NACHI releases new, free, online Structural Issues for Home Inspectors course.

[Larry D. Kage]

Originally Posted by phinsperger

http://education.nachi.org/show.php?...7&course_id=22
'A lock of collar ties (see picture to right) can cause rafter spread' should be ' A lack of collar ties.... '

Typically, collar ties are placed in the upper third of the roof. Collar ties are not meant to prevent the horizontal spreading of the rafters or act like rafter ties - they are intended to prevent uplift from wind and really nothing more.

Rafter ties, which can be ceiling joists are placed near the bottom of the rafter and help to prevent the horizontal spreading of the rafters .

[Nick Gromicko]

Paul, thanks... all good points. Thanks! Here is my thinking on it:

I normally find cold joints in poured concrete to be diagonal, not horizontal. This being because the chute of the concrete truck stays fairly stationary and so the concrete slumps to the sides on an angle then cures some before the next truck arrives.

I can't find anything on having crawl space vents not being recommended, even if northern climates.

I like the idea of adding a reminder for inspectors to look for missing blocking, like under walls running parrallel but between joists. I can't find anything about cross bracing though. All I find are arguments that cross bracing does nothing structurally other than perhaps disperses some of the live load over a greater number of joists. I can't find any reference to them being required anywhere.

We tried to use the structural course to first and foremost remind inspectors to stick to the SOP and not crack analysis which is not their job (and in fact is probably a criminal offense) to do perform any sort of crack analysis (engineering services).

[Nick Gromicko]

Larry, here is my thinking on collar ties:

They help prevent rafter spread. First of all they are called ties, implying that they are under tension, not compression. Second of all, code permits them to be 1" by 4". Now a 1" by anything is useless under compression but fine under tension. Third, having lived in snowy parts of the world all my life, my own roof's collar ties are under tremendous tension when the roof is supporting a heavy snow load. The ties suffer tension because the rafters are trying to spread.

[Larry D. Kage]

Originally Posted by gromicko

Larry, here is my thinking on collar ties:

They help prevent rafter spread. First of all they are called ties, implying that they are under tension, not compression. Second of all, code permits them to be 1" by 4". Now a 1" by anything is useless under compression but fine under tension. Third, having lived in snowy parts of the world all my life, my own roof's collar ties are under tremendous tension when the roof is supporting a heavy snow load. The ties suffer tension because the rafters are trying to spread.

Research may educate and change your mind.

http://www.iccsafe.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=10;t=001392



http://www.iccsafe.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=10;t=001142



This site uses a definition that helps your use of “collar ties” but it is atypical, IMO.
http://www.ahfc.state.ak.us/iceimages/reference/log_construction_appderef.pdf

“Collar tie. A horizontal member used to tie a pair of opposing rafters
together. May be high to hold the upper joint together or may be low
to serve as a ceiling joist. Also called a collar beam.”


Edited to add links.

[Robert J. OConnor]

Here is a link to a good discussion on the difference between "Collar Ties" and "Rafter Ties" ...

http://www.nachi.org/bbsystem/viewtopic.php?t=4514

Collar Ties up near the ridge should not be in tension from rafters spreading unless there is a problem ... like a change to a cathedral ceiling which removes ceiling joists acting as Rafter Ties. Common problem with retrofit cathedral framing when the ridge plate isn't swapped out for a posted ridge beam.

JMO ...

[phinsperger]

Originally Posted by gromicko

I can't find anything on having crawl space vents not being recommended, even if northern climates.

While it may still be a 'requirment' in some building codes, it is outdated building science. Origianlly it was thought to vent the crawl like an attic because both are outside of the conditioned air envelope. That thinking is flawed because we are dealling with two completly different temperatures relative to outside.

Basically in they are useless in the winter because they are covered by snow. In the summer you have warm moist air outside. As it comes onto a vented crawl the moisture condenses out of the air because the crawl space is cooler. So it is bassically dumping moisture into the craw. Attemps at cross ventilation doesn't work.
Here are a referance I found quickly;
http://www.basementsystems.com/crawl...space_vent.php
I know from personal experince as well not to ventilate a crawl. I have a 137 year old crawl with a dirt floor, stone rubble walls. Not venilation, and even now in the wet spring season its dry as a bone. This referance ( http://www.buildingscience.com/bsc/topten/south.htm ) says not to vent crawls in the south but I have no personal experince in the south so I don't know. Maybe one of the southern guys can comment.There are other factors as well such placement of vapour barrier, pressurization, etc. that also need to be taken into account, but in general I do not recommend venting crawls.

Originally Posted by gromicko

I like the idea of adding a reminder for inspectors to look for missing blocking, like under walls running parrallel but between joists. I can't find anything about cross bracing though. All I find are arguments that cross bracing does nothing structurally other than perhaps disperses some of the live load over a greater number of joists. I can't find any reference to them being required anywhere.

Ontario Building Code 9.23.9.3 and 9.23.9.4

I have also seen section of floor systems with a bit more bounce where they have been removed to make room for duct work.

Originally Posted by gromicko

We tried to use the structural course to first and foremost remind inspectors to stick to the SOP and not crack analysis which is not their job (and in fact is probably a criminal offense) to do perform any sort of crack analysis (engineering services).

While I agree it is very important to stick to the SOP I just think that information belongs in the SOP course. A slight reminder at the beginning and/or end of the technical courses is fine but I felt it was a bit much IMHO

[Joe Farsetta]

Paul,

Here in NY State, if the crawl is not adequately ventilated, there is no C.O. issued.

[Nick Gromicko]

Larry and Robert... good links. However, they are wrong and I believe I can prove it with a simple question:

If collar ties don't help prevent rafter spread and only serve the purpose of holding down the rafters to the ridge board, then why are they required when the joists (rafter ties) are perpendicular (not parallel) to the rafters (no rafter ties)? If they don't help prevent rafter spread, why is their installation required when rafter ties are absent?

It's cute to have little rules that say at some magical point (top 1/3 of the attic) rafter ties become collar ties and no longer help prevent rafter spread but the engineering truth is that a rafter tie is nothing more than an optimally placed collar tie and the higher you lift it, the less effective it becomes at preventing rafter spread (or as Erby called said "the less leverage it has"). The function of effectiveness goes from ultimate at the bottom (as joists) to zero at the very top by the ridge board with it being helpful to some degree depending on its placement.

[Nick Gromicko]

A way to look at any engineering problem is to look at it backwards. Instead of asking "Does the tension in collar ties prevent rafter spread?" ask this: "What can I do to collar ties to create rafter spread?" The answer is to lengthen the collar ties (push out on the rafters). The compression caused by the lengthening the collar ties could only be relieved by rafter spread (assuming the rafter/ridge joint holds)*... thus... rafter spread is resisted by their tension.

*Looking at the problem forwards we can assume that during rafter spread the rafter/joint will not open as rafter spread causes this joint to tighten, not loosen.

[Nick Gromicko]

Totally off topic... but I once built a new home designed to look like an old cottage (like a Ginger Bread house). The client wanted the ridge to intentionally sag in the middle. I had to build the ridge board out of overlapping plywood (3 layers) cut in the shape of a big long curve. Every rafter had to be measured and cut to a different length (shorter in the middle, longer near the gables). She wanted the ridge to sag enough so that it looked clearly intentional (and wouldn't be mistaken for an actual structural failure which it was attempting to imitate by design). Anyway... you should have seen my roofer trying to lay the shingles .

[phinsperger]

Originally Posted by jfarsetta

Paul,

Here in NY State, if the crawl is not adequately ventilated, there is no C.O. issued.

Venting of crawls is required by our code here as well (OBC 9.18.3) but as we all know, the codes are slow to change and this still reflects old building science.

[Larry D. Kage]

Originally Posted by phinsperger

Venting of crawls is required by our code here as well (OBC 9.18.3) but as we all know, the codes are slow to change and this still reflects old building science.

I agree with Paul. Venting the crawlspace is old school.

However Joe F. is correct. Even around here, unless you have them, no CO is issued. We put them in then close them pemanently after the CO is issued. I like heating the space actually.

[Larry D. Kage]

Nick,

I’m not an engineer and I don’t want to play one so I’ll leave that to Robert.

Collar ties may help to some minor degree but that is not their intended function. However, if rafter spread is desired just remove the rafter ties, which are intended to prevent that spreading, and wait for loading.

As indicated in my first post, some people refer to any horizontal member, used to tie a pair of opposing rafters together, as a collar tie but that is atypical.

[Nick Gromicko]

I have another inspection question:

1/2 the sides of a hip roof are made of rafters that don't run parallel to the joists. Are rafter ties and/or collar ties required if the floor joists don't switch direction at the gables?

[Larry D. Kage]

Originally Posted by gromicko

I have another inspection question:

1/2 the sides of a hip roof are made of rafters that don't run parallel to the joists. Are rafter ties and/or collar ties required if the floor joists don't switch direction at the gables?

The diaphram of the ceiling material nailed to the ceiling joists (no matter what direction they run) and attached to the top plate acts as the rafter ties in the short distance needed.