Fiber-cement panels over plywood


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Old 01-05-10, 04:28 AM
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Fiber-cement panels over plywood

My house has existing T1-11 plywood siding without sheathing behind it, only builder's paper. I'm in FL and this is OK with code. It's a 2-story 2x4 framed house about 15 years old.

I'd like to install fiber-cement panels ("vertical siding"), which needs sheathing behind it, directly over the plywood with new 30# felt behind it. A few questions about this:

1. Other than budging things out a bit (like window trim), do you foresee problems with doing this?
2. Is weight of the cement siding a problem?
3. To make the panels easier to handle, I'd like to cut them in half lengthwise (2x8). My thinking is the plywood is providing the structural. Problems with this?
4. Is there a problem with having the plywood sandwiched between two layers of builder's felt?
5. Any other concerns about cement siding I should consider?
6. Are power shears suitable for cutting panels or are they primarily for short cuts?
 
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Old 01-05-10, 06:29 AM
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1. Other than budging things out a bit (like window trim), do you foresee problems with doing this?

The area around windows will be very susceptible to leaks. You will want to get nice tight fits around your window trim and caulk with a caulking that specifically says it is for fiber cement on the label. Vulkem 116 is one of these. You should also install new drip cap flashings over your windows.

2. Is weight of the cement siding a problem?

No.

3. To make the panels easier to handle, I'd like to cut them in half lengthwise (2x8). My thinking is the plywood is providing the structural. Problems with this?

Yes, don't do that. Your house is likely framed with studs 16" OC. Cutting the siding into 24" strips would mean that the edges would not be nailed into the studs. This is not allowed. If your house is 2 story, you need to break the top and bottom edges of the siding on solid wood as well, which means locating the rim joist. If there is a seam in your plywood siding, your seam will likely be in the same place. You will need to use a z-flashing (drip cap) between the 1st and 2nd story layers.

The sheets of cement siding are very heavy. You will want an additional person (or two) to help you.

4. Is there a problem with having the plywood sandwiched between two layers of builder's felt?

No. You will just want to make sure that no water is getting behind the felt paper near the top of the buiding envelope. So don't forget to caulk to the top of the siding where it meets the soffit and/or rake fascia.

5. Any other concerns about cement siding I should consider?

Wear a dust mask when cutting, let the wind blow the dust away from you.

6. Are power shears suitable for cutting panels or are they primarily for short cuts?

They are for short cuts. Unless you are getting paid by the hour, then go crazy. LOL. Seriously, you will want to use a skilsaw with a fiber-cement blade for any rips.
 
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Old 01-07-10, 07:08 AM
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Originally Posted by XSleeper View Post
3. To make the panels easier to handle, I'd like to cut them in half lengthwise (2x8). My thinking is the plywood is providing the structural. Problems with this?

Yes, don't do that. Your house is likely framed with studs 16" OC. Cutting the siding into 24" strips would mean that the edges would not be nailed into the studs. This is not allowed.
Well that was a pretty dumb question - you're right that the studs are 16" OC. But what about 16" and 32" strips, at least for the second story? Vertical seams would be covered by battens and yes horizontal seams would have z-flashing.
 

Last edited by suobs; 01-07-10 at 07:47 AM.
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Old 01-07-10, 09:13 AM
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You could, but I think you would be asking for trouble. By ripping them into strips (which in reality would probably be 15 7/8" wide) you could potentially run off layout pretty quickly unless your rips are PERFECTLY straight, and unless you maintain an exact 1/8" gap between panels to make up for the kerf of the saw blade. It's also VERY hard to get a nice straight cut on fiber cement. As the blade gets dull, your cuts will start to wander. Not having perfectly straight cuts can also cause problems. You would also have to hope that your framing is exactly spot on. Extra seams double the leakage potential and you could potentially have trouble nailing those vertical panel edges, unless your seams are EXACTLY in the center of each stud. Panel edge nails must be a minimum of 3/8" away from the edge, so your nail pattern up each stud will be 7/8" wide (3/8" from edge + 1/8" gap + 3/8" from edge). So if you line everything up PERFECTLY (which you will have a hard time doing since all the studs are hidden- covered by your t1-11 siding) those panel edge nails will be biting into 5/16" of stud on each side. (you can nail at a little angle but not much, since the nail head has to be flat) If you deviate even a little, you risk missing the stud. So it will help if you go around the entire house, verify exactly where the studs are, and chalk vertical lines on your housewrap for your center of stud locations. As long as your chalk lines are accurate, it will really help avoid some of the problems mentioned above.
 
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Old 01-07-10, 01:24 PM
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I get what you're saying. Those are problems nailing any panel to a stud even if the studs are visible. And I'm multiplying the problem by increasing the number of seams. Plus I can't look at the inside to see if nails missed. Maybe screws would be better, since I would feel a missed stud?

I just don't like the idea of maneuvering a 4x8 sheet of cement board around on a ladder or scaffold, even with 2 or 3 people.
 
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Old 01-07-10, 09:25 PM
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May want to check with your local Building Department as to what you plan. May be requirements for the ripped panel as now they are hybrid with special nailing in your high-wind area. Have to pre-drill each batten nail or risk cracking the panels. Screws may not be allowed.... Prime the backs and edges, remember concrete products are like sponges. Use real builders paper: http://www.fortifiber.com/pdf/fortif...sk_vs_felt.pdf

Be safe, Gary
 
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Old 01-08-10, 08:25 PM
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I'll do that. But prime the backs and edges? I thought it was preprimed and waterproof anyway . . .

Now that you mention it, what exactly happens when a fiber-cement panel gets wet?
 
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Old 01-08-10, 09:05 PM
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Its not exactly "waterproof". Moisture will cause the product to expand and if it is continually wetted, (too close to ground, too close to a roof, etc) it will eventually disintegrate. They also say to keep building materials dry (covered) on site, and to not put up the product if it is wet.

Additional priming is always a good idea but to my knowledge isn't required by most manufacturers. With battens over the seams there is a lot of leak potential there.
 
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Old 01-08-10, 09:17 PM
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Could I just dip my house in a few inches solid plastic?

Seriously, the main appeal of fiber cement was avoiding rot. I suppose there had to be a downside . . . but I'm not sure there is a functionally useful distinction between something that can rot (exterior plywood) if it gets wet and something that can disintegrate (fiber cement)! The end result is the same - me fretting over cracking paint and fixing it again and again.
 
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Old 01-08-10, 09:23 PM
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The only cases where I have seen it disintegrate is where installers did not follow the mfg. installation instructions, putting it in contact with a roof, or too close to a roof, or sidewalk, or steps, etc. Or bad flashing details where kickout flashings were not installed.

Such as that shown in this image link:

http://paragoninspects.com/images/wa...ber-cement.jpg

It's a good product so I wouldn't fret too much. But to say it's waterproof would be stretching the truth. In many cases, field cut edges (like you would create if you ripped a panel) don't get primed and painted as the mfg. recommends and that is the start of the problems. Follow their directions and you have held up your end of the deal.
 
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Old 01-08-10, 09:32 PM
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Thanks, just kidding.

Am understanding this right - you disagree that the edges and back need to be primed? Doing that nighmarish little project in my garage is one of the main things I'm trying to avoid by switching to fiber cement. I want something I can put up, not obsess over whether it rains once before I paint it, and then paint it and call it done.
 
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Old 01-08-10, 09:45 PM
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Well, yes and no.

I am saying that yes, the product usually comes ready to use, preprimed. And no, in that the mfg. does not usually mandate backpriming. But if you were to add additional primer as GBR recommended, it certainly couldn't hurt. Especially since you are in such a rainy climate.

Even in your garage it wouldn't be a big deal. Take a 4" roller and quickly prime the perimeter (outer 4") of each sheet on it's back side, and just transfer the stack from one spot your garage to another. This technique is kind of a throwback to the best methods used when installing 303 plywood siding. Backpriming makes wood products last a lot longer by controlling the amount of moisture they soak up. In theory, the same would apply to fiber cement.

But you DEFINITELY have to prime and paint every new cut edge that you create. Regardless of whether or not it is exposed, or is covered by your battens. Failure to do so would certainly void the mfg. warranty. This is something that HAS to be done as the product is installed. If you don't, you may well have problems a few years down the road, as was shown in the picture in that link above. The final coat of paint can be applied later, but field cut edges should be coated right away, or they could easily be missed.
 
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Old 01-10-10, 06:36 AM
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Thanks for the good advice and I recognize that you're helping to extend the life of the product.

Just FYI, it does say in the Hardipanel installation instructions to prime or paint cut edges, but there's nothing about backpriming. They do say the bottom of the siding should be min 2" above roofing or other surface. However, as near as I can tell from what's in the stores, only the front (exterior) is primed, suggesting that the manufacturer doesn't recognize an issue with the back or edges.

They say that unprimed products should be painted within 90 days, and the primed products within 180 days, and that it should not be painted when wet, all suggesting that it will tolerate some rain, painted or not.
 
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Old 01-12-10, 09:14 PM
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Sorry to worry you so. Back-priming is not required, but a good idea. We are talking fiber cement here, porous like a sponge. It is called a moisture reservoir cladding, one that would be best not coupled to the building.

Have you looked into rain screens? http://www.greenhomebuilding.com/pdf/RainScreen.pdf

http://www.nchh.org/Portals/0/Contents/Article0440.pdf

Be safe, Gary
 
 

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