Author Topic: Roof Ridge Connections  (Read 43315 times)

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Offline Don_P

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Roof Ridge Connections
« on: October 14, 2009, 09:14:42 AM »
In the interest of explaining the correct way to frame a ridge board roof for those interested, I thought it might be a good idea to post what the codebook asks and try to explain a little bit about the reasons why.

For those wanting to read along this is in chapter 8, R802.3 Framing Details.
https://www2.iccsafe.org/states/Virginia/Residential/PDFs/Chapter%208_Roof-Ceiling%20Construction.pdf
Rafters shall be framed to ridge board or to each other with a gusset plate as a tie.
The top ends of a rafter need to be attached to a ridge board OR if there is no ridge board they can be connected directly to each other with a plate across both faces of the joint, think about how a truss is built for that connection.
Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter
A ridge board is typically 2x material but it is perfectly ok to use 1x (3/4") material. A ridgeboard is not a beam, it is simply something to nail the rafters to. The ridgeboard needs to be the full height of the plumb cut to give full bearing to the rafter. This is to prevent the possibility of splitting of an unsupported end. I have seen a 2x12 rafter split and fail when the split began at a gap at the unsupported bottom edge and followed sloping grain till it ran out at the top edge some feet away. In that case the lumber grader had passed a marginal timber and the carpenter failed to provide a fail safe.

That section goes on to discuss valleys, hips and low slopes.

To continue discussing the top end of the rafter I'm going to bounce to the end of the next section R802.3.1
Collar ties or ridge straps to prevent wind uplift shall be connected in the upper third of the attic space. Collar ties shall be a minimum of 1x4 inch nominal and shall be spaced not more than 4' on center
This is a fairly recent addition to the codes in the wake of high wind roof failures. If you remember the news footage of roofs unzipping at the ridge and both halves flying through the air, this little bit would have prevented that embarrasement. Under the ridge, in the upper third, a 1x4 is connected across from rafter to rafter OR a metal strap is connected over the ridge from rafter to rafter to prevent the top connection from blowing apart if wind either gets inside or a very high wind blows across the roof trying to lift it apart. This locks the ridge connections together. Not required in all areas, sure is easy insurance to install.

Stepping back in the last section it discusses the bottom connection;
Ceiling joists and rafters shall be nailed to each other and the rafter shall be nailed to the top wall plate. Ceiling joists shall be continuous or securely joined where they meet over interior partitions and are nailed to adjacent rafters to provide a continuous tie across the building
I left out the details here and those are important, please go back and read this section before building. The intent of this section is that the bottoms of the rafters need to be well tied to each other across the building to prevent them from spreading.

Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice
If the rafter feet cannot be tied to prevent spreading then the rafters should be hung from the top end. A load bearing wall works or a ridge beam (as opposed to a ridge board). If a beam is used it needs to be sized to support the roof load. Where a ridgeboard carries no load a ridge beam supports half the weight of the roof. This distinction is important, make sure you understand it before omitting ceiling joists or rafter ties.

Hope this helps

Offline jdhen

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Re: Roof Ridge Connections
« Reply #1 on: October 15, 2009, 02:25:32 AM »
Thanks, Don.  Timely information for me.
 As it turns out, I rechecked the height of my temporary poles using another method and realized I'd made a small mistake.  Since I had to get up there and trim it down anyway, I decided to cut it down a little further so that the gap at the ridge board would be at the top as you and Red suggested.  Just to be sure- it's ok to leave the gap at the top or should I fill that in too?
Jesse

Offline Don_P

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Re: Roof Ridge Connections
« Reply #2 on: October 15, 2009, 04:10:10 AM »
Now we're crossing into the area between the code and physics, I don't mind the gap at the top because it's on the compression edge and won't induce splitting. You can't go wrong filling it and I think that's where the wording in the law comes from.

This is yet another method to check your poles;
http://windyhilllogworks.com/Calcs/ridgehgtclc.htm

I will often set mine a little low and have a couple of shims in my bags to "tune" the height when I bring in the first rafter pair at each end. It beats the heck out of climbing up there with a saw and trimming a high pole down.


Offline Woodswalker

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Re: Roof Ridge Connections
« Reply #3 on: October 15, 2009, 04:49:16 AM »
Thanks Don for the good info about ridge boards, beams, and rafters.  Should be very helpful for those not yet at that stage in their projects.  Good to know that I met the requirements when I built my cabin a couple years back.  Used 2 x 10's as collar ties, far exceeding the 1 x 4 standard, but spaced them 4 ft OC, just meeting that one.  Not sure if they are quite in the upper 1/3 of the "attic" space, but mine is a "vaulted" setup with no ceiling joists or real attic.  Imagine if one were using trusses that were engineered and built off-site they would meet all of the standards as long as they got attached properly to the top plates of the walls.  When I was up on the roof installing shingles and a chimney, the whole thing seemed very solid.  I did use 5/8" sheathing, which also helps.

A little off-topic, I agree completely with your opinions about lofts.  Don't quite understand why so many folks posting here are building cabins with lofts.  I suspect most have not actually climbed a steep ladder leading to one and tried sleeping up there in a cabin heated by a wood stove.  My experience years ago was that you get so hot you throw off all the covers, and maybe open a window up there to get to sleep, and then you wake up in a couple of hours shivering.  Of course, a good ceiling fan would help - if you have electricity.

Offline MountainDon

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Re: Roof Ridge Connections
« Reply #4 on: October 15, 2009, 06:14:42 AM »
Don't quite understand why so many folks posting here are building cabins with lofts... 

Those were all reasons we went with a normal flat ceiling. I must admit that I like the outside look of a steeper pitch roof and the open interior appearance. But I also didn't lie the prospect of working on a steep pitch roof.

Ceiling fans would be a help as would opening skylights of good quality.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline Bobmarlon

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Re: Roof Ridge Connections
« Reply #5 on: October 15, 2009, 04:28:09 PM »
Also if you you have a dormer that meets at the ridge board the rafter can be set a little above the top of the ridge board so both of the peaks meet at the same place.
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Offline Don_P

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Re: Roof Ridge Connections
« Reply #6 on: October 15, 2009, 04:48:20 PM »
I don't have a problem with lofts, I don't think  ??? I'm mostly interested in passing on the few tidbits I've learned over the years on good sound construction whatever the building style.

In that regard I think I missed the mark here. In a ridgeboard (as opposed to a ridgebeam) roof, the code requires a rafter tie in the lower third of the rafter height to prevent the rafter feet from spreading. These are on each rafter pair and may be floor or ceiling joists or a tie raised as much as 1/3 of the roof height, but no more. Additionally as the rafter ties are raised the allowable rafter span is decreased because the raised tie is putting an increasing bending load on the rafter as it is raised.

A collar tie is in the upper third, can be tight under the ridge, or a strap over the ridge, and is to prevent the roof from being blown apart at the ridge. This happens in high winds, usually if a gable window or wall fails and lets the wind in. It can however happen if a high wind roars over the roof and gives it lift similar to an airplane wing.

If a roof cannot have rafter ties across the lower third of height a ridge beam is used. If the ridge can't sag, the rafter feet can't spread, and the walls won't bow out. A collar tie or strap is still required in the upper third or over the top of this roof.

Here's one tabletop experiment that should help explain the difference in those two roof types. Dig out an old greeting card set it on a slick tabletop in gable roof fashion. Push down on the ridge, the feet will slide outward. Now tape a couple of pieces of thread across the "feet" as rafter ties, set it on the table and push down again, the thrust has been restrained by the rafter ties. If the thread pulled out of the tape remember that when you are nailing those ties in real life and don't skimp on that connection, it can be highly stressed.

That was our ridgeBOARD model, now for a ridgeBEAM. Now find a new pencil and some books or chunks of wood. Remove the ties and set the gable roof up again Slip the pencil through under the fold and support each protruding end of the pencil on your blocks or books. Push down on the ridge again, no thrust. If the ridge can't settle there is no thrust. If you broke the pencil and the roof splattered bear that in mind if you try to use a ridge board where you should have used a ridgebeam.

For those of you that are really bored and still with me we can keep playing with the model. You'll need a needle and thread this time. Pierce through the card 2/3 of the way up the roof on each side near each end and thread 2 high ties through there. Don't use the pencil, this will be a ridgeboard roof with too high a tie. Push down on the ridge, notice the bending stress you put into the rafters. A high collar tie having to act as a rafter tie is potentially a very overstressed member and is also stressing the rafters unduly.

Offline davidj

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Re: Roof Ridge Connections
« Reply #7 on: October 16, 2009, 05:30:39 AM »

A little off-topic, I agree completely with your opinions about lofts.  Don't quite understand why so many folks posting here are building cabins with lofts.  I suspect most have not actually climbed a steep ladder leading to one and tried sleeping up there in a cabin heated by a wood stove.  My experience years ago was that you get so hot you throw off all the covers, and maybe open a window up there to get to sleep, and then you wake up in a couple of hours shivering.  Of course, a good ceiling fan would help - if you have electricity.

I think the reason there are a lot of lofts is that they end up happening by default with a lot of these cabins.  A great room for kitchen/living is pretty much a given, and a steep roof has lots of advantages for shedding snow.  If you then want the cathedral ceiling, which gives the space a nice feel, then you either have weirdly high bedroom/bathroom ceilings or a loft.

We're not planning on using ours much, but there's typically either 2 of us or 8-10 people staying - for the latter situation the loft will help a lot, even if it's not the greatest place to sleep.

Offline poppy

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Re: Roof Ridge Connections
« Reply #8 on: October 16, 2009, 10:59:43 AM »
Don_P, you continue to entertain while supplying sound info.  [cool]

Talk about your "house of cards."  :)

Offline Bobmarlon

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Re: Roof Ridge Connections
« Reply #9 on: October 17, 2009, 12:15:19 AM »
This kinda of ties into the what makes a cabin a cabin thread,  We could of just had two stories but the loft just makes it a cabin.
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Offline Texas Tornado

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Re: Roof Ridge Connections
« Reply #10 on: February 16, 2011, 02:09:19 AM »
In the interest of explaining the correct way to frame a ridge board roof for those interested, I thought it might be a good idea to post what the codebook asks and try to explain a little bit about the reasons why.

For those wanting to read along this is in chapter 8, R802.3 Framing Details.
https://www2.iccsafe.org/states/Virginia/Residential/PDFs/Chapter%208_Roof-Ceiling%20Construction.pdf
Rafters shall be framed to ridge board or to each other with a gusset plate as a tie.
The top ends of a rafter need to be attached to a ridge board OR if there is no ridge board they can be connected directly to each other with a plate across both faces of the joint, think about how a truss is built for that connection.
Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter
A ridge board is typically 2x material but it is perfectly ok to use 1x (3/4") material. A ridgeboard is not a beam, it is simply something to nail the rafters to. The ridgeboard needs to be the full height of the plumb cut to give full bearing to the rafter. This is to prevent the possibility of splitting of an unsupported end. I have seen a 2x12 rafter split and fail when the split began at a gap at the unsupported bottom edge and followed sloping grain till it ran out at the top edge some feet away. In that case the lumber grader had passed a marginal timber and the carpenter failed to provide a fail safe.

That section goes on to discuss valleys, hips and low slopes.

To continue discussing the top end of the rafter I'm going to bounce to the end of the next section R802.3.1
Collar ties or ridge straps to prevent wind uplift shall be connected in the upper third of the attic space. Collar ties shall be a minimum of 1x4 inch nominal and shall be spaced not more than 4' on center
This is a fairly recent addition to the codes in the wake of high wind roof failures. If you remember the news footage of roofs unzipping at the ridge and both halves flying through the air, this little bit would have prevented that embarrasement. Under the ridge, in the upper third, a 1x4 is connected across from rafter to rafter OR a metal strap is connected over the ridge from rafter to rafter to prevent the top connection from blowing apart if wind either gets inside or a very high wind blows across the roof trying to lift it apart. This locks the ridge connections together. Not required in all areas, sure is easy insurance to install.

Stepping back in the last section it discusses the bottom connection;
Ceiling joists and rafters shall be nailed to each other and the rafter shall be nailed to the top wall plate. Ceiling joists shall be continuous or securely joined where they meet over interior partitions and are nailed to adjacent rafters to provide a continuous tie across the building
I left out the details here and those are important, please go back and read this section before building. The intent of this section is that the bottoms of the rafters need to be well tied to each other across the building to prevent them from spreading.

Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice
If the rafter feet cannot be tied to prevent spreading then the rafters should be hung from the top end. A load bearing wall works or a ridge beam (as opposed to a ridge board). If a beam is used it needs to be sized to support the roof load. Where a ridgeboard carries no load a ridge beam supports half the weight of the roof. This distinction is important, make sure you understand it before omitting ceiling joists or rafter ties.

Hope this helps


Don can you do a visual ie: pics?

Offline Don_P

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Re: Roof Ridge Connections
« Reply #11 on: February 16, 2011, 06:24:04 PM »
I'll add sketches or pics as I can here;

This is a sketch showing a ridge board, as opposed to a ridge beam. The board is supposed to be the same height as the cut end of the rafter. In this case the rafter is a 2x12 so the ridgeboard is actually 2 boards, a 2x12 and a ripped down 2x6 to create the needed height.


this sketch shows a few ways to satisfy the collar tie requirement. I drew them on three conecutive rafters for clarity but they only need to be every 4'. First is a 1x4 tight under the ridge. Second is a meatl strap over the ridge nailed to both rafters, third is a 1x4 dropped beneath the ridge but still in the upper third of roof height. Below them you can see the rafter ties on every rafter pair.
« Last Edit: February 16, 2011, 06:39:01 PM by Don_P »

Offline John Raabe

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Re: Roof Ridge Connections
« Reply #12 on: June 08, 2011, 08:09:59 AM »
Great work on this thread Don.  :D :D :D

Some of the older standard framing techniques (such as ceiling rafter ties at 4' spacings) that used to allow for open cathedrals are no longer kosher. This is due to greater concern over potential high wind damage. With the tornado damage we have seen this Spring that concern has been heightened.

The solution for most owner-builders is to order trusses (that are often delivered to the wall top) or to go with a structural ridge beam. The trusses are engineered for local loads by the truss company and the ridge beam can often be calculated by the lumber supplier - so these are not difficult modifications. (The ridge beam does have to be designed with solid bearing support that follows all the way down to properly sized foundation footings.)
« Last Edit: June 08, 2011, 08:23:37 AM by John Raabe »
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Offline Texas Tornado

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Re: Roof Ridge Connections
« Reply #13 on: June 20, 2011, 01:47:16 PM »
John or Don,
Can you explain in greater details what this is?
The ridge beam does have to be designed with solid bearing support that follows all the way down to properly sized foundation footings.

Offline Don_P

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Re: Roof Ridge Connections
« Reply #14 on: June 20, 2011, 06:04:17 PM »
On a typical gable roof with a ridgebeam the rafters are basically hanging from the ridgebeam and are resting on the walls. The ridge is supporting half of each rafter with the walls supporting the other half of each rafter. As you look down on it from above each wall is supporting roughly 1/4 of the roof load and the ridge is supporting half of the roof load. The posts that support each end of the ridgebeam, assuming there are only two posts, are supporting 1/4 of the roof load. This is a significant load coming down each column. Those columns can be quite tall and must be checked for crushing and buckling. At the bottoms of those columns there needs to be an adequate footing to safely distribute that load over enough ground area to avoid sinking.

A quick example, take a 20x28' building. The ridge is 28' long, lets assume a 2' gable end overhang. Half of the 20' width, or 10' of width is being supported by the ridge. One half of the 28' length of the ridge is being supported on the end column, 14' +2' or 16'. An area of 10' x 16' is supported by each column. In my area the combined dead + live load is 35 pounds per square foot. 160 square feet x 35 lbs per square foot = 5600 lbs. In my area the soil is generally considered to be good for 2000 pounds per square foot, 5600 lbs/2000 lbs/sq ft=2.8 square feet. The footprint required to support just the ridgebeam is 2.8 square feet... or about 20" square. Chances are we are also supporting a girder with a floor load or maybe the load of 2 floors added to that 5600 lbs. These would also be factored into the footing requirement.

Offline John Raabe

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Re: Roof Ridge Connections
« Reply #15 on: June 20, 2011, 07:57:13 PM »
Good overview Don.

Ridge beams and internal beams and posts require a "load trace" as Don mentions - following the loads as they accumulate from the roof and floors down through the walls and into the foundation and any internal piers. These loads can be generalized in stock plans but need to be checked when heavy snow and other unusual loads need to be considered.

Anyone designing their own house would be wise to have a local engineer, architect or building designer do such a load trace and then size the supporting members.

This load trace is simplified considerably when you have a perimeter concrete foundation and full span floors and roof trusses to carry all loads to the outside walls. This is why I use that configuration for most of my smaller stock plan designs.
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Offline Texas Tornado

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Re: Roof Ridge Connections
« Reply #16 on: June 21, 2011, 05:27:11 AM »
Thank You both for those answers!!  [cool]
How does it play out on when a place is built on just posts?

Offline John Raabe

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Re: Roof Ridge Connections
« Reply #17 on: June 21, 2011, 06:26:36 AM »
Then you are into timber frame construction or pole framing where loads are traced down from a combination of beams carrying loads to posts or beams and ultimately into the foundation and ground beneath the building. Most modern buildings are a combination of frame walls and built-up posts.


An oversized header (beam) is carrying the load of the roof beam to support posts on either side. This from the Victoria Cottage construction photos
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Offline Texas Tornado

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Re: Roof Ridge Connections
« Reply #18 on: June 21, 2011, 01:26:59 PM »
Thank You John!
Does putting cement under the post keep it from sinking into the soil?

Offline John Raabe

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Re: Roof Ridge Connections
« Reply #19 on: June 21, 2011, 01:30:05 PM »
A concrete or sharp gravel footing under a foundation pier spreads out the load over a larger area of the underlying soil. Think snowshoes.  :D :D :D
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Offline Texas Tornado

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Re: Roof Ridge Connections
« Reply #20 on: June 22, 2011, 11:43:28 AM »
 [cool]
OK what if building a 20X20 single story what size beam would be needed and what kind of weight are we talking?

Here's a pic just for example:
« Last Edit: June 22, 2011, 12:15:16 PM by Texas Tornado »

Offline John Raabe

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Re: Roof Ridge Connections
« Reply #21 on: June 22, 2011, 02:25:24 PM »
A little single story house like this would most easily be built with a truss roof system or a standard framed rafter system (with rafters tied together with ceiling joists).

However, if you want to do a more expensive ridge beam supporting a single rafter (cathedral) ceiling, then you need to first determine the roofs (pitch, which way the ridge goes and what it lands on). You have to design a roof before you can trace the loads.
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Offline Texas Tornado

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Re: Roof Ridge Connections
« Reply #22 on: June 22, 2011, 03:24:46 PM »
4/12, cathedral, over fireplace and kitchen... Can you explain "trace the load"?
Thank You so much for teaching!!!

Offline UK4X4

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Re: Roof Ridge Connections
« Reply #23 on: June 22, 2011, 04:06:41 PM »
snow load
static load
type of roofing material
angle of roof

here's a basic example- I have a thread somewhere where Don kindly explained a lot of beam sizing- he's the expert and not me !- i'm just learning

This is what is in my house plan

Ridge beam calcs
32 ft long by 24ft wide
Ridge beams North South will be 2 off 16 ft long supported in the middle by the central pillar to basement
Area supported = 16 long  x 12 ft wide =192 ft squared
75 snow load plus 15 for roof weight = 90# per sq ft
192X 90 = 17,280# total weight

Beam moment
17,280 X 16ft long /8 = 34,560#

Big beam 7” X 11 7/8”   LVL has a moment of   41,130# so way sufficient

Not including slope and roof material

When I had it engineered it came out to be less due to heated slipery roofing - 5.75 X 11 7/8 LVL

being that it will be mostly empty and not a primary residence I'll be installing the beam I calculated

Offline John Raabe

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Re: Roof Ridge Connections
« Reply #24 on: June 22, 2011, 04:19:48 PM »


Here's a mythical load trace on this house.

In a 20'x20' design the ridge beam is carrying 10' of the rafter load (5' from each side). If we are in a 30 psf (pounds per square foot) snow load and we add 10 psf for the dead load weight of the roof structure, we come up with 400 plf (pounds per lineal foot) falling on the ridge beam. Looking in a nearby table for glulam beams I find that a 3-1/8th inch x 13-1/2 inch glulam will span 20' and carry 427 plf. A 5.5" x 12" will also work. The short headers over the doors must also be checked as they are carrying over 4000 pounds that lands in the center of the header. A built-up solid post under each end of the beam and the sides of the headers carry the loads down to the foundation wall.

If the foundation is pier and beam the piers need to be located and sized to get this into the underlying soil.
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