shed roof room addition

[edwinc]

i am adding a 14' x 12' room with a shed roof to my cabin. i am cementing 2- 6" x 6" pressure treated posts 48" deep 14' apart 12' from the cabin wall. i am using 3- 2" x 10" x 14' with 1/2" cdx between each pair of 2" x10" glued and nailed together for a beam on top of the posts. i am using 2" x 8" x 12' roof rafters 16" oc. the pitch is 21" for the twelve foot run. i will roof with 1/2" cdx plywood and standard shingles. i will stick build the 3 walls, cover with 1/2" cdx and i plan on putting in large windows in each. there will be 2" x 8" lumber nailed around the outside of the beams at floor height and on the two sides at beam height

questions:

1. is the beam strong enough for the load? would 3- 2" x 8"s be strong enough?

2. are the 2" x 8" rafters strong enough. the cabin is above the 45th parallel and gets lots of snow. a similar room i built on the cabin 29 years ago with the same pitch and a 10' run using 2" x 6" rafters 16" oc has not given me any problems.

3. how many 3.5" nails do i need use on each end to toenail in the 2' x 8" rafters. they will be attached to a 2" x 8" x 14' nailed to the wall studs of the cabin and to the face of the beam.

4. how many nails do i need to use to secure the beam together? could i use dry wall screws instead?

5. would using a rafter hanger with adjustable bottom be better than toenailing? do you know if h depot or lowes carries them?

5. are there any possible problems in this construction that i am missing?

[Don_P]

1&2.. where are you, or more importantly, what is the snow load?
3. if you're attaching a heavily loaded rafter to the beam face, not typical, it does need a hanger rated for the load calculated from the info above
4. drywall screws or most screws are brittle and are not for structural applications, they snap under load. Best practice is nail every 16" along each side staggered top to bottom, a pair at each end.
5. yes
5b. oh yeah...It sounds like you have a beam up high to support the roof and no floor beam, unknown attachment of roof, inadequate pitch, unknown/inadequate footing. We don't know if the existing cabin is up to sharing the load. This won't work safely as described. Needs to be though out better with quantified knowledge of local conditions.

[edwinc]

the snow load is 60 in northern michigan. the foundation depth is 42", the pole barn depth is 48". the cabin is on a floating slab and 2" x 4" walls. the floor is attached to a 2" x 10" attached to the floating slab with lag bolts. the 12' floor joists are attached to a double 2" x 10" across the outside 14' span. the ground here is a mixture of small rocks, stones and course sand. the ground is dry with excellent drainage and no clay. the top of the beam is approximately 9 foot off the ground.

i am aware that the pitch is low but it is all i can fit in. the roof on the back screened in porch has the same pitch and it has held for 29 years with the original shingles. i am replacing the shingles on that roof this summer.

thanks edwinc

[Don_P]

This is set up for what I believe your rafters/loads to be, you can adjust the inputs as needed or confirm it;

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Spruce-Pine-Fir&size=2x8&grade=No.+2&member=Rafters+%28Snow+Load%29&deflectionlimit=L%2F240&spacing=16&wet=No&incised=No&liveload=20&snowload=60&deadload=10&submit=Calculate+Maximum+Horizontal+Span

The rest will take a bit..

[edwinc]

the horizontal span is actually at the maximum span of 11' 5". do you think i need to go to 2" x 10" roof rafters to be safe. the cost increase is minimal, but it adds weight to the roof.

i was planning to sandwiching 1/2" cdx between the 3- 2" x 10"s for the 14' beam. i have read that since the i have to use an 8' and 6' piece that it will not add any strength to the beam, even glued and may even weaken it. do you know if that is true? i know how to feather the ends to overlap them 5" or 6" before gluing them, i did this on a project building an eighteen foot kayak. would this strenghten it enough? i am using the spacers to make the beam the same width as the post. i don't know if i could get a 5/4" board 14' long to make up the inch. i don't want the beam to be wider than the post either. any suggestions?

thanks edwin

[Don_P]

You're at the limit for #2 SPF, its up to you, a better grade, or stronger species, or deeper lumber, certainly won't hurt anything. It sounds like you're fighting a height problem so it all factors in.

If you check the foundation section and the girder table in the codebook you're into engineer territory. An engineer asks the question "how would you like to frame it?" I checked the spans and loads and would ask him to see if this would work.
http://i180.photobucket.com/albums/x109/windyhilll/piernbeam2.jpg
There need to be 3 posts, notched 3" deep to accept a double 2x12 girder. Apply the first 2x12x14 across the posts, nail the joists, apply the second ply of the girder, nail it well and then apply the 2x14 ledger snug under the joists, nailed well. You can use hangers there also.

Subfloor notched around posts.

There is not a need for a beam up top. Walls could be framed in between the posts with them cut off at top of wall height, double top plates across top of studs and posts. The rafters sit on the wall with a birdsmouth and tail as normal. The windows then get headers in the frame wall. There is nothing wrong with an upper notched in beam and let it tie it together and act as the header.

[edwinc]

i was planning on installing a 9' window in the 14' wall. if i cut off the beam at wall height, notch it 3", nail in 2- 2" x 10"s across the span, and frame the two 2' wall sections on each side of the window with double 2" x 6"s next to the opening, will the 2- 2" x 10"s be strong enough to act as a header for the 9' window?

i did not want to notch the posts at floor level because i thought that it would weaken the post too much, but i can see that it is the 2" x 12"s that will support the roof through a supporting wall. am i supposed to nail the joists through the ends? if so, do i angle them in and how many per joist end?

do you see any problem with my attachment of the floor or roof to the cabin wall. i could add cemented posts next to the wall for added support, if you think it is needed.

thanks for the help
edwinc

[Don_P]

If you cut off the midpost just under the subfloor only keeping the corners full height and run a 2-2x12 with wall framing, the 9' span with #2 SPF works for me, double 2x10 failed. The load dropped although its still alot of snow but the span increased. The narrower you can get that window would help the fastest. A double 2x10 runs out about 8'6".

The worst case scenario I can make is about 1000 lbs on each joist end area, but the girder is actually taking most of the roof load. If there were about 10 nails end nailed through the girder 1st ply and ledger in that area you wouldn't be wrong. Then the plies of the girder need to be well attached to each other. You could also look at the specs on Simpson hangers; strongtie.com and use them.

You could bolt 4x6's to the posts to carry the girder and avoid notching them. You are not allowed to carry the girder on bolts.

It wouldn't be wrong to make the thing self supporting and just bolted onto the cabin to provide lateral alignment, with all vertical loads bearing on a fresh set of posts. Otherwise you need to satisfy yourself that the existing work is up to the job.

I missed one of your earlier questions. I've seen the thickness of the plywood plies that are running along the length of the beam used while calculating the moment of inertia in the beam, not using the thickness of the crossbands. I don't count it for anything.

[edwinc]

i can easily reduce the width of the window to 8'. i could cement in the added 4" x 6" post after bolting to the original but that would move the floor girder out 3" and increase the the horizontal span. i think i will go with your suggestion of notching the corner posts for the 2" x 12" girders and framing the wall for roof support with double 2" x 10"s as header. you discussed cutting off the middle post at subfloor level. i was not planning on a middle post. do i need one to support the load on the double 2" x 12" girders? the only difficulty with installing posts on the property is that the gound is extremely hard to dig. it takes 2 to 3 hours to dig a 1' x 4' hole.

it is difficult for me to judge if cabin will support the load. the cabin is solidly built. i was planning on running a pair of 2" x 10" verticals between the upper 2" x 10" and the 2" x 12" bolted to the foundation. i feel all of this will work, my only concern is that this is a floating slab. i am bolting it to the 12" x 12" part of the foundation that is 16' along the edge. when i had it poured i had the standard foundation mix increased by 2 bags of cement per yard with rebar. it is extremely stong and is 30 years old. do you think i need the extra two corner post?

i wish to thank you again for you advice.
edwin

since the above i decided to just get it over with and i dug the two post holes for the supports next to the cabin the posts will be cut off just below subfloor. i am going to dig the outside wall middle post hole, tommorow. i am going to use a 2" x 12" bolted to the foundation, then i will add a second nailed to the first. i will then support the two ends with a notched 6" x 6" nailed to the double 2' x 12's and then cemented in. i am going to use a single 2" x 10" nailed to the cabin upper wall to take the rafter hangers. i don't see any advantage to doubling this. i am still going with the 2" x 8" rafters and this allow me to nail a 2" support below the rafter hangers. i am nailing a 2" x 8" or 2" x 10" against the cabin between the upper 2" x 10" and the coner posts to help transfer load to the floor. did i understand you, that i need the joist and rafter hangers all need to be rated for 1000lb?