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blackmetal19 03-11-2013 07:16 PM

Roof construction for a shed….Please explain
 
I am currently planning to build a shed this summer and have run into a bit of confusion when it comes to the roof. I have a decent knowledge base when it comes to construction, framing, etc but I have never done a roof before.

After some research I have found that a typical roof should consist of one of these two designs. 1. A ridge board supported by posts going either to the floor or the supporting walls. All rafters would then be running off the ridge board. 2. Rafters connected to each other with collar ties and rafter ties. Another option would be to use Trusses, which from all the ones I have found seem to have some sort of horizontal support that forms a triangle.

My confusion came up when I started looking at shed designs. I went to a few places around town and they all seem to use the same design. I also found similar designs on the internet. From what I have seen none of these designs incorporate rafter or collar ties. They all have an open design to provide more head room. So what keeps the roof from collapsing or pushing out the walls?

Here are a few links that show what I am talking about:
http://www.shedking.net/How_to_build_a_shed_roof.html
http://www.weaverbarns.com/standard-features.php

Thanks in advance,

Matt

sixeightten 03-11-2013 07:30 PM

Those links show a very basic shed. Collar ties can be eliminated if the ridge is properly beefed up. We call this a ridge "beam" instead of a ridge "board". How big of a shed do you have in mind? What pitch of roof are you considering? A lot of lumberyards offer packages including the plans.

Daniel Holzman 03-11-2013 08:18 PM

sixeighteen is correct, the diagrams you looked at are not suitable for heavy roof load (snow). Collar ties do very little except distribute roof uplift due to wind load. Rafter ties on the other hand, which are typically at ceiling height, keep the walls together.

If you incorporate a ridge beam, you need to size it for bending due to vertical load from the roof. The advantage of a ridge beam is that there is no lateral thrust on the walls, so you can use a vaulted ceiling with no rafter ties. Course you need to support the ridge beam with posts.

I built my shed roof with a ridge beam, worked pretty well.

cjohnsonmn 11-18-2013 02:59 PM

I have a follow-up / related question for a slant shed roof (ie half-gable). Why do you have to plumb-cut the top end of the rafters and tie/toe nail them into the ridge beam; why cant you just run the rafters on top of the ridge beam and tie them down with hurricane straps? If its because of twisting - the rafters wanting to fall over - a nailing block between rafters (like is done with floor joists) would seem to take care of that. Just seems easier, so what am I missing? I dont see this mentioned in any books as an option, there must be a reason.

tony.g 11-18-2013 03:37 PM

Probably because the traditional way is the easier way.

If you run the rafters over the top of the ridge board, you would still need to plumb-cut them so that opposing rafters can meet up and are in the same plane. Alternatively, if you overlap them, presumably the ceiling tie connecting the feet of the rafters would be on the twist. And you'd get a raggy-looking ridge unless you cut the overlapping parts off flush, = more work.

Then you would still need to fix the metal ties between the ridge and each rafter. It would look - and be - a mess.

So much easier just to cut them plumb and nail them to each side of the ridge.

cortell 11-18-2013 06:34 PM

Quote:

Originally Posted by cjohnsonmn (Post 1268288)
I have a follow-up / related question for a slant shed roof (ie half-gable). Why do you have to plumb-cut the top end of the rafters and tie/toe nail them into the ridge beam

Woah. I must be missing something. What's this talk of a ridge beam in a lean-to roof? That doesn't make sense to me. A ridge (board or beam) only comes into play in a symetrical roof (gable, gambrel, hip, etc). A lean-to-roof simply has rafters that run from one wall to the other. There effectively is no "ridge".

cjohnsonmn 11-18-2013 06:51 PM

Thanks for the response. I think your points are well taken, but I dont see that its that much extra work. I should have mention that the reason I was considering this method - rafter resting on ridge - is that I'm working alone and its kind of a temporary makeshift shed. Its kind of like the timberframe style common rafter with a ridge beam underneath; or more accurately a stand-alone a lean-to shed.

cjohnsonmn 11-18-2013 06:59 PM

Let me elaborate further. In my lean-to conception, the high end wall is actually an open 12' span between two 12' tall 6x6 posts. (This lean-to is intended to closed off the rear end of a tarp covered "high tunnel" shed I bought and assembeled from a kit with 1-1/4 inch pipe hoops). So across the span I have a double 2x8 "beam" instaed of a wall.

cjohnsonmn 11-18-2013 07:18 PM

A picture might help. This is looking from the back wall toward the front span, and then through the high tunnel shed. The posts and beams are traced in red the rafter in yellow.
https://dl.dropboxusercontent.com/u/.../lean%20to.jpg

cortell 11-18-2013 09:41 PM

Hm. That is interesting. I'm not even sure what to suggest here, but here goes. So, do you need to plumb cut the rafters at the top end? No; you can leave the ends square. Do you need a board (fascia) at those rafter ends? Well, if you don't, what's going to keep the rain from just riding down the rafters back into the structure? The board should be deeper than the rafters, so that rain cannot reach the rafter bottoms (if exposed) or soffit (if covered).

But let's forget the roof for a second. Hard to tell from the pic...what's holding up the 2x8 beams? It looks like it's just fastened to the column side...with nails. Are my eyes playing tricks on me?

hand drive 11-19-2013 08:21 AM

the beam can serve as a wall but with that application the beam the rafters rest on would be a problem if it twisted. make sure the beam does not twist from the load of the rafters

cjohnsonmn 11-19-2013 12:03 PM

Thank you gentlemen. These are all exactly the kinds of considerations and suggestions I was hoping for.
Cortell, that is exactly what you are seeing in the back girt, but these horizontals are only 2x6's nailed in as a temporary brace to hold things square.
In the front girt that you cant see very well, I've inset one 2x8 on the front side of the 6x6 upright posts and fastened with two 1/2" lag screws on each end. To beef it up to beam standards, I plan to add a second 2x8, partially inset on the backside of the uprights (and down a little to allow for the roof slope), and two knee braces.
If I can do it this way, then for the knee braces I thought could use either 6x6's with shoulders or 4x4's (or triple 2x's) through bolted to the beam members. My thinking is that the additions of knee braces will prevent the 2x8 beam members from twisting - as well as reduce the span load force factor.
This enclosure will end up being about 12' by 12', so my rafters will run 16' and think I will probablly need to add another post/beam girt across the middle given our snow load here in Minnesota. (To help with that, I envision the roof material will be a slippery and translucent corrugated polycarbonate)

Thanks again. I will take your advice about the fascia board on the high end.

Curlie 11-22-2013 05:52 AM

I seccond that motion
 
Quote:

Originally Posted by cortell (Post 1268429)
Woah. I must be missing something. What's this talk of a ridge beam in a lean-to roof? That doesn't make sense to me. A ridge (board or beam) only comes into play in a symetrical roof (gable, gambrel, hip, etc). A lean-to-roof simply has rafters that run from one wall to the other. There effectively is no "ridge".

Cortell - I second that emotion! I just joined and I'm searching for some input / experience dealing with framing the end walls of just such a half gable or "simple shed" roof. The ridge beam or ridge board comments seem so out of place.:)

Curlie 11-22-2013 06:49 AM

This discussion still looks alive and well so I'll throw in a couple of comments for cjohnson mn. First - a 16 ft span for rafters can be done with a lightweight roof. Check rafter span tables, but with that light a load and assuming (we know 'bout that word), the tables might, maybe, perhaps indicate 2X8s spaced at 16". That's using the SWAG method (scientific wild ass guess). I'm almost sure that 2x10s spaced at 16" would work - BUT - please be sure to use span tables that consider a "live load" for I believe that your snow load will be quite a factor in MN. Good tables let you consider differing "live loads" and local builders or code enforcement can tell you what your locale requires there. Give careful consideration to roof pitch with that loading from snow (snow that can turn to ice and refreeze in the next precipitation event - that's heavy). - and yes - plumb cut your rafters, at least on the upper end, nail a board horizontally to keep rafter twisting to a minimum, and flash over the top with wide metal onto the roof and over the edge vertically helping to keep water away from rafter ends. An example If roof pitch is 4/12, rafter cuts will be about 15 degrees to vertical. Subtract that from square or 90 degress and you have a 75 degree angle for the flashing bend. The last 1/2" of the flashing in the vertical mode should "kick-out" to help direct drip away even from the fascia. BTW, after you plumb cut the top end for moisture and flashing reasons you may want to cut the lower end to match. A good drip edge along the lower edge of the roof is cheap and may help some.
- The other rafter scenario would be to place another beam, as you suspected, at the mid-point of the 16 ft span and use lighter rafters, remembering those snow loads.
- I know this is lengthy, but one more consideration, since I threw drip edge into the mix is corrosion in a high moisture area. Today's treated lumber formulations have a higher concentration of copper, so when I build sheds, I use a galvanized drip edge along the rake and the bottom "nailer". I buy a roll of the door and window membrane that self-sticks and wrap just enough of the "fly rafter" (treated lumber) and the fascia board (treated lumber) - so that the drip edge is not directly in contact with treated lumber. For this same reason - I do not use treated lumber for "nailers" when using metal roofing. I do not want sheet metal touching treated lumber. The asphaltic membrane sold as window and door flashing comes in handy wherever you do not want metal windows doors, flashing to touch treated wood. I find it economical to buy wide rolls and cut the lengths and widths I need with a razor knife and straight edge. Yeah - quick reply - right.
- If you do span the full 16 ft with rafters, blocking between them at the mid-point will help to unitize the roof. I like to alternate between rafters and offset consecutive blocking by 1-1/2" so I can easily nail through the rafter and into the end grain. A pipe clamp or two as used in woodworking come in very handy where you have to get a little twist out.
- Sorry - don't mean to run on so much, but I've had my coffee quota and not my morning walk yet. All the best with your project!


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