Pole Barn Roof Questions

[freefour]

I want to build a small three sided equipment shed to house my implements. I helped a friend tear down a metal shed last year that got partly damaged (built around 1980). He gave me all the metal and decent 2x6 lumber from it. I been checking with my local coop and finally got some utility poles. I have enough room for a 16'x 40' shed (open on 40' side). I am trying to do this as economical as possible so I am going with a mono slope roof. I got the poles already set on 10' centers, 8 feet high in the back and 11 feet in the front. I live in southern Illinois and we do not get much snow. However, in the 10 years I have lived here we have had a foot once or twice.

What I need some advice on is the roof. My thoughts is to go with 2x10x10' across the front and back of the building (girders?) then stand 2x6x16' rafters every four foot on top of them front to back. I was then going to lay down 2x4x12' purlins every two feet from front to back (overlapping them) from side to side for the metal to lay on. Will that be strong enough? Do I need the girders on the backsides of the poles too?

I have one friend that say he wouldn't go less than every two feet for rafters. That seems like over kill to me but I am not sure. Another buddy says I should build it like his lean to and skip the girders and bolt 2x10x16' from front to back directly to my poles then stand up the 2x4 purlins on top of that. He said he was told that would hold much more weight. Ten feet seem like a long span for a 2x4 but that is how his is built. It had no problems with those foot snows we had.

Anyway, my reason for using the 2x6x16' is because I already have 10 of them. I can use them elsewhere if it isn't strong enough for rafters.

Any thoughts are appreciated.

 

[POC]

I'll try to post some pics from a book I have...

 

[POC]

 

[POC]

This might be helpful too. This is for a 30x12 shed roofed building.
Your roof is not going to be even a 3/12 pitch like the chart above shows though.

 

[ustmd]

Here is a span calculator for rafters:

Maximum Span Calculator for Wood Joists and Rafters

Here there are in a chart:

Rafter Span Tables

I know you are trying to do this inexpensive, I would follow the advice of your friend. Looking at the span calculations you have to space the rafters every 12 inches with an absolute min load to span 16 ft.

I would recommend sizing up to 2 x 8s on 24 inch centers.

 

[freefour]

Thanks everyone. Is it better to put the rafters on top of the girders or should I attach them to the side with joist hangers?

If I stand them up on top, what is the best way to keep them in place? Should I use one of those diamond hangers that has the notch on the top?

Should I have a girder on the front and back of the utility pole?

OR...is there a better, more cost effective way?

 

[blackdog2086]

Cut a birds mouth in the bottom of the rafter and use the storm collars (the bracket in your picture) to hold them in place.

 

[carol palmer]

It is the most simple shed design. Poles are fixed into the ground, secured at top which are then supported by a roof. I guess you can DIY this pole barn. Just ensure few things like getting the right supplies,wood(2x4s and 2x6s to frame structure) and plywood for the walls. Choose the prefered roofing. Some commercial roofing company prefer metal roofing since they are cheap and long lasting. Else if you prefer rust free roofing, go for shingles. Get the other supplies like concrete, gravel, galvanised screws, and the tools for fixing them. Calculate the area, clear it, level it and create a string line. You are ready to go. Best wishes

 

[LD1]

There is no simple solution to your problem.

Lumber is not all equal. There are different grades, and different species. So to say a 2 by whatever can span xx feet on xx" centers.....cannot be done without specifying lumber species and grade.

In the midwest here, shopping at box stores like menards, home depot, carter, lowes, etc......there are really only two species. SYP and SPF. And most commonly in #2 grade....but can sometimes be found in #1.

Then there are varying charts.....that can be found online.....and they all differ. Do they spec L/180 or L/240 design. 5psf or 10psf dead load?

According to illinois.gov....southern illinois should be designed with 20psf snow (live) load. And if going metal......you wont have more than a 5psf dead load. If plywood and shingles.....10psf is the normal.

Also....no chart or calculator will allow for more than a 24" rafter span. Its just not common and not wise to do so.

So with that said....lets look at some options.
What you need is a 20 psf live....5psf dead load roof. With no ceiling underneath...L/180 is common design parameter. All spanning 16'

#2 SPF 2x6 on 12" centers are good up to 16'3" span......
#1's wont gain enough to stretch to 16" centers
#2 SPF 2x8 on 24" centers are good up to 15'7". I'd probably call this good.....since the actual span length is probably gonna be closer to 15'....with a header board on both the inside and outside of the posts.
#2 SYP 2x6 is the same.....12" centers max
#2 SYP 2x8's are actually rated less than SPF......only good for 19'2" centers. #1 SYP 2x8's will get you to 24" centers though.

So for your rafters.......for a 40' building.....you either need 41 2x6's or 21 2x8's. Not sure how many 2x6's you have already.....but 2x8's are usually only a few bucks a board more than 2x6's......no where even close to double the cost. Which they would have to be for it to be a break-even point since you can use half as many of them. But also consider thats half as many hurricane ties (the metal bracket you linked above), its half the labor, half the cuts to notch for the header, half the nails, etc.

Now on to the header (that you refereed to as girder earlier). Headers tables usually spec "allowable" load.....or allowable weight per foot.

Each header is supporting 1/2 of the 16' roof. So you have 8 square feet of roof supported by each lineal foot of header. And we know that the roof is designed for 25 pounds per square foot (20 live/snow + 5 dead load). So...what we know is
*Header must span 10'
*Must be able to support 200 plf....or 2000# total (per 10' section)
A 2-ply 2x10 SYP #2 fits this bill You can put one on the inside of the post and one on the outside of the post.
A single #1 grade SYP 2x12 would also work.

And a final note that I didnt see you make mention of.....is post to header connection. It is highly advisable to notch the tops of the posts to sit the header on, that way the nails, lags, bolts, screws, whatever....are not holding the sheer load. Which would be 2000# at each post. All the fasteners would be required to do is hold the header into the notch. But no shear.

Good luck. And dont skimp on the design. You will regret it and it will cost more money in the long run to make it right. If you build below the minimum.....best case scenerio is the roof sags and just looks like crap. Worst case is obviously failure....and damage to whatever or whoever is inside at the time.