flattest roof pitch for metal roof?

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
i'm finally going to build some storage racks with a roof. very tired of plastic and wet wood even though things have seasoned for two years.

i'm planning to use this product for my roof:
https://www.homedepot.com/p/Metal-Sales-5-ft-Classic-Rib-Steel-Roof-Panel-in-Charcoal-2313117/204254754

i'm going to use a single sloped roof, pitching away from my house. the product cut sheet says 3:12, but that seems unreasonable to me. can i go with something as low as 1:12 and maybe use silicone at the lapped edges between adjacent sheets? would like to save on headroom as i'm building the rack such that it's underneath some windows on my house.

sketch is shown below. i didn't include the deck for the wood to sit on in the sketch, but there is one.
shed.jpg
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
You didn't say how far out that overhang will go from the house unless I missed it.

Generally I don't like to below 3/12. But we have snow.

My only thought would be if you are not going tk follow what the manufacturer says I would at a minimum use ice and water shield instead of tar paper for your underlayment. Flash where it meets the house real good and also use a good bead of roofer grade flex seal or something similar as a bed for your flashing.

I don't think I would use silicone really. I'm not sure its needed. Between ice and water shield on the whole thing, roofing screws with rubber washers, and maybe error a little heavy on the pitch I think you would be ok. I would not do that for a roof on a house but for an open area I think it should be fine.
 
Last edited:

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
i might be mistaken, but i don't think an underlayment is needed for this, right? doesn't the metal sheet just screw directly to the purlins?

for overhang, i plan to hang off the back a few inches, and off the front about 10" before it drops into a gutter. the roof of my house has a 2ft overhang and it's about 12ft above this shed roof level. it's a relatively protected spot. no snow.
 

Bad LP

Minister of Fire
Nov 28, 2014
864
Northern Maine
1:12 is pretty shallow. I have a 3:12 pitch on mine and it works just fine. So well in fact I'll bet a 2:12 would work but the draw back would be my snow load. To save height I only used 2X6 joists.

I don't see any mention of sheathing on your cross section and IMHO any snow load is going to rip that gutter right off. Why such a big overhang?


We posted at the same time. The flatter the roof the more important plywood is going to be.
 
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iron

Minister of Fire
Sep 23, 2015
568
north of seattle
gutter would screw to a fascia board that is secured to the vertical 2x4s shown in the sketch.

no snow.
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
gutter would screw to a fascia board that is secured to the vertical 2x4s shown in the sketch.

no snow.
I assumed....stupid me...that you were going to be using plywood/osb and then an underlayment, and then metal. That's standard building practice where I'm at.

With a 1/12 pitch with no underlayment and plywood I would almost guarantee it will leak.

Silicone does not hang on real well to metal especially when it gets cold. I would not rely on silicone to keep water from going between your sheets of metal.

I think I would pay a little extra, use osb, and ice and water shield, and metal on top.
 

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
hmm.
https://www.englertinc.com/blog/choose-the-right-underlayment-material-for-your-metal-roofing-project/
Structural metal roof panel systems are designed to span structural supports without requiring a structural deck. Hence, they do not typically include roof underlayment since the installation lacks a continuous substrate or deck to support the underlayment material.

http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/26838/sheathing-metal-roof-necessary

sounds like it's optional and mainly used for condensation purposes. if the inside/outside temps are the same, i don't see why it would condense. i know the single piece of poly roof deck i have on one of my stacks has no underlay and it's fine. might be apples and oranges though given the material difference
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
hmm.
https://www.englertinc.com/blog/choose-the-right-underlayment-material-for-your-metal-roofing-project/
Structural metal roof panel systems are designed to span structural supports without requiring a structural deck. Hence, they do not typically include roof underlayment since the installation lacks a continuous substrate or deck to support the underlayment material.

http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/26838/sheathing-metal-roof-necessary

sounds like it's optional and mainly used for condensation purposes. if the inside/outside temps are the same, i don't see why it would condense. i know the single piece of poly roof deck i have on one of my stacks has no underlay and it's fine. might be apples and oranges though given the material difference
Structural metal roof systems are also known as standing seam and they do not require underlayment but every roofer I know including my father uses underlayment.

The link you provided did not look like standing seam metal. It looked like the regular ribbed metal. Which is probably why it says to use with 3/12 or steeper.

Most structural metal (standing seam) is good down to like less then 1/12 if I remember correctly.

I am away from wifi so I'm having a hard time getting your link to pull up again but definately double check that. I almost think you are mixing up 2 different materials but I've been wrong before.
 

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
just read through the install manual (skimmed). it says:
Metal Sales’ panels are designed to be installed over open framing and/or directly over a wood substrate (minimum 5/8”) with 30# felt moisture barrier (or an Ice and Water Shield when required by Local Building Codes).

therefore, i think it's okay to screw directly to the wood without an underlay.
 

mike van

Feeling the Heat
Apr 24, 2013
359
Kent Ct
A 5 ft run on that roof, I sure wouldn't worry about the seams. The 2x4 flat in your sketch does need to go all the way out to where you wrote 'gutter' It's a wood pile, if a small amt. of rain ever got up & around the lap, who cares? Do 1" pitch, it'll work where you are. And no underlay, I agree, purlins on all mine, some 30 years now.
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
just read through the install manual (skimmed). it says:
Metal Sales’ panels are designed to be installed over open framing and/or directly over a wood substrate (minimum 5/8”) with 30# felt moisture barrier (or an Ice and Water Shield when required by Local Building Codes).

therefore, i think it's okay to screw directly to the wood without an underlay.
I just got back home. Now I can see the link again.

If you look at the load calculations they provide you then it shows that if you space your purlins 1.5' OC your max load ends up being 142 pounds if Ia looking at the right product in their load calc sheet.

The way I understood your original question was that you were trying to keep water out.

The layout the guideline of 3/12 in order for water to continue to flow down and not get under the metal. If you do not care about water getting then it should be fine if you don't walk on it.

I don't think you will have a ton of water getting in but I do think you will have some at some point in time.

I was helping with a roof the other day that was about a 2/12 pitch with the classic ribbed metal with 30# paper under it and it was leaking into the restaurant and making a mess. It had been there a couple years but it was still leaking.
 

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
why do you think the 2x4 FLAT needs to cantilever out the full length?
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
Yeah, I'm not following the flat 2x4 needing to cantilever oit either. Typically you would run your rafters one way vertical and cantilevered out and then run your purlins the opposite way flat.
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
image.jpg
 

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
i'm going to use the flat 2x4 to screw to the tops of the columns that support the roof (easier to screw a flat than a vert). then, i'll put a vert 2x4 on top of the flat and run vert purlins between each vert 2x4. these will be at the same height as the vert 2x4s to save roof height. 5ft span. might use joist hangers, probably not since i can sit them on the 2x4 flat which'll have 1" sticking out for a seat.
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
i'm going to use the flat 2x4 to screw to the tops of the columns that support the roof (easier to screw a flat than a vert). then, i'll put a vert 2x4 on top of the flat and run vert purlins between each vert 2x4. these will be at the same height as the vert 2x4s to save roof height. 5ft span. might use joist hangers, probably not since i can sit them on the 2x4 flat which'll have 1" sticking out for a seat.
Got it. That makes sense. That picture is of MY wood shed. I built it that way because I don't care if I get a little water in. My pitch is about 3/12 and I dont get very much water at all coming in. Also it's under a big cedar so it helps keep almost all the snow off.
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
And without snow I can't see the flat 2x's bending that are going to set your purlins on. In a snow area that's a different story!
 

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
it would be a very rare occurrence to accumulate more than 6" of snow here. once a decade at most.

another question: on the uphill end of the metal, do i need a drip edge on the purlin? or, should water drip off the roof before rolling down the underside of the metal panel?
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
Are you not attaching this to your house? It looks like there is a small gap between the house and the wood shed.

If that is the case, I don't know if drip edge will benefit you much. If the $5 it costs isn't a big deal then I would do it just for looks so it's the same all the way around but I can't see it being a benefit.

And quite frankly I didn't use any drip edge on my wood shed at all. I just ran about a 2" over hang on my metal.
 

iron

Minister of Fire
Sep 23, 2015
568
north of seattle
perfect. yes, gap between the house. in the event of an earthquake, i don't want to be supporting an additional 20,000lbs.

i was planning a 2" overhang at the back, so sounds like i should be good to go. thanks
 
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saichele

Minister of Fire
Nov 18, 2005
545
A 1:12 pitch should keep water moving downhill, and as was said, it's still just a woodpile - it a little seeps at a seam in a downpour it's not the end of the world. If you were really worried you could tar the seam. The biggest issue would be to keep leaves and pine needles and crud off it so water doesn't pool on the panels.

And although the snowload issue is not trivial (still just a woodshed...), my pole barn has just metal on 2x4 purlins at 16 inch spacings, and I can walk around on it as a "live" load. Six inches of fluffy Idaho snow shouldn't even be a challenge.
 

Doc C

Minister of Fire
Jul 26, 2017
666
Bonner County Idaho
A 1:12 pitch should keep water moving downhill, and as was said, it's still just a woodpile - it a little seeps at a seam in a downpour it's not the end of the world. If you were really worried you could tar the seam. The biggest issue would be to keep leaves and pine needles and crud off it so water doesn't pool on the panels.

And although the snowload issue is not trivial (still just a woodshed...), my pole barn has just metal on 2x4 purlins at 16 inch spacings, and I can walk around on it as a "live" load. Six inches of fluffy Idaho snow shouldn't even be a challenge.
LOL...fluffy idaho snow!! Been plowing for 3 days and just about got caught up! Almost 2 feet of heavy wet snow!
 

cahaak

Member
Feb 12, 2012
44
MN Twin Cities
It's not a problem using just the 2x4 for purlins on the flat and a 1/12 pitch will work fine for you. This is a metal Ag roof type installation. My woodshed has a very similar roof on it, the panels are 11' long and the storage space is 8'. Actual wood storage dimensions are 8' x 24' and I can stack 6' high. I have my uprights spaced at 8' centers, so effectively 3 individual 8' x 8' storage areas that hold 3 cords each. I have a 2 x 6 support member for the roof bolted on the inside and outside of each upright and then run my purlins across those in four places, so about 3' spacing between the purlins. The purlins are run on the flat edge and span just about 7'6" max. I'm here in MN and we get plenty of snow and there are no load issues are all with this set up. I did place a modest bead of an acrylic latex caulk down the edge of each panel where the overlap occurs. There is no leaking at all. The only issue that I have to deal with is that I have to brush all of the white pine needles off each fall. They stay up on there as it is not steep enough for them to drop. Metal roof Ag panel approach is a very cost effective roof. Mine is approx 27' x 11' and I think the total cost for that was about $200. Panels cost $0.66 / sq ft for Ag steel roofing and will be there long after I am gone.