to gap or not to gap..that is the ?
hvtrimguy
| Posted in Construction Techniques on
I just finished framing and sheathing an addition. never in my past experiences has anyone I worked for left a gap between sheets of ply. However , on the sheets it says to leave an 1/8″ gap. why? is it to accomodate lumber shinkage? surely the plywood is stable enough not to need an 1/8″. maybe a slight gap I could see 1/16″. Also, this gapping effects layout which defeats the purpose of a 4 x 8 sheet. if the ply manufacturers want this gap, why not make the sheets a 1/16″ shorter in each direction so layout can be maintained?
“it aint the work I mind,
It’s the feeling of falling further behind.”
Bozini Latini
Replies
Wars have been fought over less.
be to vent or not to vent...is that the?
This has been discussed a lot here, if you go to "advanced search" at the top of the column that lists the current discussions and click, it will ask several questions including words to look for. Words like "plywood spacing" or "spacing roof sheathing" may bring up the old discussions. I think the concern is that humidity will cause seams that are butted tightly to make ridges that telegraph through the shingles. Some plywood is made a little shorter to allow for this spacing. It's all very confusing! (Someone else may be able to explain it better, but I think it's even stamped on some plywood "sized for spacing" or something like that.)
That is for insurance in case a dry shhet of ply absorbs water and swells.
If I were to get ply direct from factory dry and install it right away, I might make an effort to keep some spacing. As it is, I get material that has had time to absorb moisture frrom the air before it gets used, and I have never seen shething that goes on perfect, so there are some minor gaps here and there anyways.
I'mm thinking maybe one out of a thousand or two roofs will telegraph the seams whicch swell a bit after installing.
if it was OSB, I would be concerned with leaving the gaps. Other thaan that, I have not seen anybody give it a thought in neaarly forty years of this work, and never have I seen a probllem from it
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I posted this before, but one time we had just finished a house and were working across the street from it on another house. About the time we'd take a break to warm up by the fire (yeah, we were whimps!) the sun would hit the roof across the street just right and you could see a ridge at every horizontal seam. We never spaced the sheathing--I'm not sure if we used plywood or OSB on that roof. The edges of the sheathing had swollen apparently.
I remember that. It was in a thread where one other guy mentioned that he's once had to replace roof sheathing because of this. Those are they only two I've heard of, but personally never saw one.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Keep in mind that, at most, you only need the gap in joints running at right angles to framing. Butt joints falling on a framing member don't need to be gapped, so it won't throw off your layout.
"Keep in mind that, at most, you only need the gap in joints running at right angles to framing. Butt joints falling on a framing member don't need to be gapped, so it won't throw off your layout."Hmmmm?AdvanTech¯ Installation Recommendations:Storage
Ship panels under tarp.
Set bundles on supports to keep off the ground.
When storing outdoors, cover panels loosely with waterproof protective material.
Anchor covers on the top of the stack, but keep away from sides and bottom to assure
good air circulation. When high moisture exists, cut banding on the stack to prevent edge damage. When handling, avoid dropping panels on corners or edges. When using a forklift, put the stack on a pallet or supports to minimize damage from forks.Panel Installation
Install panels over two or more spans with the long dimension perpendicular to the floor
framing.
Install panels with the certification stamp facing down for ease of viewing by local
building officials.Panel Spacing
Tongue & groove panels are designed to self space on T&G edges.
Space 4' panel ends a minimum of 1/8" at time of installation.
End joints of adjacent panel runs should be staggered.
Square edge panels should be installed with a minimum spacing of 1/8" on all panel
edges at time of installation. Someone's got it in for me, they're planting stories in the press
Whoever it is I wish they'd cut it out but when they will I can only guess.
They say I shot a man named Gray and took his wife to Italy,
She inherited a million bucks and when she died it came to me.
I can't help it if I'm lucky.
They're playing CYA in case the stuff gets soaked and swells at the joint. If the entire panel gains length it will just push the framing farther apart, so the gap is unnecessary.
If your view never changes you're following the wrong leader
if you use clips you will have the spacing.BOB is always right, ALL HAIL BOB
They're playing CYA in case the stuff gets soaked and swells at the joint. If the entire panel gains length it will just push the framing farther apart, so the gap is unnecessary.You know exactly how to crack me up<G> out in the field, we gap it... you should come out sometime<G> Someone's got it in for me, they're planting stories in the press
Whoever it is I wish they'd cut it out but when they will I can only guess.
They say I shot a man named Gray and took his wife to Italy,
She inherited a million bucks and when she died it came to me.
I can't help it if I'm lucky.
"you only need the gap in joints running at right angles to framing. Butt joints falling on a framing member don't need to be gapped, so it won't throw off your layout."
That's the way I look at it too.
If the plywwod expands in the SHORT (4') direction it can't stretch the framing members and make them longer. So you could get buckling if it isn't gapped.
If it expands in the LONG direction, it will just push the framing members apart a hair. So the ends dont need gapped.
My opinion only, of course...
So many lawyers, so few bullets.
That's not been my experience with buckling materials. The fact that they are on a framing member means nothing. The power of the water easily overcomes the nailing.
If two products are swelling and pushing against each other, something has to give and usually the nails are just pulled out or through the sheetstock.
Or, I may be misunderstanding what you guys are thinking.
jimfka (formerly known as) blue
Let me try it another way - If a sheet of ply on a roof expands in the 4' direction - Let's say it swells up to where it wants to be 4' 0 1/8" wide. If there's not gap between it and the sheets above and below, it either has to compress, buckle, or make the framing members longer that it's nailed to.
The early bird gets the worm?
That's just about the worst incentive program I've ever heard of
Aha! I agree. I was misunderstanding.
jimfka (formerly known as) blue
Yeah, I'm thinking that gaps falling on framing members only help compensate for end swelling due to rain exposure -- don't do anything for bulk material changing dimensions with temp/humidity. And even a half-inch gap won't help much when the ends start swelling in the Z dimension.I can see that a sufficiently large gap would prevent water from being trapped following a rain, but seems to me that it would have to be closer to 1/4" for that -- 1/8" would just about "optimize" trapping.
If your view never changes you're following the wrong leader
Read this it touches on spacing:
http://www.coastalcontractor.net/cgi-bin/article.pl?id=167
I always make it a habit to space the osb on roofs at least 1/16". I space it both ways. The ply clips don't quite space it 1/16, so I make a consciense effort to slightly increase the gap. The end gaps on the roof sheathing usually are set at 1/8".
I can't say that it causes a problem on the layout because if you are sheathing a roof 40' long, you will only have four joints. Theoretically speaking, if you start with your first joint spaced perfectly in the center, you would end up having to push the truss over about 3/8" on the far edge seam if you still insisted on the joint landing perfectly in the center of the truss. Personally, those types of variations don't register with me. Even if we are sheathing a 200' roof, I simply would make a saw cut every 5th or 6th sheet. I've never sheathed a continuos 200' run, so I'm not losing much sleep over that.
I developed my habit because of real world experiences dealing with buckled roof sheathing back in the 70's. I've had to strip shingles and replace buckled 1/2" cdx exterior ply and the bosses firm rule after that was "if it's tight, run a saw kerf through the joint". I've followed that directive, on roofs ever since and have ordered anyone that works for me to do the same.
For the record, we used to get cdx ply in Yellow pine versions and fir. The fir was much more forgiving. The yellow pine would swell and the ply would develop valleys and humps between the roof members. It was not unheard of for us to have to take a saw up on a roof after a rain and re-cut some of the joints that we weren't diligent enough with.
I've never had any osb swell like the old cdx.
On wall, I butt the sheets snug.
jim
fka (formerly known as) blue
thanks for your feedback. Unfortunately this one is done so hopefully I won't have any issues. haven't in the past. I think from now on I will gap the roof shaething as you do. It just sounds like a better safe than sorry adjustment worth making. doesn't cost anything to do it."it aint the work I mind,
It's the feeling of falling further behind."Bozini Latinihttp://www.ingrainedwoodworking.com
You'll find out that gaping it really becomes second nature. You're probably going to be fine as evidenced by all those who never gap anything.
I forgot to mention that I always leave a slight gap on t&g subfloor too: expecially plywood! I gap it at least an 1/8th on the ends and about 1 1/16 at the tongue and groove. When the boys get the big sledge and want to pound it tight, I tell them to lay off and just tap in in the right amount.
Necomers to the jobsite will look at me like I'm crazy until I tell them that I've had plywood delaminate many times and the warranty is void unless the sheets were properly gapped. More than once, I've had lumber mill field reps inspect the installations to honor the warranty. When they come out, if they don't see the plywood gapped...I'd end up eating the labor re-installation and the builder would have to pay for the lumber.
I've also had a building inspector warn us that we have to gap the roof sheathing (OSB) and he actually wrote a violation to several framing crews in a sub. I think the guy just finished an OSB seminar or something. Anyways, one time, he noticed that we had a few tight joints on the inspection and I told him "It's a good thing we gapped them when we installed it because it looks like the rain has swollen them tight." That comment actually saved me from having to remove shingles and run a saw kerf in the plywood.
Anyways, I wouldn't lose any sleep over this if you didn't gap. Just keep it in mind in the future if you build in damp regions.
jimfka (formerly known as) blue
I definitely will keep it mind. It's strange how many things you don't give a second thought to initially and then it comes up in a discussion such as this. next thing you know there is a wealth of information and past experiences surrounding that very small detail that could easily be overlooked but at the same time cause all kinds of issues down the road. Thanks for giving me all that feedback.I used the advantech stuff on this job and I was amazed at how nice the t and g joints fell together. just a little tap with the sledge and that was it. nice stuff.later,Jason"it aint the work I mind,
It's the feeling of falling further behind."Bozini Latinihttp://www.ingrainedwoodworking.com
I agree Jason. I really appreciate hearing "real world" experiences because if you listen the the sales reps, everything about their products is foolproof and perfect.
Advantech has internal spacers which automatically spade the tongues and grooves. They also have built in drains to keep the water from puddling. I think.fka (formerly known as) blue