12 x 16 GARDEN SHED NE TN

[John Raabe]

I'm worried about the loft joists. They look like 2x4 @ 24" with a working span of 11'. On my table @ 24" spacing for a 2x6 joist (not 2x4) will span only 9' to 10' (@ 30 psf load). Think about sistering in another 2x4 if you can.

I also hadn't noticed the notches in the trusses. It complicates things. Can you reinforce those? Will you be sheathing the roof?

[NavyDave]

I planned on using the "lofts" as glorified shelves and not necessarily a space to walk on (except while hanging the roof). I'll sister in another 2X4 though if there may be a safety issue here.

I don't plan on sheathing the roof. I am running the purlins the length of the building in 12 different places to reinforce the roof. My dad and I did talk about re-inforcing the center notches on the top of the building by installing gussets on both sides of each truss.

[John Raabe]

Doubling the 2x4 will stiffen the lofts. You might see some sagging if they carry much load.

Also consider a good sturdy metal connector between the trusses and the top plates. I don't want to suggest what might work in your situation, but you can look at the listings here: http://www.strongtie.com/products/category_list.html You might find something in the DIY projects section.

[NavyDave]

Thanks for the advice John. I thought I might go with adding 2 2x4's to each loft and make them 12" OC in stead of 24" OC. Would this be better than doubling the 2X4s or is it the same thing? Or is doubling better?

Top plate connectors are a good idea, and pretty cheap also......good insurance thanks.

[John Raabe]

12" spacing for the loft joists would work better for the OSB span and while still under sized for a bedroom level load, it's probably fine for light storage use.

[NavyDave]

Worked between the raindrops again today but managed to get the rest of the siding on and cut the openings for all of the doors. I also followed Johns advice and added 2 more joists to each loft to make them 12" OC instead of 24".....huge difference in rigidity!!






12" OC loft joists

[Redoverfarm]

Dave enlighten me.  Why did you comprimise your rafter by cutting 1-1/2" X 3-1/2" notch for the purlins.  Not too much a problem on the side roof but the top section it looks like it would weaken it considerably. 

[NavyDave]

Redover,

The answer to your question in short is that I really wasn't thinking about the depth until it was already cut. In the future if I were to notch rafters i'd oversize them and use 1X material instead of 2X or just not notch them at all and allow a space between the rafter and the roofing. I'm planning on reinforcing the top notches in the same fashion that I did the corners, which should stiffen them up considerably.

[Redoverfarm]

Redover,

The answer to your question in short is that I really wasn't thinking about the depth until it was already cut. In the future if I were to notch rafters i'd oversize them and use 1X material instead of 2X or just not notch them at all and allow a space between the rafter and the roofing. I'm planning on reinforcing the top notches in the same fashion that I did the corners, which should stiffen them up considerably.

I am not sure (might have been mentioned) whether you are going to use  metal roofing or shingles.  For metal top mounted purlins would be OK but shingles not so much.  An alternative would be to split the difference 1/2 dado for the rafter and 1/2 for the purlins.  But you could probably accomplish the same stiffeners by attaching them on the attic side w/o a dado.

Even with the notches it should not be an issue in TN as your snow load would probably not be that great.  I was just curious. Thanks for the reply.  Keep on hammering. 

[NavyDave]

Red,

I'm going with a metal roof. I've heard that if you screw into OSB sheathing and overtorque it even just a little that the screws tend to back out over time. My metal roofing material is 36 inches wide so I wouldn't be able to screw the ends into a solid piece of wood on every sheet with a 24" OC roof. That's why I went with the purlins.

Today I took a couple hours and built a "temporary catwalk" to be able to work on the roof while standing in the loft. It's perfect height for me, I can actually set my chin on the peak of the roof while standing on it. Tomorrow hopefully I can get all of my purlins in and I'm actually gonna invest some time to make some reinforcments for the top notches. Those trusses are pretty strong, I actually hung on one of the peaks today and it held my 170 lbs without any flex at all. I'll definitely sleep better though with some reinforcments installed especially if we get a freak snowfall on the east coast like we did in 2009.


Catwalk is made of 4 16' 2X4s at 12" OC and 2 pieces of 7/16" structural OSB. It is reinforced in 3 places with 3 moveable posts.

[MountainDon]

Dave, I've been mulling this notched rafter thing over in my head for a time, and I have come up with some comments. Please accept this constructively and as an explanation / lesson to future builders reading this today or "tomorrow".

I do not think even notching a rafter for a 1x, top or bottom, is a good idea at any time. If we look at the cross section of an "on edge" length of 1x, 2x, or whatever x, the strongest portion of that member is the upper edge and the lower edge. The middle is not imparting as much strength as the upper and lower edges. Think of an I-joist; a thin plate with a thicker flange top and bottom. Similar to a metal I-beam, the web (the thin part in the middle), simply holds the flanges apart and in place. An engineer would likely have a more concise explanation.

By cutting those notches the rafter is basically reduced to little more than a 2x2.  The purlins inserted across the rafters in the notches don't come close to re-strengthing that section of rafter. The grain of the purlin runs across the notch; wood compresses across the grain much more readily than against the end grain. That is why properly sized floor joists include the minimum bearing area where they rest of a support. Even if the purlin is a very tight fit there will likely be some unwanted movement because of the compression. If/when that happens the purlin will not spring back.

It would probably be safest to double up a rafter2x along side every notch and sheath the roof with OSB. Then apply the weather resistant barrier ("tar" paper) and the metal roofing. As for the screws into OSB issue mentioned my belief that has some possible truth behind it, but we have two metal roofs over OSB here in NM. I have been over them with the proverbial fine tooth comb this spring. No loose screws anywhere. The screws were tightened just enough to flatten the neoprene under the washer "just so". I don't anticipate any problems now after 5 full seasons of exposure on the oldest roof.


EDIT: "...aimed at everyone out there...  I meant to mention that OSB sheathing over the rafters, any rafters on any roof for that matter, builds a strong roof diaphragm that is able to better withstand forces of wind. I believe that many builders who use this metal over purlin technique do so because it saves money. With that in mind I also believe that those same builders them, more often than not use light gauge big box off-the-shelf metal panels that are NOT structurally rated. meaning that under strong wind load they may deform which is not good.

I also try to "look down the road" for "what if's".  Who knows what events may transpire between now and sometime later.  Who knows when someone hires a 250 pound roofer to make some repair or alteration on that roof and they don't look inside and up before climbing a ladder to the roof? 

[MountainDon]

NOTE: some messages have been  split off into a separate topic.That may be found in the general Forum as Building Codes: TX   Part of this one moved, two following responses copied/moved.

[NavyDave]

Don,

I appreciate the time it took to explain the "engineering" behind how the grain of wood compresses and the I-beam comparison really was a good example to illustrate the point. It was definitely taken constructively and it was helpfull in making my decision on how to move forward with my project. Thanks.

I figure at this point there are two options:

1.  Pull down my rafters and reconstruct them without notches with my initial intention of sheathing the roof and living with the occasional loosening screw in 8 to 10 years. OR

2. Accept the fact that I've weakened the trusses and try to re-inforce them in a manner that will restore them to as close as possible to original strength and move forward using the structural sheathing and 30# tar paper that I already have.

I've decided against reconstructing the rafters for the simple facts that: This is not a dwelling but a garden shed AND the snow load where I live is rated at only 10 PSF.

A lesson to anyone who's willing to learn from my mistake:
A funny thing about this situation is that I originally ordered the material for a sheathed roof without purlins. It wasn't until I read/heard of the overtorqued screws backing out of OSB material that I dreamed up a way (in the form of purlins and notches) to prevent this from happening. I plan on making this property a final homestead and want to construct buildings that will withstand the test of time so I assumed with a bit of constructive engineering that I'd be able to keep from having a weaker roof over time. I do believe that with these purlins i've solved the problem of the loosening screws but have compromised some strength of the actual structure as as a cost. An additional cost to include the actual time of notching rafters, constructing purlins and reworking the mistake was approximately 3 days. This time would have been compounded if the structure were a dwelling because I would have opted to rework the rafters from scratch. I'm not at all convinced that this building will fall down because of my mistake but it isn't as strong as it could have been if I hadn't wasted a couple days "fixing" backing out screws.

I've put the shed kind of on hold for the last two days. My father flew in and we have been "wasting time" We did get some reinforcments cut and have the purlins started but have mostly been gardening and uuuuuumm fishing in the pond What can I say the bass have been teasing me! We'll be mixing concrete for my Victoria Cottage piers tomorrow and working on the shed roof while the concrete sets up.

Even with the occasional dumb mistake life can still be good

[Don_P]

I was debating reworking the roof of our shed when we moved it. I just toenailed the roof back on with 4 nails and was going to make a decision that week. It went about 75 yards and was going for a record when a pine tree jumped in the way. I still intend to redo that roof when I get round tuit. We're always learning 

[NavyDave]

OK we are back to work on the shed. We got in all of the purlins and the reinforcments for the top pitch of the roof. We are supposed to get quite a bit of rain in the next 3 days so we will probably only have a chance to get the side reinforcments in and put in the top plate connectors....possibly will frame the gambrel ends too but that depends on the weather. Sheathing is next then we'll tackle the metal!








That 16X20 tarp looks like it was made for this building!

[Bob S.]

It looks to me like you have fixed the problem with the notched roof trusses in a very satisfactory way. It should hold up very well.

[NavyDave]

Bob,

The roof is definitley solid. I contribute most of the rigidity to the new OSB sheathing but the notch reinforcments make me feel better also.


Sheathing and tar paper on, should be seeing green metal up there real soon!

[NavyDave]

Here's the metal! It's Hunter Green. We are probably gonna build the doors next so I can lock 'er up.

[NavyDave]

Got the first door built today.....took WAY longer than I wanted but I got it swinging nicely. Played around with the window trim too.....I wasn't trying to achieve an oriental look but that's kinda what it reminds me of.

[Redoverfarm]

Are you planning to put a door in the gable end for storage access?

[NavyDave]

Yep, gonna put doors in both ends. I think it will be nice when it gets hot to just pin those doors open in the gambrels and let them vent out the heat. Still contemplating on what to do in the gambrel ends though. Not sure if i'm gonna side it with the t-111 or get a little artsy and sheath it with OSB and tar paper and rip the T-111 to make shakes. I think a shake end would look real good and be unique for the area but this project has taken twice as long as it should've already. I have a house to build too! I'll probably take the longer harder route.....I like to do that usually 

[OzarkBrandon]

I have learned that making and fitting doors usually turns out to be much more of a chore than expected, but those turned out fantastic.  Keep up the good work, I've been enjoying your build.

[NavyDave]

I've learned the same thing! They were definitely trying....luckily my dad was here to help me hang them. He's a perfectionist so I'll attribute the great look to him.

Thanks Brandon for the positive words!

[John Raabe]

Nice looking shed. Coming along nicely.

[ColchesterCabin]

Looking awesome!