Calculating largest lean to I can manage

[Whiskeymike]

I have a 30x40 metal shop and I'm looking to add a lean to on each side to add additional space. The roof line at the peak is 16' and I think the outside edges are 12'. I'm trying to figure out how wide I can go from the sides. Originally was thinking 20', but wondering if I could go 25'.

So I pulled up a roofing calculator. I'm in Austin, TX, so I figured a pitch of 3/12 was the best way to go. (is it? is 2/12 acceptable) But I think that comes out to a 5' rise. Which would mean my end of the building would be 7' to get up to the 12' edges. Is that correct?

If that's correct, 25' out would make it a 6' rise and put me down to 6 foot which would not be acceptable.

To determine current height, do I measure to the outside roof line, or the wall?

Beyond the roofline, are there other factors that would limit how wide I can go?

 

[larry_g]

A couple of pictures of the eves would help. Are you looking for a fully enclosed addition or open sided? Do the floors have to be on the same plane? A flat roof you can run infinitely .

lg
no neat sig line

 

[astroracer]

What kind of rafter/truss are you looking at? You have to remember the roof support will impede into your headroom by however tall it is.
You may want to simply "T" into the existing building with a new wing. Would be easier and the cost will be about the same as two lean to's.
mark

 

[Voi]

If I'm understanding correctly it sounds like you want to ad two, single slope lean-to type additions to each eave of your shop?

I just got done with a single slope roof project and from the numbers I ran for our snow loads exceeding 20' would have started to get cost prohibitive. I priced it with beams and purlins, I-joists, 2x12 lumber and even SIPs and 20' always seemed to be the point before costs started to creep up.

I went with 2:12 pitch but did have to pay a bit extra for steel roofing rated for that flat of a pitch.

All of the affordable metal roof options I found were only rated down to 4:12.

 

[MScott]

Depends on where you want to start the top of the lean-to. You can start under the eaves, at the end of the eaves or on the roof itself all the way to the peak. If the latter choice you will have to consider the load on the rafters or trusses.

 

[wyo george]

This is something I'm trying to figure out too, I'll be watching.

 

[hnsy]

Me too!

This is something I'm trying to figure out too, I'll be watching.

 

[NorCA]

Me three.

 

[matt_i]

I think 3:12 is about the lowest one can go with standard asphalt shingles. Below that metal roof will be required. I've seen some red iron buildings that were purported to be 0.5:12, obviously in areas like yours with no snow.

As far as structure design the limiting factor in my mind is the length of the rafter and the increasing expense of getting a single solid piece for each rafter. There can be joints in the rafter but ideally those would be placed over columns (or a transverse column/header system) otherwise you land in a spliced-wooden beam world that adds a lot of extra engineering costs. At some point I think I joists will overtake SPF solid as the most economical structural member but you'd have to do some more detailed study of a lumberyard's catalog to find out more.

I would spend time planning uplift load pathways all the way into the ground. A really big roof can become like a really big wing given the right gusty wind conditions.

Edit: would also be valuable to bird-proof the lean-to, in other words, enclose or eliminate areas they can build their nest under the roof. My framed, roofed, sheathed building was a magnet for birds wanting to nest until I got it closed in.

 

[brownbagg]

put a metal roof on it a go 1:12, use some metal k joist and go 24 feet easy

 

[davejo]

Yes 5/20 is the same as 3/12

no 6/25 isn't the same as 3/12 but 6/24 is

draw your building on graph paper and forget the calculator

I'm interested in the wind uplift aspect as well as I prepare to add a lean-to onto a shop building. I'm going to tie into the building wall under the current eave so I don't have to mess with the original eave components. Mine will be a 1:12 scabbed onto a 4:12

How tall do you need the sides to be, are you wanting to drive vehicles in from the sides?

 

[Orange65]

In metal, .5:12 is used on a lot of buildings- especially in no snow areas. You have to use the right spacing and size of support metal. If it matters to you, might want to draw it to scale to look at the affect this has on the look of your building.

 

[dv8customs]

I built one beside my 30x40 but it is free standing. I just tucked the edge of the steel under the roof of the existing shop so water would continue to drain. My eaves on the shop were 10' and the far side of my lean to is 8'. I used 20' 2x8's so the actual span is about 17'

I ended up closing in the side and rear to keep blowing rain out. It has been up a year with no signs of problems so far.

 

[davejo]

Why did you decide to go freestanding with your shed?

 

[Pathfinders]

dv8 do you have any sag due to the span distance? I am planning a lean to off the gable end of my garage and spanning 14' with 2x8x16

 

[RoscoTom]

Why did you decide to go freestanding with your shed?

I can't speak for him, but I'm pondering a similar lean-to.

Free standing will let me skip a ledger board and necessary wall reinforcement.
My wall is vinyl siding, I'd like minimal alterations to it.
I will attach it to the wall for stability, with weight on posts instead of the wall.

In my case,mine will be 12x24 on the gable end of my attached garage.

Tom

 

[dv8customs]

I kept it separate because I just had the shop built and wanted to avoid any issues in the future if I had a problem with the shop. If I had attached it I am certain the contractor would say any problems were related to the lean to I added.

No issues with sag so far and it has been up just over a year now.