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3 layers?


homnspector
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Uh huh,

Nope, not taking the bait on that one Les.

To answer the original question. No, now there's a tremendous amount of extra weight on that cover and the framing beneath it.

For decades it was always said that two layers was okay but three was too many.

Someone should have told the City of Seattle. I found an old tudor with about 5 (or was it 6?) layers of comp back in 1997 or 1998. 2 by 4 rafters on 24 inch centers with a chord length of about 24ft. (They made 2 by 4's that long back in the day.) I called the code boys down at city hall and wanted to confirm that it was a violation of local code. "Nope, not a violation of code," the guy said, "If they want to put 10 layers on they can and they're doing it at their own risk."

I remember somewhere around 1997 where truss manufacturing engineers were finding problems with trusses built in the 60's with two layers of comp on. The folks in my franchise network were told that the industry was revising its thinking and no longer felt that two layers on a truss were okay - that more than one would cause damage. The franchiser issued a memo about it. I'll see if I can find it somewhere.

ONE TEAM - ONE FIGHT!!!

Mike

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I'd call it as 3 w/a rolled mineral edge "starter".

As much as I hate to say it, # of layers is only important as roof pitch. Steep pitch makes excessive load less a consideration. I see a lot of very steep 3 & 4 layer roofs that perform just fine. Multiple layers precludes ice & water shield though. [:-irked]

That said, anyone that'd install 3 layers is despicable, and should be cast into the 7th level of Hell. Caulk the edges to prevent wind uplift my ass......

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Oh yeah - I forgot to tell you to wire brush the edges, so the caulk could stick.

I have seen roof surfaces with five layers on a 9/12 that have pulled the fasteners toward the drip edge at least 3/4".

To my knowledge the issue always has been weight - 1915 thru 1941 tab shingles were approx 285lbs per sq. Then dropped down to abt 235 per sq, then 225 per sq then 185 per sq. I know there were and are exceptions, but the difference is great; 1lb per sq foot. Rafter design and truss design relied on shingle weight, decking weight and pitch. Work these variables around and you can see what can happen with multiple layers.

That is why you should caulk between each layer at the edge, stick the nozzle of caulk gun under abt 3inches and inject caulk then run a course of brick along the edge to hold them down, until the caulk is dry.

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I'm with Kurt. More than two layers of asphalt shingles is wrong, but I have yet to see a roof with 3 or 4 layers that seemed to suffer because of the additional weight. I inform my clients of the number of layers, but I don't get my knickers in a twist over it.

You've got to admit, it's pretty damned hard to make a roof with 4 layers of shingles leak. (Flashings are another matter entirely)

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The picture is a house I am inspecting today, I stopped by for a quick look and saw the multiple layers. I know it is recommended not to go over 2 layers and it probably voids the manufacturer warranty, but my real question if there is no visible truss sag, the fasteners are long enough to penetrate the sheathing, no significant sheathing sag, what would you recommend? Seems excessive to call for stripping the roof. Outer layer of shingles is maybe a year old.

Les, caulk is pretty messy, especially roof cement. Couldnt they just duct tape the exposed edges?

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Use all weather type and you should end up with a nice looking edge!

Seriously, I strongly recommend never going three layers, regardless of pitch.

When I see three layers and it is flashed ok and surface looks good, I tell them "this shingle will not perform as intended".

If you think in terms of: 1st layer 235lbs less wear = 200lbs, 2nd layer 235lbs less wear = 200lbs and 3rd layer 185lbs less no wear = 185lbs; then you have 585lbs per 100 sq ft, plus any applicable snow load and/or live load, decking weight +, and 30lb felt, then folks begin to understand the consequence of three layers. On a 4/12 roof that load, plus me prancing around up there gives the buyer a good visual. Also I always slip in "When you get ready to replace shingles, it will cost you lots more, maybe twice as much".

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Hi Les,

Yeah, I've had those roofs. The upper layers slipping so badly that I'm not even sure it's safe to walk across the roof for fear the whole thing will pull away from the layer below and begin an avalanche. Usually find 1 inch nails being held in place by asphalt only. Then there's the listing agent (and sometimes the buyer's agent) - "What?! That roof was just replaced. What do you mean it's no good??? My inspector would never make such a ridiculous call!"

ONE TEAM - ONE FIGHT!!!

Mike

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About 6-7 years ago I was inspecting a house while there was a third roof layer being installed. The listing agent was very proud of the fact that the seller was paying to install the new roof as part of the deal.

I mentioned to my client that a third layer is not recommended and was not typically allowed since 1990. I recommended that he call the town to check if the required building permit was obtained to install the new roof.

During the afternoon I got a call from my client and he told me that a permit was not obtained and that the local construction official had gone out to the site and issued a stop work notice. I found out later that the brand new shingles had to be stripped along with the two existing layers and another roof layer was installed.

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Homenspector,

Give me the details of span, pitch and size and I'll run you an engineered truss profile tomorrow with 3 layers of comp. compared to standard design loads. I can post the results or even email you anything interesting that comes up.

For anything more specific I need to know if it's hipped and if there are ceiling conditions within the house. A tray or cathedral will make a HUGE difference on what materials might "bump" up to in design.

I don't know what your loading is in AZ, but here in OH it's 25, 10, 10, 90mph gust/wind unless specified otherwise. I expect your wind may be less.

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Kurt,

As an engineer for a truss manufacturer, I agree with your assessment in general. BUT, let me give you guys a typical scenario that could happen to any house.

Let's say I'm designing a truss to span 48ft and is tied into a girder that spans across the garage 16ft. into the garage. I decide to pick up the garage/entry wall as bearing to reduce the load on the girder so I have a three point bearing truss. (outside wall-garage wall @32ft-girder=48ft.)

Now the big problem here is uplift. With 3 points I've essentially created a lever. As a "standard" designed truss, this will never work. The truss will fail, upgrade lumber, require a specific web pattern. I take that into account but may still have to work to get it above industry standards.

Add 2#s/f (maybe less)and that whole dynamic has changed. The wind hitting the far side is acting like a kid pushing down on a seesaw and now has a multiplied effect and in extreme cases can rip the girder off the wall. In the very minimum case, any hurricane tiedown is now inadequate.

That got me thinking, since I'm new, maybe I can help earn my keep. I can post a list of material loads if you guys think it might be useful. Asphalt shingles, slate, 5/8", 3/4", concrete flooring, comp. shingles, sheetrock, yadda yadda. I've got a huge design list I go through every hour.

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jseddy,

That's good info; I like having a truss engineer in the mix here. No argument; I defer to the engineer.

Then, there's the world I live in, where 4 layers of rolled roofing on the original cedar shingles can be found on roof's built w/ 2x4's on 32" centers. Balloon framed, no less. Horrifying, but the damn thing's have been sitting there for 100 years. They survive because the pitch is about 10:12. And, the lumber is old growth.

That doesn't stop me from harping on my customers to tear it all off though; there's few things nastier than 3 layer roofing.

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Kurt,

WOW! You didn't walk that did you? BALLS if you did.

I tell you what, in defence of that situation, I can't say anything "engineered" today could stand up to that. SPF and Hm-fir is NOT oak or what anyone 60 years ago would have chosen.

slightly off topic- if you think about it, they don't even really build in europe. They don't have a "housing explosion" to speak of.

even more off topic- what do they call a 2x4, 2x6.... in other countries? I'm serious. I've never thought of it till now. I asked a buyer at my Co. and he didn't know.

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Originally posted by jseddy

slightly off topic- if you think about it, they don't even really build in europe. They don't have a "housing explosion" to speak of.

even more off topic- what do they call a 2x4, 2x6.... in other countries? I'm serious. I've never thought of it till now. I asked a buyer at my Co. and he didn't know.

Well, not an explosion; more of a controlled expansion. Just read about a project in the Netherlands where a developer is building a large floating housing complex. There are mega-apartment complexes in the works throughout Europe & Asia. Only in America do we get the big box mentality on 1/4 acre lots.

2x4's in other countries are called "firewood". (Yes, I know, non-PC.)

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Hi,

My vocabulary is rusty, but if memory serves in Germany studs are typically referred to as bretter. You have Wandbretter (wall studs), Fussbodenbretter (floor joists) and Dachbretter (ceiling joists). Size is normally expressed in millimeters, not centimeters.

I remember that the singular was Brett and I believe that the plural was Bretter but it might be Bretten instead. It's been about 15 years since I've cracked a Duden open or carried on more than 10 minutes conversation in German.

ONE TEAM - ONE FIGHT!!!

Mike

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Your both right,....sort of

From the Certainteed Single Applicators Manual:

ROOF-OVER INSTALLATIONS MAY BE CONSIDERED WHEN:

  • No more than one layer of shingles (not including a layer of sawn square butt wood shingles if local codes approve this as a substitute roof deck? are in place on the existing roof
  • After careful inspection, the roof deck is found to be strong and to provide a good nailing base.
  • The combined weight of the first and second layers of shingles will not exceed the rated carrying capacity of the deck.
  • The contractor is certain all roof system components, especially flashing and valleys, can be properly repaired or adapted to the roof-over installation.
  • Manufacturer's instructions do not prohibit roof-over installations.
  • The cost to dispose of old roofing materials would be prohibitive.

    NOTE: Concern is growing regarding the disposal of old shingle and roofing materials in landfills. Some landfills prohibit or restrict such disposal. Others charge a premium. Research has been underway for some time to develop recycling methods. Some progress has been made in the process of turning roof waste into a component for road construction. However the profitability of recycling is yet to be proven. Some believe that roof-over is a desirable approach with the hope that one day an economically viable recycling method will be found that solves the problem. In the meantime, they believe old roofing is best stored on the roof.

  • Another argument in favor of roof-over is based on the belief that the first layer is additional insurance against leaks.
  • The homeowner enjoys a clear price advantage by avoiding the added cost of tearing off.
INTEGRITY ROOF SYSTEM REQUIRES TEAR-OFF

ShingleMaster or SELECT Roofer companies choosing to install a Certainteed Integrity Roof System are aware that one of the system's requirements is that, whiteout exception, underlayment and shingles must be installed over a clean roof deck. The two primary reasons for not allowing a "roof-over" are that they:

  • Are more prone to workmanship errors.
  • Are more likely to hide decking defects. (By tearing off the original layer, a more thorough inspection of the roof deck condition is possible.)
When Roofing Over Existing Shingles...

If old roofing will not be torn off, check local building codes for the maximum number or roofing layers allowed (usually two, sometimes three) and maximum weight per unit area. Check the underlying deck to be sure it is sound and will provide good anchorage for nails. Here are the requirements for specific types of shingles:

  • Shingles weighing more than 350 lb/square (such as super-heavyweight Grand Manor Shangle and Carriage House Shangle): If the old roof consists of two or more layers of standard-weight shingles or one layer of heavyweight shingles, it is required to tear off existing roofing, repair decking and/or install new decking.
  • Other asphalt roofing shingles (except lock-type or dutch lap): Make the old roofing surface as smooth as possible by replacing missing shingles, and splitting, nailing flat and securing all buckles, raised tabs, and curled shingles. It is recommended to cut old shingles back flush to the rakes and eaves. Another recommendation is to apply corrosion-resistant drip edge along the rakes and eaves to cover the edges of the old shingles. Use no underlayment over the old roof, and apply roofing in accordance with product application instructions.
  • Square-butt, sawn-wood shingles: Apply beveled wood strips to all courses to obtain an even base.
  • Lock-type, dutch lap or wood shingles (other than sawn square-butt style): Remove the existing roofing, and follow tear-off instructions.
I think saying that "Most shingle manufacturers will void any warranty if their product is installed on a preexisting shingle layer," is too strong. Some do but I think most don't. How do you know when you look at a shingle that's 10 years old and installed on a roof-over what the manufacturer's specs are? Answer - you can't know unless you're willing to do a whole lot of stuff that inspectors don't do to find the answer. I'm not willing to do that and I bet neither one of you are either. I think it's fair to warn the client that some manufacturers don't warranty their product on a roof-over, as long as you understand that the some is in the minority and most will honor the warranty as long as the rules outlined above - and any manufacturer-specific rules - have been followed.

ONE TEAM - ONE FIGHT!!!

Mike

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