question on roof framing

[dug]

Trying to work out some details (they seem endless) and I have a question about framing lookouts. I plan about 18 in. overhang at the gable eves and have seen several methods for doing this. I am thinking about doing it this way-



It seems fairly simple and strong. I would extend the ridge board to the ends and use gable walls for support instead of the lookout rafter they used. Is this a good method or is there reasons for doing it a different way that I am not thinking of?

Also- 18 in. overhang looks good on paper, but would it make more sense to go with 2 ft. as far as working it out with the sheathing?

Thanks!

[dug]

instead of the lookout rafter they used.

sorry- gable truss, not lookout rafter

[MountainDon]

That looks good.

As for the amount of overhang, what are you planning on using for the soffit material? Can it be ordered in a size you can use at 18". If not how much does the waste add up to?

I calculated our eve overhang by the amount of shading I wanted on the wall at the peak of our hot weather. That created a small issue when it came to enclosing the area under the eves. Standard cement board soffit material was not quite wide enough. So if you calculate and measure carefully you could avoid that trap. On the other hand it was not impossible, it was just a little harder, more steps.

[Redoverfarm]

Dug I would undersize them some.  Say if you were going with 24" go with 23-7/8".  CAUSE if you you cut 3 pieces off at 24" you will be short by 3/8 of an inch on your last piece.  Remember that the saw kerf should be included in your measurment.  Depending on how you trim it out you may get by with the short piece if using a trim board to hide.

[dug]

if you you cut 3 pieces off at 24" you will be short by 3/8 of an inch on your last piece. 

Sound advice. It's easy to forget that sawdust flying is part of the board, that is until you measure the last piece!

Do you think 24 in. overhang would look OK on 20 by 30? I thought that usually you matched the overhangs on all sides and with a 12/ 12 roof I was thinking that might be a lot on the long sides. It's easy to see what looks good in a photo but hard to take measurements from them!

[davidj]

Do you think 24 in. overhang would look OK on 20 by 30? I thought that usually you matched the overhangs on all sides and with a 12/ 12 roof I was thinking that might be a lot on the long sides. It's easy to see what looks good in a photo but hard to take measurements from them!

I went 18" overhang all around on our 20 by 30 and it looks good.  24" would probably work at the ends but would lose you a lot of light down the sides with a 12/12 roof.  Of course, with an 18" overhang you end up with something like 16'3" x 33' sections of roof, which is a nightmare from the point of view of saving plywood.

As an inexperienced framer, having done the two types of lookout on my shed and garage (dropped rafter and vertical lookouts vs. notches and horizontal lookouts), I think the notched lookouts are easier if you aren't using manufactured trusses.  With the dropped end rafter I found that things all seemed to end up 1/4" out even when the math was right.  On the cabin I'm doing the notches/flat lookouts (using 2x4s), but with 2x6 compression blocking under the 2x4, so you end up with a T shape lookout under the eaves.  This seems to be strong but without the issues of the dropped rafter.  That being said, I ate a lot of sawdust cutting the 28 notches!

[rwanders]

Overhangs for the rakes and eaves are mostly whatever looks good to your eye but, it also may have a utilitarian aspect.

I have very wide eaves on my 12/12 roof to handle the snow coming off the large area of a 24x34 cabin. The eaves extend four feet (5 ' on the run) from the sides to leave four feet on each side clear of snow or rain. We get a lot of snow and I didn't want it to pile up against the cabin. The overhangs at the ends is only 12" since snow buildup is not such a problem there and it looks good even though it doesn't match the eaves.

This kind of design may not be advisable if you are subject to high winds---it could result in a lot of uplift and your roof may tend to sail away.

[Jens]

if you are running a boxed soffit, you may think about adding a sub-fascia of 1x6 or 2x6 material.  On the gable ends, run blocks between the lookouts that are offset to the outside of the gable wall so that they act as nailers for your soffit.  I would run the dropped gable option if you are in a heavy snow area.  To make it easier to frame up the drop, use two of your common rafters as guides.  Set them at the end of where your gable wall will be, and build the wall beneath them.  Now you have it dropped the correct amount without worrying about the math.  of course, mistakes do still happen.  IMO, 24" overhang is better, but like already said, with a 12/12 roof...

[bayview]

   Hope this picture is of help - (one picture=one thousand words?)

   I ran the gable ends studs at 16" on center.  Then the lookouts at 16" on center.  I used Simpson hurricane ties on each lookout.  May have been overkill, but I was glad I did it.  The shed took on a tornado with over 100 mph wind and survived. 

   The gable end "truss" was 3 1/2" lower than the adjoining truss.  So the lookouts fit on top of the gable end.  Gable end was tied to wall with Simpson hurricane straps.

   I went with a 12" overhang, so I could use the hardi-plank soffit material.  I used 1X6" Hardi-plank for the fascia board.

   Roof trusses on the shed were 16" on center.  I had built our garage roof with 24" on center with 1/2" exterior plywood.  I felt like I was going to fall through the roof.  16" on center was perfect.  Very solid!

[Redoverfarm]

  Hope this picture is of help - (one picture=one thousand words?)

  I ran the gable ends studs at 16" on center.  Then the lookouts at 16" on center.  I used Simpson hurricane ties on each lookout.  May have been overkill, but I was glad I did it.  The shed took on a tornado with over 100 mph wind and survived.  

  The gable end "truss" was 3 1/2" lower than the adjoining truss.  So the lookouts fit on top of the gable end.  Gable end was tied to wall with Simpson hurricane straps.

  I went with a 12" overhang, so I could use the hardi-plank soffit material.  I used 1X6" Hardi-plank for the fascia board.

  Roof trusses on the shed were 16" on center.  I had built our garage roof with 24" on center with 1/2" exterior plywood.  I felt like I was going to fall through the roof.  16" on center was perfect.  Very solid!

I seem to be more conventional in that I have bever likes 24"OC and if possible I tend to go the extra expense of 16"OC.  I am not sure when the 24" Ctr came on board but I think they may have just pushed the envelope a little far in my opinion.  Maybe someone who is a little more familar with the building industry might shed some light.  Was it with the inception of OSB?

[dug]

(one picture=one thousand words?)

At least!

Thanks everyone for the help. I think I may yet change my mind and go with 16 in. spacing on the rafters.

Nice project davidj! Any updates or are you snowed in for the winter? I may have a question or 2 for you later if you don't mind.

[Jens]

24" oc roof framing has been going for over a hundred years, of course back then they also used 3/4" board sheeting, 2x4 rafters for big spans, and much of the time no ridge boards or collar ties.  No hurricane clips or earthquake gear, and many of them are still standing too.  Makes me wonder about all the modern stuff.  Had better lumber too though.  1/2" on 24's gets pretty spongy, even worse when OSB instead of plywood. 

[Don_P]

Not of any real use but John's comment got my wheel's turning,
There was a program in the trade journals called "Mod 24" in the mid '70's using 2x6's on 2' centers, this came out of the embargo. I remember ordering the literature out of "Professional Builder". 24" framing is not too uncommon in old work though, sawn sheathings were more forgiving. OSB came out in the '80's.

This is from my oldest codebook, I'm not calling this code now, this section is in my '85 ammended '68 and sounds original from the '58;
"Studs shall be not less than 2x4's, wide face perpendicular to wall, spaced not more than 16" on center for plaster finish or 24" o.c. for wood ceilinged or 3/8" laminated wood finish or 1/2" gypsum wallboard in one story dwellings or 16" o.c. for first floor wall framing in 1-1/2 or 2 story dwellings and 24" o.c. for second floor wall framing supporting roof and ceiling only."
It goes on to state that 2x6 studs on 24's can be used in 2 story if everything stacks.
Plaster walls and ceilings needed more support than modern sheetgoods. Many old houses did not have any plaster though, beadboard or panelling was very common. I lived in one house that had no spacing "rule". They had framed corners and openings and then just divided up what was left, often by eye it seemed. Sheathings were rough boards so trying to exactly fit a 4' sheet of something wasn't a concern.

Balloon framing is considered the first shift from heavy timber framing to light framing. In colonial Virginia carpenters had used similar techniques to quickly build shelter.

Plank houses, bargeboard, jenny lind, whatever you want to call them, had no wall framing, just a couple of layers of boards, this could also be tracked back into timberframing times where the planks were heavy 3+" thick or half logs. Frank Lloyd Wright designed and built several.

[Jens]

I don't know about the comment that most old houses didn't have plaster on the ceilings, as I have almost never seen one without plaster.  16"oc joists would have been more effective at keeping plaster ceilings from sagging though.  I don't understand why walls were framed on 16's (or so) and ceilings at 24's (with 2x4's most of the time).  Interesting how 24's could be used with 2x6's, but not 2x4's, as there isn't too much difference in their load in that direction (as far as I know).

[John Raabe]

OVE or "Optimum Value Engineering" involves lumber and energy saving construction techniques that set the framing module at 24" o/c and use other techniques such as single top plates to reduce lumber and increase savings. It is a less forgiving building system and requires better planning and layout to fully realize the full savings. Often done in production home building using a standard optimized floor plan.

May require upgrades to sheathings and interior finishes because of the wider spans. Not as naturally strong against wind and earthquake loads. See more at link below:

http://www.toolbase.org/pdf/techinv/oveadvancedframingtechniques_techspec.pdf

[Don_P]

 

Interesting how 24's could be used with 2x6's, but not 2x4's, as there isn't too much difference in their load in that direction (as far as I know).

I'm curious from a structural point of view, let's see.
Assume a clearspan 20' wide 2 story building and frame it with 2x4 walls on 16 & 24" centers. I'll use 40 psf floor and 35 psf roof, snow varies that number alot.

16" Framing
Wall-10 psf x 8' tall x 1.33' (16")=106.4 lbs
Floor- 40psf x 10' wide x 1.33'= 532 lbs
Roof- 35psf x 10' wide x 1.33'= 465.5 lbs
Total load/stud @ 16" oc = 1104 lbs

24" Framing
Wall- 10psf x 8' tall x 2' = 160 lbs
Floor-40psf x 10' wide x 2'= 800 lbs
Roof- 35psf x 10' wide x 2'= 700 lbs
Total load/stud @ 24"oc = 1660 lbs

A stud is checked as a column. The 1-1/2" direction is fully braced by sheathing so the buckling plane is the 3-1/2" direction. This is a column calculator;
http://www.windyhilllogworks.com/Calcs/columncalc.htm
Inputs I tried;
Length 96"
Depth 3.5"
Width 1.5"
Fc- 525
E- 1
Load- try it with 1104 and 1660 (16" and 24" framing loads from above)

That was not a great stud, I could make it pass with a good stud, but it shows that a 2x4 on 24's is pushing the line for 2 stories. Add notches, this wind we're getting on top of the snow load... it's getting too close for me.

I then entered 3.5 and 5.5" depth studs and looked at the column stability factor and crushing strength required of the plates, the 2x4 was pushing it and the 2x6 was good by a fair margin.

Anyway, another way to think about it  .