Roofing decisions - help!

[PlicketyCat]

Alrighty, I've figured out the bones of my 16x24 cabin, now I'm down to its skin and clothing.  The roof is causing me some issues because of the massive cost difference between materials and the utter lack of information for the combo & style I'm using.  Hopefully one of you all can help me out...

We're installing a gambrel roof, rough 2x12 rough rafters 24" oc (which is about double the depth needed to pass the span test, we just need the insulation space). Lower rafters are 75° and upper rafters are 60°... yes, a seriously steep roof on both slopes. There will be a 4' porch on either side wall, with a joining shed roof at 15° with finished 2x4 rafters 24" oc (which pass the span test since it is so short).

We will be using 29ga Norclad metal roof panels since they are made in AK (less $$$) and highly recommended for bush conditions (long life & fire resistant) and DIY. They are considered "ultra-light" from a weight perspective, but "heavy-duty" when it comes to handling 50 lb snow loads and long spans between supporting members. Coupled with a steep roof, they also are excellent for shedding snow and cleaner for a catchment system for whatever rain we might get.

So, the two issues that I'm fretting over are what to use for sheathing and underlayment.  

Sheathing:  All the info I can find says to use 5/8 OSB or 1/2 CDX for sheathing in heavy snow areas... BUT all that information assumes a heavy shingle roof and a moderate pitch. We've got a light roof and a severe pitch. We'll be using 7/16 OSB to sheath the walls, and 7/16 design values work for our spacing if we didn't have the snow load... but isn't there a pitch offset (reduction)for the snow load in our case? I can't find any definitive info about this, although there is a mention... I just can't find the formula.  Again, the 7/16 OSB is manufactured locally and the cost is less than half the 5/8 (especially since, between walls & roof, I would have a full unit and qualify for even more discount). We also won't be walking on the house roof (too steep), but will have a permanent ladder installed to sweep the chimney. We might walk on the porch roof to clean gutters or sweep off snow in the valleys where it joins the house, but 7/16 OSB passes the test for that span & pitch just fine.

Underlayment: Firstly, we have almost no rain (less than 16"/yr) and it is extremely arid, so the only moisture we're even vaguely concerned about is snow melt and some minor vapor condensation from the house-side if we don't get the interior vapor barrier perfect. Secondly, we will be installing the roofing right after installing the paper, so it won't be sitting around for weeks exposed to the elements. We know we need to use a butyl ice shield on the peak, eaves, rake edge, and slope changes; but now have to figure out whether to use 15# or 30# felt on the rest of it. The only real differences between the two is that 15# tears more easily and 15# dries faster. But everywhere I go, I keep seeing that you should use 30# (or more)... all the time... especially if you have a steep roof, but they don't explain why (water repellant properties are the same regardless of weight). Well, my roof is so steep that we certainly won't be walking on it during the roofing process and will be relying on ladders/cleats/scaffolding, and we won't be leaving the roof unfinished for a long period where the wind can get at it, so tearing isn't really that much of a concern (or is it?).  Again, the price of 30# is more than double that of 15# (extra bulk discount since we're wrapping the walls with 15# as well).

So, what do you all think?  Go hardcore and use 5/8 OSB with 30# felt? Or will 7/16 OSB with 15# felt work just fine for our application? Or some combination?

I'm not opposed to spending more to do the job right if it's really necessary for proper performance and safety; but if it just comes down to the mindset that "bigger is better" with no real improvement in performance or safety, then it's not worth the money IMO.

And before anyone mentions it -- we have no building regulations out here, so I can't just go "look at the Code".  The Fairbanks building codes don't mention anything special about sheathing or underlayment over what IRC stipulates, which is where I can't find anything definitive regarding my super-steep metal gambrel roof :(

[DouginUtah]

I have used both 15# and 30# roofing felt and I will never use 15# again.

I would probably use 7/16" OSB with the aluminum clips placed between the rafters.

[PlicketyCat]

Why won't you use 15# agains, Doug? Was it harder to work with? Did you develop leaks? Why do you prefer 30#?

[DouginUtah]

Yes. Harder to install, tears easily. Wind must be calm even with 30#.

(No offense, but roof slope is specified in rise over run, as in 4/12 (often considered minimum) or 12/12, etc.. I don't know what your 60° and 75° calculates out to in roofing terminology.)


I don't know if this will help you but you might have a look at this:
http://www.xmission.com/~sherwin/shed/shed_list.html

[PlicketyCat]

OK - in roofing lingo, the lower rafters (75°) would be roughly 45/12 and the upper rafters (30° -- I mis-spoke earlier) would be 7/12.  A standard 12/12 roof is 45° and 90° is vertical... so, my lower rafters are midway betwwen 12/12 & vertical.

[Radar67]

If it is a normal gambrel roof, 7/12 (7 inches of vertical rise for every 12 inched of horizontal run) on the top and 12/7 (12 inches of vertical rise for ever 7 inched of horizontal run) on the bottom is standard. I have built several sets of trusses for gambrel buildings.

Here is a 16 x 24 I built on my property a few years back.


It uses the above pitches for the roof.

[PlicketyCat]

Hey Radar - that looks awfully familiar  Maybe because I've been modeling, and remodeling, and remodeling my gambrel cabin for months now.

12/7 & 7/12 (60° & 30°) is pretty common for gambrels and that's what I started my design with originally.

Unfortunately, the angle of the ceiling in the loft didn't leave me quite enough headroom at the landing coming up the stairs... I couldn't move the stairs somewhere with more headroom without hitting a structural member or completely messing up the usable floor space.

So, I had two options: add a knee wall to the loft to raise the height of the roof or increase the pitch of the rafters. Increasing the pitch actually made it possible for us to have a little more floor space & headroom upstairs, to add a tie in the upper rafter so we weren't relying completely on gussets, and not be forced to add a bracing wall at the slope change.... and it didn't cost us anything more in materials (bonus!).

So we've ended up with a 12/3.25 & 7/12.

Just can't afford to lose any inches of usable space in a tiny cabin .

Radar - what did you use for sheathing and underlayment? Did you have metal or shingles?

[PlicketyCat]

Here's a snapshot of the truss from my model:

[Radar67]

I used 2x4 slats on 2 foot centers for nailers and corrugated metal. Here is a picture of the shell before the roofing.



I placed the metal straight to the 2x4s since this was just a shed. The exterior is covered with 15# felt. I like the 30# felt, but the 15 works if you nail it properly. At least here it does. I also used a 1 foot knee wall to increase my headroom. If I were to do it again, I would have went 2 foot on the knee wall. I opted for the knee wall because the standard pitches for gambrel roofs is more pleasing aesthetically.

I found a lot of good information on barnplans.com.

[PlicketyCat]

Looks like you have those trusses 4' oc.  Ours are 2' without the purlin/nailers... that's probably overkill, huh? Especially considering our rafters are twice as deep as they need to be for the snow loads?  I couldn't find a calculator to determine rafter loads with spacings other than 16" & 24", so I went with 24.  But hey, if I can halve the number of 2x12 trusses I need and just buy some 2x4 nailer/purlins instead that might save me a bit of cash (and backaches trying to get those heavy trusses up!).

I liked the look of a standard gambrel, but my steeper one actually reminds me of the "barrel" roofs that I used to see in Germany when I was growing up. I always find that knee walls on the second story make the house look funny/stretchy from the gable end... but that's just me haha

[Radar67]

Yes, my trusses (2x6) are 4 foot on center for that shed. We have zero snow load here. My shed is not heated, so I didn't use sheathing or felt for a vapor barrier. Up there in Alaska, I would definitely use a vapor barrier and sheathing on the roof. I would also stick to the 2 foot spacing for the snow loads, plus, you will need the closer spacing when you finish the inside with wood or sheetrock.

[PlicketyCat]

True - snow loads and seismic bracing put a damper what you can and cannot eliminate from your design.  Oh well, putting the trusses 4' oc ended up requiring 3/4" OSB, 5/8" if I used purlins 2' oc, which completely negated any cost savings right there!

So, now I just have to figure out whether I really need the purlins/nailers 2'oc or whether I can do away with them entirely since I will be using structural sheathing and my rafters are close enough together.

Serious, trying to design the most material-efficient and cost-conscious home that is still safe can really be a PITA!  I've rebuilt this model, redone the math, and recalculated material cost and waste so many times I think I might have to shoot myself to end the misery!

[Radar67]

We don't use purlins with sheathing down here. I've been working on a house design for almost 4 years now. I have 4 versions so far, with the possibility of more changes as the process progresses. I have decided that at some point, I am just going to have to jump in and get started. It is inevitable that there will be changes during the build. I have not done a project yet that didn't require changes.

[PlicketyCat]

Yeah, I haven't seen too many houses that used sheathing and purlins together unless the rafters were spaced farther than 2' apart or they were using some seriously heavy roofing (like clay tiles on a 4/12 or less).  I figure with the steepness of the roof slope, the snow won't be laying on the roof long enough to build up too much weight anyway ;)  Worse comes to worse, we can always get up the porch roof and rake the snow off like we had to do with the tent this winter... already have the telescoping-handle snow rake thingy, might as well keep using it .

Hmmm... wondering if the shear strength of the OSB sheathing eliminates the need for diagonal bracing against racking and swaying??

Gawd! Trying to eliminate/reduce time-consuming custom cuts (like notching in braces) and redundant lumber or excessive waste is so frustrating. If the building window up here weren't so short I wouldn't have to worry so much about getting this sucker up as quick as possible with the least amount of finesse... but when you only have about 12 weeks for two people to get the whole kit and kaboodle completed (or at least dried in with a good roof), the last thing you want to do is start out with a design that forces you to spend hours on custom cuts and intricate detailing. Getting the perfect balance between time-consuming carpentry and budget-consuming connectors is driving me crazy.

I'm sure I had hair and sanity before starting this whole thing smiley_dizzy

[Jim_Rogers]

You should also consider, on the porch roof, the snow load from sliding snow off the upper most roof.....
That snow will slide off on a warm day in the sunlight and it will land on the porch roof.... I know you said you have double the load for the porch but you have to consider this added weight as well as the drop of the snow from above. This is what engineers do, so insure that the porch roof can handle this extreme load.....

Somethings to consider....

[1938farmall]

pc,  another reason for the common use of roof purlins is to better hold the roofing screws.  i think you will be very unhappy when you see how little screw holding power you have with the 7/16 osb.  perhaps consider some 1x3 strapping over the osb to screw into if you don't want to use 2x purlins.  you might also use the 1x3's inside the roof 16" o.c. to mount your ceiling material.  wouldn't it be nice to do that first, then fill the 12" space with insulation from above, and then put on the roof?  maybe too much weather risk.  i envy your back-country adventure.  best of luck.  al

[PlicketyCat]

You should also consider, on the porch roof, the snow load from sliding snow off the upper most roof.....
That snow will slide off on a warm day in the sunlight and it will land on the porch roof.... I know you said you have double the load for the porch but you have to consider this added weight as well as the drop of the snow from above. This is what engineers do, so insure that the porch roof can handle this extreme load.....

Somethings to consider....

Good point Jim. I was concerned about that as well, so I factored the porch roof for 100 lbs. We'll be putting a 2x4 porch rafter on either side of each of the 2x12 house rafters so they brace each other (plus blocking & seismic angles). If you trust the calculators, that combo with 7/16 OSB should support 128 lbs live load (which coincidentally is how much I weigh haha). Hubby did his physics thing and calculated the impact force of 50 lbs of snow hitting the porch from an 8 ft fall and it was about 70 lbs... so, if there was already 50 lbs on the roof and another 70 lbs hit it, that's 120 lbs giving us 8 lbs to spare.

We considered increasing the pitch on the porch roof and/or using deeper rafters, but ended up without enough head room on the porch unless we dropped the deck below the house sill beam and made it two steps up into the house.  That might still be an option.

[Larry]

I built one about 20 years ago with standing seam metal...held on by screws and covered with a "U" shaped extrusion over the seam.  Just finished a new one with conventional metal screwed down.  Both houses were 7/16" OSB and 30# felt.  Never had any trouble out of the first one...to soon to tell on the last one.

Most metal manufactures have specs on what they recommend for underlayment.  Might check with your company just to see what they say.

[PlicketyCat]

pc,  another reason for the common use of roof purlins is to better hold the roofing screws.  i think you will be very unhappy when you see how little screw holding power you have with the 7/16 osb.  perhaps consider some 1x3 strapping over the osb to screw into if you don't want to use 2x purlins.  you might also use the 1x3's inside the roof 16" o.c. to mount your ceiling material.  wouldn't it be nice to do that first, then fill the 12" space with insulation from above, and then put on the roof?  maybe too much weather risk.  i envy your back-country adventure.  best of luck.  al

I think adding 2x4 purlins 4' oc, or 1x3 strapping 2' oc, would work fairly well. Putting them closer together would be a little overkill since my house is designed on a 4x4 grid to be a perfect match for sheet goods no matter which direction I nail them. Anything nailers I add will be on the exterior since I'm more worried about the roof blowing off than the ceiling drywall falling in ;)

It would be nice if there was an easy way to assemble the walls and roof so you could insulate conveniently. But since we're blowing in dense pack cellulose (highest affordable R-value we can get our hands on) we need the sheating, the interior vapor barrier and most of the drywall installed before we can do that. The only things we can insulate ahead of time are all the nooks and crannies that the blower can't get to, which we'll be stuffing with fiberglass or foam as we go.

[PlicketyCat]

Norclad recommends 30# felt; but the manufacturing rep that sells and installs them up here says that 30# felt can make your roof rot up here because it doesn't dry fast enough if there is vapor transmission from the inside... which is much more of a concern in our climate that rain. We get some fairly nasty wind gusts here, but not too much on the sustained wind front, and he said that the proper flashings and trim details eliminate that concern once the roof is installed.

::SIGH::  So I'm stuck deciding whether to do it per the product recommended instructions or per the advice an experienced roofer who takes local conditions into account. I wouldn't want to void my warranty, but I don't want to chance damaging my roof because the warrantied recommended install isn't appropriate for my area.  This is the story of my life lately -- do the "right way" for the Lower 48, or do it the right way for Alaska.  Makes me want to drive a fork into my forehead.

[Larry]

You need to check out Building Science.  The principal Dr. Joseph Lstiburek has done a lot of research on moisture control.  Many building codes are based on his findings.

If you are near a library of any size you will find books written by him...and he covers climate issues all over the nation.  I bet even in Alaska.  As your aware there an be extreme differences.

Just last month there was an article in JLC.  An architect designed the house...I bet similar to your climate as it was in the UP of Michigan.  The insulation contractor...speaking from experience...convinced the home owner to change roof venting details.  Unfortunatly the changes caused an ice dam which flooded the house.  Lawsuit time. 

To much money at stake to be taking the word of your experienced roofer...unless he will be around if things go south.

[PlicketyCat]

I've read the Extreme Cold article on Building Science and agree with all the principles described. They agree with almost everything that the Cold Climate Housing Resource Committe (CCHRC) has come up with.  Funny thing about principles is that you can apply more than one solution to them... you don't necessarily have to build *exactly* the way that they built, you just have to address the same problems adequately.  Unfortunately, not all of us have the thousands of extra dollars in our budget that the research folks do.

We'll have a double wall system with staggered studs filled with blown cellulose, so no thermal bridges there. We won't be having the double roof because we simply cannot afford it, so there may be some minor thermal bridging at the rafters but (hopefully) no vapor migration. We'll be using at least 6 mil plastic barrier on the entire inside (warm side) of the house that is uninterrupted across the floor, up the walls and up the roof framings behind the interior finish materials. There are only 3 penetrations in this plastic bag -- 1 in the floor for the combustion air intake, 1 in the floor for the sink drain, and 1 in the roof for the stove pipe... those will all be sealed with proper sealant. No penetrations in the exterior wall for plumbing or electric runs because the tiny bit of plumbing is on an interior wall and all the electrics are going to be face run in chases. The entire outside of the house will be wrapped in felt (uninterrupted across the crawlspace "ceiling" to the roof ridge).  Between these two membranes, weather isn't getting in and vapor isn't getting out... same principle that their system accomplishes, just done differently.

I fully agree that you need to protect the investment in your home and you need to build it so it will be safe and durable. If I had all the money in the world, I would certainly build it closer to the "ideal" model... but, I'll just have to build it the best that I can within my budget, and reserve the big $$ for only those items that give substanstial performance and safety improvements. Sometimes you have to have to do the best you can with what you got.

Now, back to the 15# vs 30# (or two layers of 15#) debate, even the CCRHC agrees that heavy felt doesn't really get you any significant improvements other than it's slightly eaiser to work with, but that the lighter felt's drying ability is a good bonus if you do get condensation and/or ice build up on the underside of your metal roof... especially if you use ice shield on your eaves and valleys (tacked down or applied over the felt - it's too cold for it to stay sticky-stuck to the sheathing in the winter). One thing they do recommend, that we'll be doing, is priming your OSB with waterproofing (especially any cut edges) before installing the felt.

The need for vented roofs, especially over cathedral ceilings or heated loft living spaces, is a nasty debate that has some very valid points on both sides. A lot depends on how much care you take in installing your barriers and insulation. I'm particularly OCD about detailing, much more so than the average contractor for sure. A lot also depends on whether condensation or heat loss is your main concern... vented roofs can be a major source of heat loss unless you can afford to spend a lot of cash on thick rigid foam panels in a double roof system. But it is possible to get an acceptable "cold roof" without the foam and double roof or venting if you apply your barriers and blow in your dense pack cellulose properly (or could afford spray poly urethane in the rafters-- Lord, I wish I had that kind of money!). The important principles to address is stopping vapor from migrating from the warm side out and condensing/freezing on the underside of the roof panels, and minimizing heat transfer from the house to the roof panels melting the snow and causing ice dams... but there is more than one way to skin the cat