2x4 load question

diyroofer · 03-29-2012, 07:59 PM

Hi all,

I am building a chicken coop and I (read: my wife) wants to put a 'green roof' on it. I have already started building it, not planning on a green roof and I am wondering if the 4 - 2x4's that support the coop will be able to handle the weight of the roof.

I will put up a couple sketchup pictures of the framing I am using for reference. I have never framed anything before as will be readily apparent, but it is a chicken coop so I wasn't too worried, until the whole green roof thing came up.

So, the 2x4 corner posts are attached with two 3.5" bolts (each), face to face to the 2x6 'foundation' which is 4'x10' and resting on leveled brick pavers. The approximate dimensions of the coop structure are 3.5 l x 4 w x 6.5 h. In terms of weight, I have leftover 3/4" plywood that I am going to use for the walls and roof, when that runs out I will switch to OSB. So in all there will be approximately 3 full sheets of 3/4" ply and/or OSB on the structure. The green roof is going to be about 3-4 inches deep and should max out at no more than 400 lbs when wet.

Can it handle the weight?

I was thinking the quickest fix could be to cut the 2x4s at the floor of the coop and put it on 4x4 posts, attaching those to the 2x6's, but only would do that if it is absolutely necessary.

OR do you think I need 4x4 going all the way up to the roof?

Thanks!

Jackofall1 · 03-29-2012, 08:10 PM

If you have it built already just add 2x4's from the bottom of the coup to the base basically making a 4x4. What the worse that can happen, scrabbled eggs?

Mark

abracaboom · 03-30-2012, 02:40 AM

I think your 2x4 corners will be plenty strong; what might collapse is the area in between the posts. I would tell your wife that the whole area around the coop is already going to be "green", so overburdening the poor chicken coop's frame doesn't make much sense.

diyroofer · 03-30-2012, 10:40 AM

Once I applied some physics to my own question, this was the conclusion I came to as well. I am going to add a post on the three sides that have 2x6 under them and add 2x4 angled corner bracing to the corners of the 'fourth side'. I think this should distribute weight sufficiently.

Thanks for the replies! I needed the reassurance that it wasn't going to totally buckle, as I'm really green when it comes to this stuff.

jklingel · 03-30-2012, 01:02 PM

You first need to determine the weight of this roof; sod? You can then look up what a 2x4 of the length you are using will carry. Bear in mind that an 8 or 9' 2x4 will support about 1/3 or 1/4 of what a 2x6 will support, given that each are braced laterally (good sheathing). I see a huge problem with the "cripples" at that window(?) not being full length. Your failure will be there if you don't have a header in there, as far as I can see.

diyroofer · 03-30-2012, 02:11 PM

Thanks for the reply. I've changed designs so many times this sketchup model is really only an outline at this point. What looks like a window is going to be a nest box. Those 'cripples' will be full length but spaced a little farther apart, see paint image below for up to date wall plan.

As I described in a previous post, I also plan on adding a 2x4 post from the floor (directly above the "30 in" in the drawing) to the foundation at the center of this side. The reason the coop is built up on stilts is because it allows the chickens to access the "run" underneath the coop, so I can utilize the entire 4x10 footprint as a run. Also, it makes it much easier to get in and clean, etc. that if it weren't up on stilts.

I figured the roof at about 5x5 max with overhangs. I am planning on an inch of pumice for drainage and roughly 3 inches of soil/potting soil/vermiculite mix. 1 sq ft, 1 inch deep of standard top soil is approximately 4 pounds. Mine should be lighter, but worst case scenario would be 400 pounds. With all the other lumber, water in the soil, etc. my best estimate is that the posts will have to support 600 pounds maximum. The best info I can find says at 8 feet tall a 2x4 wall with 24" spacing can hold 479 pounds per linear foot. The 'walls' of this structure are just under 6.5 at the front and 6 at the back, and 2x4 get stronger as they get shorter. So, by adding the extra posts I will be close to 24" spacing. Accounting for rookie building and calculation errors I divide that by 4 - so 120 pounds per linear foot. The walls are 4 x 3.5 for 15 linear feet total x 120 pounds per linear foot = 1800 pounds. Divided by two because I still don't know what I am doing and I am at 900 pounds. I think it will be able to take it.

Thanks again to all.

jklingel · 03-30-2012, 04:17 PM

I believe this is more accurate for the weight a 2x4 will carry, used as a column. It is a quote from a guy named Robert Riversong, posted on the GBA forum. No one there argued w/ it, and it mimics what I have seen elsewhere. If we were at 479 plf, no houses would stand. "A single SPF#2 9' 2x4 that's restrained laterally by sheathing or blocking can support 1820 lbs and a similar 2x6 can support 7061 lbs without buckling (almost 4 x as much)." Your shed is tiny; 2x4's are fine, but any openings need a small header.