While it may still be a ‘requirment’ in some building codes, it is outdated building science.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/crawlspace/crawlspace_products/crawlspace_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.
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.
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
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.
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.
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 :roll: .
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.
I’m not an engineer and I don’t want to play one so I’ll leave that to Robert.
[size=2]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. [/size]
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.
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.
So if the attic has no flooring, just the ceiling drywall underneath substitues as rafter ties when the joists run perpendicular to the rafters? Seems wack if you ask me.
Collar ties (up higher near the ridge), gusset ridge plates, or metal ridge straps (in addition to the typical rafter/plate toenails) are needed no matter what direction the ceiling joists span to keep the top of the rafters from coming apart in higher winds. Many local framers consider use of both metal ridge straps and 2x4/2x6 collar ties good practice (in addition to the rafter ties).
Sometimes if the ceiling joists are framed perpendicular to the rafters, and a structural ridge beam isn’t used, the wood collar ties will go into tension. But the usual wood collar ties closer to the ridge with a few 10d nails at the connections just doesn’t have enough strength … and the connections start splitting and/or pulling apart which allows rafter spread and ridge sagging.
I have seen it happen on many houses I have done evaluations on which only had typical collar ties higher up near the ridge. Usually requires additional larger and more closely spaced tie members with heavier bolted connections, or installation of a structural ridge beam to fix.
That is also why IRC 803.2.1 requires ceiling joists or rafter ties (continuous subflooring or wood/metal ties) … parallel to the rafters at the wall top plate level, and attached to the rafter heel … with fairly heavy connections per IRC Tables R602.3.1 & R802.5.1.9 … or a structural ridge beam or special rafter tie design by a PE/RA is required.
Requires a posted ridge where the hip rafters frame into the ridge (required per IRC 802.3) … or a special design is needed for the first few rafter ties near the hip ridge, as they will be more heavily loaded.
JMO & 2-nickels …
P.S. I agree the typical sheetrock ceilings don’t make an effective roof rafter tie.
Certainly not ideal but that is how the drywalled ceiling is acting.
A better feature in your design example would be 2x stock nailed perpendicular atop the ceiling joists parallel to and below the hip common rafters and connected to the exterior wall top plate. A diagonal brace or anchor point would be needed also to prevent the ceiling joists from moving all at once.
I just ran into this same issue on a gable roof, the load bearing walls run parallel to the rafters and the ceiling joists run perpidicular to the rafters. The rafters are 2x6 with an approximate 14’ span. Some of the rafters are cracked and some have stress cracking the roof is extremly wavy on one side.
Is it in our best intersest to explain to the customers the two different ways to remedy the problum or just refer them to a contractor or engineer?
