Constructing Roof for Shed

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So this is my first ever shed construction, about 20' x 10' by 8' high at the front, built from 4 x 2 s. Slope is about 1 in 9.5 or 6 degrees.
I plan to use 4 x 2s on the edge for the roof rafters, and sheet with metal cladding. At a 2' spacing, and with a 10 foot span, 4 x2s seem strong enough with not much sagging. I haven't decided yet what to use for the purloins, maybe 2 x 1 1/2 or 3 x 2 which would give better support for walking on the roof if any maintenance needs to be done, plus less likely to be split by the Tek screws.
So some questions:
When I rest the upper top plate on the lower one, it doesn't sit properly because the top plate of the front wall is level (see 4th and 5th photos). Should I worry about that or just clamp everything down and nail? I could perhaps mitre the top of the stud at the back wall so that the plate seats better.
For a 4 x 2 what's the recommended depth of the heel cut of the birds mouth? I don't want to weaken the overhang of the rafters if I cut in too much. I've seen figures of 1/3 or 1/4 of the width of the timber mentioned as a rule of thumb. Other suggestions are to make the seat cut the width of the top plate ( 3 3/4" in my case). I tried this on a test piece of timber and the heel cut is quite short for the slope of the roof, about 3/4 inch.
When attaching the box profile sheets, I'm going to space the purloins about 3 feet apart. So do I need a purloin at the front and back of the shed in line with the top plate, or would the sheets bend down to the level of noggings which would be just above the top plate? (do I need noggings here or just rely on a board nailed on to the overhanging ends of the rafters?)
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Eugbug


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Eugbug wrote:

I never use noggins since I don't know what they are, other than the part of my anatomy that sits on my neck.
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wrote:

For some, e.g. those without period or shift keys, that would be their cheeks.
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On 10/8/2016 8:44 AM, Eugbug wrote:

I would recommend 2x6 for the roof. You might want to hang something from them in the future.
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On 10/8/16 10:39 AM, Leon wrote:

I second that. 2x4 isn't strong enough for that length, especially if you get any snow at all. And especially if you plan on walking on it for maintenance, as you stated.
By the way, I can't tell if there's a moisture barrier above and/or below that concrete slab. But don't be surprised if you get a lot of rot in your sole plates. Perhaps you put a foam sill plate sealer under your walls that isn't visible in the pictures.
As to your cap plate. You should've make the non-sloped wall high enough the rip a bevel along the length of its cap plate to avoid that gap. What's done is done. Just nail it where it's flush. But that leaves you with a wedge shaped section (same shape as that gap) of that cap plate that will extend above the adjacent cap plate. You can belt sand that down flush, or make a bevel cut to take the excess off.
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wrote:

A useful link: <http://www.awc.org/codes-standards/calculators-software/spancalc

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'krw[_6_ Wrote: > ;3633707']

> (http://www.awc.org/codes-standards/calculators-software/spancalc )[color=blue][i]

Ok, thanks, will check out that link.
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On 10/8/2016 10:51 PM, krw wrote:

That's a nice calculator, but it does not take into account the pitch or style of roof. A flatter roof requires more support. A barn style roof, less, as most of it is very steep, might not require 2x6 where a normal pitch might. At any rate, I read the ops original 4 x 2 and went ... whattttt??? you mean a 2x4!!!
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On 10/9/2016 2:38 PM, woodchucker wrote:

Also the op never specified snow load or not.
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I assume you're just doing a simple shed roof, with the rafters spanning from the front wall to the back (no trusses)?
If so, for a 10' span with 24" spacing, you should at least use 2x6 rafters. Even then, depending on your climate (snow loads) and the species of wood, even a 2x6 would be undersized.
You could either switch to 2x6's at 16" spacing, or use 2x8's at 24" spacing.
You can't judge the rafter sag just by standing in the middle. You probably weigh less than 250 pounds. Imagine how much a foot of wet snow weighs "on every square foot" of that roof! If you use 2x4 rafters you may come out one winter to find your roof has collapsed.
Also, if you ever want to insulate your shed, 2x4's won't give you much space for insulation. And as you discovered, 2x4's don't have enough depth to cut bird mouth's.

Use the same size lumber you use for your roof rafters. This will provide support for your metal roofing, and will prevent the rafters from twisting as they dry out. Stagger the blocking so you can nail through the rafter into each solid block, or use joist hangers.

At this point, the way you have it built I would probably skip the top plate and just use metal plates to tie the walls together.

Use deeper rafters (2x8's) and cut the bird mouth the width of the top plates. This will give a flush intersection on the interior and exterior of the wall.
Other issues I noticed:
1. I don't see any jack studs under the door header. You have a doubled king stud on each side, but nothing actually holding up the header?
2. What are you sheathing the shed with? If you're not using plywood, you should add some kind of diagonal bracing let into the studs. This will prevent the shed from racking side to side.
3. The doorway seems narrow? If you are planning to store lawn equipment (wheelbarrows, riding lawnmowers, etc.) you may want a wider door.
Anthony Watson www.watsondiy.com www.mountainsoftware.com
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On Saturday, October 8, 2016 at 9:17:11 AM UTC-5, Eugbug wrote:

As Leon suggests, use 2X6 roof rafters, not only for his reasoning, but cut to fit/accommodate that top plate issue.
I don't see any rough window framing, but I also suppose you're not finished with the framing. I think you gonna want a window or two, even if the opening would be a door-like/wood shutter type opening (no glass).
**Wider than normal doorways really really really accommodate moving larger projects in and out easily!!!
Though it's a small shed, I'd recommend putting a trimmer or jack stud under the doorway header.... *hint, hint.... now's the time to widen the doorway, if you'd like.
Sonny
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I agree with many others...make the door wider, as wide as possible, maybe even double doors or a sliding "barn door".
Ditto on windows. Light is always nice, not to mention ventilation. No idea where you live or what you intend to put in the shed but ventilate in some way.
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Ok, thanks everyone.
We don't get much snow, never more than 6 inches, but usually about 4 inches for a week at most in a year, every 10 years or so.
There's a moisture barrier under the slab. don't know how effective it will be because its below ground level. However I trimmed the edge of the membrane flush with the edge of the slab so it doesn't catch rainfall, and the metal box profile cladding will extend downwards to the bottom of the slab. There's about 6 inches of sub base composed of small broken concrete pieces below the edge of the slab at its perimeter, extending about 4 inches away from the slab, so hopefully this will accommodate any water without flooding. The surrounding ground is a sand and gravel mix with great soakage and never floods. Also I built on top of an existing floor with a strip of about 9 inches width of concrete added around the edges, (membrane lower at the perimeter) so water can't travel inwards and upwards, except by capillary action. I could add a strip of concrete into the trench around the slab which would slope downwards at 45 degrees, diverting water away from it and the damp membrane, and also securing the base edge of the sheets so they can't be peeled off by burglars. There's a 4inch strip of damp proof course (the stuff they use under concrete walls) nailed to the underside of the sill plates with galvanized nails.
As regards the 4 x 2 sizing, my existing 70 year old shed which had a span of about 7 feet, only had 3 x 2s on the flat spaced every 4 feet and there was no issue about walking on it. Granted however, there was a 3 x 2 purloin on the flat half way down the the span and the shed was only about 10 foot long. The roof was made from corrugated iron (the old type stuff which was heavier gauge and perhaps had more structural strength than modern box profile cladding).
I won't be doing any insulating, maybe perhaps add a damp proof membrane under the sheets on the roof. I wonder would I get condensation on this?
What's the idea of jack studs? The header is securely nailed to the king stud, plus I've added another stud adjacent to this also.
The walls will be sheeted with box profile metal cladding which will hopefully prevent racking. The prevailing wind will hit the front long wall so I was thinking about bracing both short end walls with diagonal corner to corner or "V" braces (i.e. braces from top two corners to middle of sill plates). Also would it be a good idea to use short braces from the top of the wall studs to the rafters?
BTW, I think "noggings" is another term for "blocking"
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Building codes are based on "worst case" scenarios. For example, what is the deepest snow you have received in the last 100 years? You might not have any problems for 10-20 years, but when that "big" storm hits your light roof could come crashing down.
Also, rafter span tables are based on the weakest lumber. It's unlikely you will have boards with perfectly straight grain. More than likely it'll have knots, voids, notches, splits, or other imperfections that can weaken the rafter.
The difference in cost between 2x4's and 2x6's will be minimal for a shed that size. Why risk it? Personally, I would go with 2x8's.

The jack studs sit under the header to actually support the weight. That header carries the weight of the structure above, as well as any snow or other loads that may be on the roof. Nails alone are not enough to support a header.
Granted, your shed is a small structure so the loads will be minimal. But again, why take chances? It's just a couple of 2x4's.

I'm not sure what box profile cladding is, but it doesn't sound like a structural panel. If you don't have plywood sheathing nailed every 6" around the perimeter and 12" in the middle of each sheet, you should add diagonal bracing to the studs (cut notches in the studs to let the bracing sit flush).

What part of the world are you in?
You are investing time and money to build your shed. With just a little effort you can build a safer and longer lasting building.
Anthony Watson www.watsondiy.com www.mountainsoftware.com
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On Saturday, October 8, 2016 at 11:48:53 PM UTC-5, HerHusband wrote:

I whole heartedly agree with Anthony. A few simple improvements/modifications would do wonders for your shed and for your peace of mind, if in doubt.

No need for that. What you've done, already, is okay. If still in doubt, I would recommend at least a 16" overhang of your roof, all around, to further keep any suspect rain/water from your foundation, = little cost.
Sonny
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+1
http://www.met.ie/climate-ireland/snowfallanal.pdf

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HerHusband;3633727 Wrote: > - > The jack studs sit under the header to actually support the weight. That > > header carries the weight of the structure above, as well as any snow or > > other loads that may be on the roof. Nails alone are not enough to > support > a header. > - >
In my case, the header has no structural function. It's just a panel to keep burglars out should they pull of the small metal sheet above the door.
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On 10/10/16 8:48 AM, Eugbug wrote:

There are easier ways to break into that shed, believe me. :-)
I hate to keep bringing stuff up, but you asked. That header *is* doing something structural. It's taking the weight of the roof over the opening and transferring it to the studs and down to the foundations.
I realize this is just a shed so it'll probably be just fine. But I'm looking at your pictures and just realized how far your stud spacing is. If that structure is indeed 20' long, then those studs are 32" apart, twice as far as they should be. It looks tome like the building is actually about 17' long and studs are 24" apart giving you a 30-32" door.
In either case, those studs are undersized. But as I said, it's a shed, not a home. I do, now recommend some diagonal bracing on all the walls. The minimal would be a half sheet of 1/2' plywood on all 8 corner wall ends. You could also put 2x4 diagonals on the insides of the walls, similar to the bracing you show in your pictures.
Just a suggestion.
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On Monday, October 10, 2016 at 12:18:55 PM UTC-4, -MIKE- wrote:

I just don't understand why you keep suggesting sturdier construction techniques. I hopped into my time machine, took a quick trip into the future to check out his shed and everything is fine. See:
http://www.buellinspections.com/wp-content/uploads/2014/04/sway_back2.jpg
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DerbyDad03;3634339 Wrote: > -

LOL!, looks a little like my other ship lapped shed (or what it will like if I don't sort it out!)
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