rafter sizing

[burr]

Longtime lurker here, just in the initial planning stages of finishing a dream (in a different fashion) started decades ago.  I have a concrete block shell 90% complete, built on a solid footing that measures 30'x15'.  I am planning on a 12/12 metal gable roof on this shell. 

Question is rafter size, nailing to ridge board and top plate on block wall. I have questions on the Maximum Span Calculator for Wood Joists & Rafters provided on this site and do not understand.  Ex. - the spans are designated 'maximum horizontal span' and I am looking for lumber size for the rafter on a 12/12 (not horizontal).  Dead loads will be 10lb, I think, basing on plywood under metal roof.

After all the discussion I have read thru on all subjects, I know I have probably seen something on this but I haven't been able to search it out.  At this point in planning, particulars are not important, just trying for a kinda close BOM for cost estimates. 

   

[MountainDon]

The simple answer is don't worry about the roof pitch. The AWC calculator is going to work when you are using ranges of pitch. When the calculator or rafter sizing table states "rafter span" it means one half the horizontal distance from one side wall across to the other. You have 15 feet, call the rafter span 7'5". It is actually less because of wall thickness, but using 7'5" allows more safety factor.

Look up the snow load for your area. It can usually be found in the IRC, section 3. Some mountainous areas require local info. Use a 20 psf for LL (live load) even if it doesn't snow where you are.

[burr]

Ok, thanks Don, it requires 2x6 for the 7'6" span, no.1 and no.2.  What is the 'select structural' , what the span sits on ?

[MountainDon]

#2 is the grade most carried in the box stores and lumberyards.
Select Structural is the very best grade, more perfect, stronger.

You need a top plate secured flat on top of that block wall. The rafter tails will attach to that and then the rafter ties / ceiling joists span from one sidewall to the other. The same calc can be used to size the ceiling joists... different LL for different purposes.

Finish the rafters off with collar ties in the upper third of the rafter triangle.

[Squirl]

The calculator is a bit more convenient to use.
Here are the IRC sections.
http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec002_par023.htm

You will also have to size the rafter ties. In most houses these also fill the role of ceiling joists. The beginning tables are for ceiling joist spans (rafter ties).
Nailing requirements are at the end in TABLE R802.5.1(9).

The rafters can be toe nailed to the ridge board.

Collar ties must be in the upper third of the rafters.

The ridge board must be the full length of the rafters. Because the rafters are cut at an angle, the ridge board has to be longer than the depth of the rafters.

[burr]

OK, I'm learning to post photos so it's a testy process at this point.  I'll settle for these two to start, just to maybe clarify my plan.  Right now, my main goal is to top off the blockwork, top of block is one coarse higher than this close up photo, 1- photo of many years of neglect and forgotten dreams and 2- after a little initial clean-up.

At the far end, I'm gonna widen that door opening to include the window opening (beside it) to create a garage door wide opening.  The length from a foot this side of that opening will be the enclosed dimensions of the dwelling app 17Lx15W, using the attic space above also. 

[burr]

Top photo is the back door / window that will be garage door placement.  Second shows scaffolding set-up to lay corner blocks.  These window openings on each end will be lots smaller than this original plan shows, probably 2x2.

[burr]

Ive searched the topic of loading block walls; filling the concrete block with runny mix or grout or whatever may be used, and there is limited info on this subject, mostly filling the top to anchor the bolts (top plate).  When originally started, I was gonna load the entire wall height and add vertical steel.  Does this add to the physical integrity of a concrete block wall (8 ft height) ? Is it required or a common practice ? 

I am nearing the completion of blockwork, progress is slow.  I'm  doing this project solo, at least this blockwork and only using one section of scaffolding which is about 6 blocks laid before moving the set-up.  Hoping to get the block topped out and the floor paved before it gets summertime down here.

Input is appreciated, thanks.     

[alex trent]

The best way to add strength to what you have is to run a horizontal concrete column all the way around.Need to build a good form.  Put rebar cage inside.  Tie to the existing wall as best you can by putting rebar inside the blocks and filling with concrete and having the veritical rebar in the blocks come into the concrete cap..  Don't have to put rebar in every block...do like every three or four.  Do you have any rebar that runs up from the footing?  If so, make sure these are tied into the cap even if you have to extend the rebar with a spliced piece.

Corners are critical, so if you can, put rebar cages in those blocks all the way down and fill with concrete.

[Squirl]

I thought I had responded to this, I guess I didn't.

You had a lot of questions.

You can fill the walls with grout, mortar, or concrete. You can also add rebar.  The most common is spacing every 48"  This does add strength, and is common amongst professionals.  I believe it is only required by code in seismic zones. 

For the top course it is common to top the blocks.  That is to block off the interior core and fill the top block with mortar.  People have used newspaper or old mortar bags for years.  That far above grade you probably would not have a problem.  Some reports of foundations have the paper rotting and causing mold.  Then you would place a J bolt (10" - 7" in the mortar 3" out) every 6 ft and bolt a plate to that.  It is easy to nail the rafters to the plate.

http://countryplans.com/smf/index.php?topic=10998.msg147130#msg147130

The horizontal bond beam is another way to finish the top.  It is more common among drystack methods, but can be used on any block wall.  They make special blocks that are U - shaped and sometimes have knockouts for rebar.  You would lay them around as the top row of blocks and fill with rebar and concrete.

http://www.oneontablock.com/block.html

[burr]

Squirl,  you're right, I have a lotta questions, and the more I read this site, the more I question myself and what I thought I already knew (and the more questions Ive got).

OK, thanks, guys, and curiously, I have been pulling the re-bar I had placed every third or fourth block many years ago for loading purposes.  I will probably use these same sticks (only the bottom ends are affected by corrosion).  The footing was well built with plenty steel and first couple courses are tied in with it, and every 6 ft or so.  Corners are more solidly built, as mentioned above.  J-bolts (still stored in the barn) in the loaded block will provide proper attachment to the roof.

These responses are what I wanted to hear.  It makes finishing the blockwork are a little further out but Ive gotta goal.  Ima gonna have to buy me some newer scaffolding;  this rickety old section is as old as the project and this 55 yo is not as agile or light on the feet as he once was...

thanks

I cant move this topic to the owner builder projects and it probably needs to be.  I would re-title as '15x30 basement conversion'

[alex trent]

Sounds like you have a good base on which to build. Best is a poured concrete beam with rebar inside for the wall cap.  Blocks don't come close to cutting it as far as strengthening the wall or providing an anchor for the wall.  The U shaped blocks to hold the poured concrete may be OK but that leaves a narrower beam  and likely less bond to the blocks below.  But, it does get you out of building forms, which can be a pain.

I'd use all the spaces I have between the top course of blocks and lower courses to get concrete down in them when I poured my concrete beam rather than blocking them off to keep the concrete from running inside.  For a few bucks worth of concrete and not real trouble you add a lot of strength if you stick a piece of rebar inside.

That's the way we "retro"-build where we have tremblors s every week.

[Don_P]

These are my state's codes which are pretty stock if the ground doesn't shake frequently.
Start at section 404 here;
://ecodes.biz/ecodes_support/free_resources/Virginia2009/09Residential/PDFs/Chapter%204_Foundations.pdf

And then section 606 here;
Begin where it says professional design not required if you do it this way.

http://ecodes.biz/ecodes_support/free_resources/Virginia2009/09Residential/PDFs/Chapter%206_Wall%20Construction.pdf

Masonry doesn't take kindly to roof thrust, the ceiling or floor joists sit directly on the top of the wall and are tied tight to the rafter feet for best results.

[burr]

DonP,
after looking around 606, I found this typ wall section and it is my idea of roof / wall attachment design.  thanks.
Now if I could only figure out how to copy and paste it here.   

[burr]

dang, I think I did it !  OK, DonP, this is the typ wall section offa sec 606.

[Squirl]

That is exactly it.  It looks like the roof connections require only 4 inches depth, 8ft on center for the J bolts. It might save you $10-$20 over how they are done at foundation connections.