Do you have a reason for wanting to cut the bird's mouth notch in them? It's unnecessary, and will, effectively, lower the load bearing limit... but not all that much.

BTW... 2x8's running a full 19'? You checked that out on a span chart? I wouldn't plan on storing anything up there.

 

Thanks for the reply. No particular reason at all, just looking for the more stable method. I was told that 2x8's SEL STR DF S4S at 16" on center would span 19'. 5" and be able to hold drywall and insulation.

 

I haven't looked it up, but even at 12" OC, I'm pretty sure 2"x8"s (ceiling only, no floor above nor storage) can only span about 18'-9". And at 16" OC, 2"x8"s are limited to just a hair over 16'.

Of course, my memory could be off a little (I'm old), so go look it up for yourself.

No big deal. It probably only means the ceiling will sag... probably not break.

 

Notching the beam at the end won't increase the tendency to sag, and also won't affect the bending stress in the beam.
All notching or chamfering does is to increase the shear stress at the bearing but in domestic situations, and particularly with ceilings, the loads are so low that shear is very rarely an issue.
There is little diference in the two methods, but personally I would be inclined to go with the top one. Occasionally the abrupt change in section that you have in the bottom pic can occur in a line of weakness in the grain and cause localised splitting, but that's probably splitting hairs!

 

I would use no notch method in #1. The square notch looks like a potential fracture point to me.
One advantage is that it leaves the top plate to use for drywall attachment of the walls.

 

s4s???

Surfaced 4 Sides

 

I understand the term, just not why he would use the s4s.

 

no notch, you want the bearing point to be across the bottom of the joist not up 1 1/2" into the joist. whatever notch you cut into it will then qualify as the actual width of the joist from the bearing point of the notch to the top of the joist, so a 1 1/2" notch would be like adding a 2 x 4" joist up there because the bearing point is 4" from the top of the joist.

 

You cannot span 19 feet with a 2 x 8. Well, I guess you can if you want, but it is going to deflect pretty badly.

 

that is an issue... probably best to double up some of the joists and add blocking between them mid span to add some re enforcement to the span...

 

notching will reduce ceiling height. here is a link to the 2009 International Residential Code span tables for ceiling joists. pick out which one applies (storage in attic or not) and it will give you the allowable spans. http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_par027.htm

good luck!

 

Where do you get 20 foot 2x8 anyway? 16 are the longest you get here without special order and high expense.

 

Joed, The local lumber yard here has them up to 32'. The 20' are $17.00

hand drive, Yes I was already planning on adding blocking.

GBrackins and jagans, if I am reading the chart correctly Douglas fir Larch SS without storage can span up to 24' 8" and with limited storage 19' 7" all at 16" on center

I am just gonna go with the 2nd method and avoid loosing ceiling height. You guys are great and your feedback is very well appreciated. Now to figure out how the heck am I gonna get those joist up there :laughing::laughing::laughing:

 

you could probably even split some 16 foot joists on those 19' ones and add 2' finish out pieces to add somewhat of a double to cover the span. just nail them really good together to make the double count.

 

There is not enough wood to effectively support a 2x8 by its self. You will likely have to build your own truss system in place or support the 2x8 properly with an angled brace or gusset to support that area. The angle I indicated in the picture shows how that would be placed.

As far as the span goes...to far without a truss system.

 

There is not enough wood to effectively support a 2x8 by its self


View attachment 70268

At 3" deep, there is sufficient timber in the cross-section to adequately take the shear stress at the bearing without adding a gusset plate (assuming normal domestic ceiling loading).

 

You could use a hanger under the bottom if notching more than acceptable (though it appears you may be good); Fig.4. The taper-cut has a maximum, Fig.7; http://www.google.com/url?sa=t&rct=j&q=taper-cutting%20joists%20or%20beams&source=web&cd=3&sqi=2&ved=0CDUQFjAC&url=http%3A%2F%2Fwww2.wwpa.org%2FPortals%2F9%2Fdocs%2FPDF%2FTG7.pdf&ei=lsN0Ua_LJuaLjALlzYHgDg&usg=AFQjCNHaOdPmo_RigJQnnROM_AZnR8yI9Q&bvm=bv.45512109,d.cGE&cad=rja

Remember the height/slope measurement is at the back of the 1-1/2" minimum bearing, not the very back of the joist.

Gary

 

At 3" deep, there is sufficient timber in the cross-section to adequately take the shear stress at the bearing without adding a gusset plate (assuming normal domestic ceiling loading).

According to the site posted by "Gary in WA",

Quote;
"Tapering Joists and Beams
It is sometimes necessary to taper the ends of a beam or joist to keep it under the slope of the roof. But reducing the depth of these members also reduces their load bear- ing capacity.
If joists must be tapered, make certain the length of the taper cut does not exceed three times the depth of the member and the end of the joist or beam is at least 1/2 the member's original size."
End quote.

So, if the load bearing point (as shown in my diagram) of the joist where it meets the plate is 3", this joist cannot be effectively used without proper trussing or bracing. Especially not over a 19' span.

If a taper cut is used for a 2x8, the point at which the joist meets the top plate must be minimum 3 3/4". The length of the tapper cut appears to be within the minimum (3 x the dimensions or 22 1/2". The criteria of both rules must be met. It appears to me that only one is.

 

MJ;

Codes are generally written as easy-to-follow rules of thumb so that architects, contractors, framers, self-builders and inspectors can work to an accepted standard without the need for further structural appraisal or calculations.

But, consider the OP's case;

On a span of 19ft @16" centres, and assuming a dead load of 10 psf with no storage, the total load on each joist will be about 260 lbs. This means the shear force at the bearing will be 130 lbs, and the shear stress on the reduced section of 1.5" x 3" will be 43 psi. The allowable shear for a timber such as Douglas Fir is around 100 psi. Even this figure can be increased by 10% because the ceiling joists form a load-sharing system. There willl also be a further increased allowance because the cut-out is at the top of the joist, rather than the underside. Without working it out fully, my guess is that the reduced section will be taking considerably less than one-half the allowable shear stress.

The point I am making is that to go outside a code does not imply inadequacy; it merely means that an inspector may ask for justification. In the OPs case, the cost of structural calcs would probably not be worth it.
As one poster suggested, it might be better to cut notches from the wallplate to maintain a greater depth of joist at the taper, and stay within code.

 

As these appear to be ceiling joists, which require X number of fasteners into the rafter (heel/joint connections, bottom chart; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_par027.htm) to prevent wall spreading, I would suggest a different approach; add a 2x4 ledger board to the top plates, notch the joists to fit, you will have code-approved bearing of 1-1/2" and meet the "tapered top" rule. Use a drill hole at the notch apex to retain strength/prevent splitting; pp.3; http://www.finehomebuilding.com/PDF/Free/021184090.pdf

Gary

 

Of course, this COULD be done the simple way... if you can stand to lose some head room....

Just install regular hangers, 7-1/2" down from the underside of the roof (or deeper if you choose to go with 2 x 10's), and put squared-off joists right in the hangers.

 

But how would he tie the rafter feet together to stop roof spread?

 

MJ;

Codes are generally written as easy-to-follow rules of thumb so that architects, contractors, framers, self-builders and inspectors can work to an accepted standard without the need for further structural appraisal or calculations.

All due respect. As architects, engineers and builders we interpret the codes, regulations and rules as they apply to our unique situations. Every aspect of construction would be difficult to write as an easy-to-follow, unless your Ikea. And even those simplified instructions can be misinterpreted.

100 carpenters could build the same house 99 different ways.

 

I do not disagree with the gist of what you are saying, but my thesis is that if a detail does not follow the code, it does not necessarily mean it is inadequate.

As a code is likely to be used by people whose skill varies from experienced framer to ham-fisted DIYer, it has to be (1) simple to follow and (2) set standards greater than the minimum, to allow for poor materials or workmanship.

The OPs case is a very simple one, and if the timber stresses are within the limits I don't believe any engineer would see a structural problem. The problem lies in the cost of producing calculations to prove it! This is why in most cases it is easier just to follow the code.

But what a dull world it would be if we all had to build to a rigidly prescriptive code, with no flexibility allowed!


(by the way, getting a doctorate in finite element analysis is easier than following IKEA instuctions).

 

A big problem for builders, (DIY to GC), is the lack of imagination, vision, or sometimes even "knowledge" on the part of inspectors. They arrive at a job site armed with an old, weather-beaten drawing that shows them how someone, somewhere envisioned a particular phase of construction, and "By God, this is how it's done... and no other way."

 

Board strecher everyone should have one :thumbup: Just a joke but with that span 2x8 is small for the load.

 

It might bend a bit, but it won't collapse :thumbsup: