self vent through the ribs.

Are the ribs the perlins or the raised parts of the metal roof material?  I'm thinking you are referring to the roof material.

At this point I see purlins that are perpendicular to the joists, which allows for the 1 5/8" venting on the sides like you mentioned.  I'll screen them like you said. 

I had planned to continue the earth plaster from the external and internal walls up to the rafters and tie it in with stucco lathe.  If I do that, it sounds like I need to protect the top of my wall between the rafters since the venting will leave it open tot he outside.  Under a 2' overhang, yes, but still to the outside.  What would you do to the top of that wall?  Merely plaster it?  Lay plastic sheeting on top, then lathe, then plaster (But the water has to go somewhere)?  Or would it be simply to nail some 1/4" plywood to the underside of the rafters over the walls?

You can tell I'm paying attention, right?   

[glenn kangiser]

Okay, I'm going through this carefully so I understand you properly.

Our givens -
Roof pitch: 2:12
Building size: 18'x24'
Roof size: 22'x28' (i.e., 2' overhang on each side, not counting the slope).

You would need purlins every 4 feet minimum over your joists - 30 lb or #30 felt vapor barrier under the sheeting on top of the purlins.

The sheeting here refers to the metal roof material, not OSB sheathing, right?  Besides the felt vapor barier, there is nothing else between the purlins and the metal roof material?

Correct, and thinking about this a bit I think I would decrease them to every 30" allowing a good lap for your felt as it comes in 36" rolls and is a better leak stopper running horizontal than vertical- if your joists are every 2 feet I think you would really only need 1x4 purlins but 2x4 would give you more venting and probably wouldn't break the bank.

Corner trim - drip edge flashing at top to keep rain from running down the underside.

Is the "top" the highest edge of the roof, or are you referring to the top of the roofing material?  I am imagining a long (28' in our case) strip of flashing folded at a 90 degree angle that would fit over the roofing material, the outermost purlin, and at least part of the rafters.  Is this right? 

Correct - You should put a facia board up there too, but make sure it is wider than the tops of the rafters or water will run down it's face then toward the low side of the house and follow the rafter to the strawbales or inside.  There may be other trims available that would suit this better.  This is who I deal with locally -http://www.borga.net/index.aspx  I think I have their catalog here.  Not much online.

Rake trim (steel bldg term ) over the sloped ends of the roof above the endwall.


Not sure how this is specifically different than the corner trim above - Probably due to my lack of absolute clarity on the meanings of rake trim and endwall.  (Google gets me close, but not close enough.)  Rake trim is like corner trim but usually has one leg wider to go farther down the side.
Endwall is the narrow wall with the sloping top.  Rake trim on a steel building connects and flashes the roof to the side wall or can transition to a wood facia.  Sidewalls are the lower usually longer walls on a steel building that are the same height all along the top of them.  With steel siding you have eave trim that usually goes on top the sheets and single leg over the eave purlin to close the sheet rib tops and trim the area,  It would be the horizontal line where the wall meets the roof underside.  It usually comes down the wall about 6 inches and over the sheets about 2 1/2 inches


It would pretty well self vent through the ribs.

Are the ribs the perlins or the raised parts of the metal roof material?  I'm thinking you are referring to the roof material. Correct - the high ribs - usually on 12" centers - some are 6" centers.

At this point I see purlins that are perpendicular to the joists, which allows for the 1 5/8" venting on the sides like you mentioned.  I'll screen them like you said.  Correct.

I had planned to continue the earth plaster from the external and internal walls up to the rafters and tie it in with stucco lathe.  If I do that, it sounds like I need to protect the top of my wall between the rafters since the venting will leave it open tot he outside.  Under a 2' overhang, yes, but still to the outside.  What would you do to the top of that wall?  Merely plaster it?  Lay plastic sheeting on top, then lathe, then plaster (But the water has to go somewhere)?  Or would it be simply to nail some 1/4" plywood to the underside of the rafters over the walls?

I think stucco lath would work great.  You might consider a layer of felt over the bales - plastic could hold moisture - felt would shed it but draw moisture out if it got there somehow.  Blocking at the top outside edge of the wall to terminate the wall -attach lath- seal the rafter cavity,  and maybe drill about 3 -1 inch holes and staple screen to the back also to vent the top better.  Just added easy ideas.

You can tell I'm paying attention, right?   

Good boy Drew, good boy.  pat pat pat 

Others may have other better ideas -- I'm just thionking of things that could work with a metal roof - house combo.  Also -- get the roll mastic tape seal -- was called Tacky Tape -- to seal the sheet edges on the low pitch roof.  You will put stitch screws in the sheet edges every 2 feet also.  Most like to precisely pre-drill holes also.  I like to pre-center punch - does 5 or 6 sheets at a time in a well aligned stack and doesn't leave a hole if you screw up -- on most of the sheets.