holes through joists...

[virgilns]

What's the reason for holes in joists being in the 25-40% range and notches in the 7-25%?

Can anyone explain why a hole in the centre weakens the joist less thana hole near the end?? Or is this just a historic finger in the air rule of thumb??

I ask as Ideally i need to put a hole near an end.....

 

[tony1851]

The diagrams showing optimum positions for holes and notches are generally based on rules of thumb, with an allowance for a safety factor.

If you want to go outside those rules, you need to check by calculation. It depends on the span of the joist, the loading, the depth and position of the notch/hole etc etc.

Generally, you don't notch near the middle because the bending stress is greatest there; a 2" notch mid-span on an 8" deep joist will effectively turn it into a 6" deep joist. A notch near the end is not so critical because the bending stress tends to zero at that point.

Holes are a little more complex.

 

[Blagard]

Can anyone explain why a hole in the centre weakens the joist less thana hole near the end??

It is not really a case of weakening but the type of forces the joist is subject to and the regulations imposed for safety

What happens is the forces at the ends of a joist are mainly in shear and the forces in the middle are mainly bending moments.

Because holes are through the middle of a joist they effect bending moments where they are smallest. Bending moments put maximum stress on the top and bottom of the joist so you want to keep those areas solid

Nearer the ends you put notches because that is where bending moments are smaller so you are chopping out where there is less stress.

Obviously you don't want notches and holes together and you avoid the middle of the joist (maximum bending moment) and the ends (Maximum shear)

Hope that helps a bit!

Edit: if you had a picture of the bending moment stress's on a joist section it would look like an hourglass with the top and bottom being wide representing the high stress's and with the middle being almost Zero width
The hour glass top and bottom would look narrower nearer the ends of the joist and wider in the middle span of the joist but always the same height

Last Edit!
It is because idiots used to put holes and notches together that they now have Zones. If you have no notches at the ends it is no problem to the joist to put a hole through. That is until some twit comes along and puts a notch in the top over your hole because the Regs say it's OK! So you are not allowed to put the hole through

Marks out of ten please for my laymans guide to structures. Just Google bending moments and shear forces to see how awkward and complicated it really is!

 

[tony1851]

I ask as Ideally i need to put a hole near an end.....

1. How deep is the joist?

2. What is the span of the joist?

3. What is the diameter of the hole you wish to drill?

4. How far from the end of the joist are you drilling?

5. Are there any other holes/notches nearby?

 

[Blagard]

I ask as Ideally i need to put a hole near an end.....

1. How deep is the joist?

2. What is the span of the joist?

3. What is the diameter of the hole you wish to drill?

4. How far from the end of the joist are you drilling?

5. Are there any other holes/notches nearby?

If you are about to do what I think you are, will you be providing calculations ? Of course if it is an old house and not subject to the building inspector then......

Long ago the rules were, if memory serves me correctly, Holes maximum 25% of joist depth, Notches maximum 1/3 depth (Yep as much as that!) but common sense had to be applied as well along the line of todays Zoning. Holes ideally through the middle of the Joist on the neutral axis almost anywhere along the length. Notches near the ends. Back then of course methods were a little different with hangers rarely in use and timber tended to be close grained and seasoned but not formally graded. And long before that the cave

 

[virgilns]

Thanks guys...

Blagard: makes sense and pretty much as I had written a reply after thinking about it more (but then lost due to my work PC crashing)

Tony:

1. How deep is the joist?
9"

2. What is the span of the joist?
2.5m

3. What is the diameter of the hole you wish to drill?
44mm

4. How far from the end of the joist are you drilling?
as close as I can get away with

5. Are there any other holes/notches nearby?
no - only nothes are right at the other end...

Would do a proper pic but hard to host from my work pc due to 'it security'

but here goes...

<-----------------------------3m----------------------->
<---------------------2.5m--------------------->
_______x___________________________________ external wall
. | |
. | |=x===========================
. | |
. | |
. | |=x===========================
. | |
. | |
. | |=x===========================
. | |
. | |
. | |=x===========================
. | | O
____| |_____________________________________ 3x9 joist
___________________________________________


=================================


=================================


=================================


=================================
__________________________________________ internal wall

__
__ is a 9x3" joist running the span of the room - 3m

| | is a 9x3" joist running outside wall to 3" joist running the span of the room (1.4m)

== is a 9x2 joist running the span of the room - the ones requiring holes are 2.5m span...

x is a requried 44mm hole for the bath waste...
O is the bath waste trap

Hope that makes sense.

Right at the intersection of the two 3" wide joists is going tobe the centre of the free standing bath (offset slightly down and right on this picture for the trap to clear the joists...

 

[Blagard]

That unfortunately is a nasty arrangement. Are you sure there is not another 9x3 trimmer joist near the outside wall picking up the end of the trimming joist (the vertical one in the sketch)?

Also the joists ends fixed to the trimming joist need to be left well alone as they will be partially cut and jointed at the ends and pinned with big nails. So you actually need to be as far away from them as possible. Certainly not close and this is obviously the oppersite of what you want. Essentially I would not do it. However, just how far from the joist ends can you get?

Better run the waste above the floor by the skirting and box it in. Raise the bath if you have to. Please don't tell me there is a doorway there!

PS by trimmer /trimming terms are a bit mixed up! - So long ago! - but I am sure you know which ones I was referring to

 

[virgilns]

3" Trimmer def runs into the wall...

I can get as far away from the 2" joist ends as needed, would just need a 90' bend (going 'up' according to the pic) at some point to exit through the wall

REALLY want to avoid raising the bath - and boxing it in above floor level is a no go as its free standing so bottom exit below floor only.

Kitchen below so no chance of going down and boxing in. Stairwell to the right of the pic and baby bedroom to the left (4m room)...so can't run the waste to the back of the house...and no drain close by...

Different bath or through the joists is the only option. And the wife REALLY wants THIS style...

Can I run the pipe 2ft into the span and maybe plate the joists with say 5mm 220 x 600mm plates both sides and bolt through??

Seems overkill though for 9" joists and i;m really only borderline outside the regs...

The only other option I just thought of is going all the way right through the wall to the stairwell and bury it int he stairwell 9" wall...but much though....

 

[Blagard]

Doing the holes 2' (600mm) from the ends you would probably be OK. Just take a good look at the joists for defects like large knots splits or shakes. If they are all sound then I would think it would be OK.

Edit: Use solvent weld pipe and joints in the floor void and a sweep bend not a knuckle bend. Possibly an anti-vac trap instead of the normal(due length of pipe run).

 

[tony1851]

IMO you could drill 12" from the end if need be. With a 9" deep joist, there's still plenty of meat left on it to take the shear force.

 

[Blagard]

IMO you could drill 12" from the end if need be. With a 9" deep joist, there's still plenty of meat left on it to take the shear force.

I would not disagree, but if traditionally fitted with a notched ends at the joint and the possibility of ungraded timber with some defects better to be safe. If modern with joist hangers and graded timber then OK Traditional joint as below changes the section at the end. There are other joint type but all reduce the end section. Shear isn't the problem

Untitled

• Blagard
• 19 Nov 2013
• 1

 

[virgilns]

Thanks guys...The joints are all tennon and mortice, joists look good...no massive knots, shakes or splits.

Yep, lots of solvent and a few joints and should be good.