Pros and cons of holding the sil back 1/2 to allow sheathing to be flush and letting overhang?
skrebs
| Posted in Construction Techniques on
Hello. I am brand new here. I am researching to build a 30 x 50 structure with 16 x 30 being livable space. THis will have a slab foundation that I intend to hire out.
I have researched this topic a lot. Read other threads on other forums, read journals, watched videos, etc. I am just perplexed. I can’t wrap my head aorund some people considering some things as being overly concerned but then those people turn around and are overly concerned about an almost identical issue.
I don’t understand so many stances when it comes to this topic and I’m hoping you can help me. The first thing is, slabs are not always square and adjustments have to be made for the walls. I understand that and just want to say it now so it doesn’t come up again and again.
Now my question is, do you let the sheathing overhang or allow it to be flush? What confuses me is people say, “Don’t let it be flush because that will allow water to wick in and allow air and so on and so on.” They also say it will have more of a chance to bow. And they bring up all of these concerns and say, just let it overhang. But they don’t mention painting the bottom edge of the sheet or protecting it in any way when allowing it to overhang. Then when you mention that they say, “You’re overly concerned.”
But aren’t they being overly concerned with the flush method? I mean the sheathing will not come into contact either way. If you allow it to be flush you’d set the sheet probably a carpentar’s pencil width above the concerte. If you overhang it, the sheathing is just as supported. And as far as wicking it water, isn’t that what weather stripping, silicone, flashing, wrap, and siding are all for? I mean if you make it flush you can cover that gap in so many ways.
Why are so many professionals concerned with the flush method and all of these solvable problems but not concerned with protectioning an exposed sheathing edge that is 5 1/2 inches from the ground and the very least assuming you have a 6 inch above grade slab? Isn’t that just as concerning? I don’t understand how one second something is a big deal then suddenly it’s, “You’re overly concerned.”
So which method is better? Which has more pros?
I’m inclided to do it like – Put down the sil sealer, align my sil plates as evenly with the edge of the slab as possible while keeping it square. Then let the sheathing overhang by about a 1/2 inch. Then run weather stripping over the concrete and sheathing. Then hang flashing with a drip edge at the bottom of the sheathing, put on the wrap, then run weather tape over the top of the flashing where it meets the wrap. The only thing I’m concerned about is the back of the sheathing touching the concrete where it overlaps.
Thanks.
Replies
HI there,
I will explain the method we use for detailing sills and sheathing on a slab foundation. Some of the details are simply hold-overs based on tradition going back quite a few years. The first detail and question you are asking about is whether or not to let the sheathing overhang the slab edge. Most builders, including myself, let the sheathing overhang the slab edge by about 1". The reason for this is to cover the joint between the top of the slab and the sill. This is a weather and wind detail that is less important now given materials and technology we have now. To me, it is still just a smarter detail than to have the sheathing cover just the sill. The sills, as you mentioned, need to be square regardless of whether the slab is. Sometimes this means letting the plates overhang the slab up to 1". All of the plates should overhang or be flush with the slab edge. Plates installed inside the slab edge willl cause the sheathing to bulge where it overhangs the slab. You are right to consider the vulnerability of the exposed sheathing edge to weather damage, but whether or not the sheathing overhangs the slab a bit or not, does not matter. All wood has to be at least 8" above finish grade regardless of how much it overhangs (or doesn't). That distance combined with proper drainage, grading, etc. should keep that exposed sheathing edge dry. As for the back of the sheathing being in contact with the slab, the contact, if there is any, should be moderate and not a concern. In really wet areas, or areas with destructive insects, it is a good practice to start the sheathing with 18" - 24" rips of treated ply. Regardless of the sheathing material, we always start with 12" Vycor that wraps the bottom edge of the sheathing, then paper, drainage, siding, etc. That's my take on it.
Wrap the bottom edge of the sheathing with flashing tape then run it an inch past the concrete. The sheathing is safe from rot and the lap prevents water intrusion.