Dangers of Installing Architectural Shingles Over a Shiplap Deck
- micahjpagett
- May 18
- 5 min read
If your house is 50 years old or older, chances are high that you have what is called a Shiplap Roof Deck. Even though this was the standard practice of the time, a Shiplap roof deck is no longer considered an acceptable surface on which to install modern asphalt shingles. We will go into the reasons here shortly, but let's define our terms first.
What is Shiplap?
Shiplap planks are individually milled boards manufactured with notches on the top and bottom, designed to overlap each other, making for a theoretically water tight connection. It is a nearly identical concept as a tongue and groove material, with some significant design differences
What Do You Mean by Modern Asphalt Shingle?
Admittedly, the term modern shingle can mean a lot of things, depending on the range of time we are considering. For our purposes, a Modern Shingle will refere to an Asphalt Architectural Composition Shingle made with a fiber glass matt.
Some key factors to consider with a modern shingle are the transition to a fiberglass matt, and the phasing out of '3-Tab' shingles in favor of the double layer laminated architectural style shingle.
Fiberglass shingles became the favored over its organic counterpart in the 1980's. They are more flexible and offer a higher fire resistance rating. Today, organic shingles are no longer made.
3-Tab Shingles are still manufactured, but are being increasingly phased out. Its modern counterpart the - architectural shingle - offers a thicker, two layer system that is designed to more closely mimic higher-end roofing material like cedar shakes/shingles. Architectural shingles will theoretically last longer and offer a higher level of fire resistance because of the increased thickness.
Why Shiplap and Architectural Composition Shingles Don't Work Together
Now that we have our terms defined, let's talk about why modern shingles don't work with that 50+ year old shiplap roof deck.
Over the 50, 60, 70 years or more those shiplap planks have been on your house, they have undergone some changes:
Plank Movement - Through expansion and contraction, board shrinkage over time and house settling, those shiplap boards have shifted since their installation. Even though the design may have once provided a tight fit between overlapped boards, this is usually no longer the case. The majority of boards will have significant gaps between planks.
Large Knots Holes - Because of wood shrinkage and plank movement, it is common to find numerous large knot holes in a shiplap deck.
Hardened Wood Splits Easily - Overtime, wood hardens as it dries. The harder the wood, the less flexible and more easily split. So even if your shiplap roof deck has minimal knot holes, and the gap between planks is negligible, it is extremely likely that during shingle installation, the new nails will cause the wood to crack, split and even break off whole chunks throughout the roof.
At this point, it may seem obvious why installing any shingle over a shiplap deck is a bad idea, but we are not quite through. Architectural Composition Shingles in particular do not work well with a shiplap deck.
A relatively minor contributor is the fiberglass matt. The increased flexibility causes the shingles to more easily form to whatever surface it is sitting on. Including gapped, knotted, warped and split shiplap. In these circumstances, water does not shed as easily - making it more likely for moisture to end up on the wrong side of your shingles.
The fiber glass matt is a minor contributor as 3-Tab shingles use the same fiberglass material and seem to have worked well over shiplap decks in the majority of instances.
The more significant contributor to architectural composition and shiplap incompatibility is the design change from 3-Tab. 3-Tab Shingles have wide gaps between each tab which allows water to more easily shed.
Architectural shingles are made of two pieces laminated tightly together. While this provides a thicker and more durable shingle, if the shingles are installed on a compromised surface like shiplap, water will not shed as easily.
The Solution
Long story short, modern shingles need to be sitting on a flat surface. The standard practice for roofing over Shiplap is to install Plywood or OSB on top of the shiplap deck. This process is what we call an overlay. This provides a nice flat surfaces that will work easily with modern architectural style shingles. The solution is simple, but it will unfortunately increase the roof price significantly as the entire roof deck must be overlaid. To give context, an average three bed two bath house will require 60-80 sheets of plywood or OSB for an overlay.
Is an Overlay Absolutely Necessary on a Shiplap Deck?
The answer to this question depends on the risk one is willing to take. It would not be true to say that 100% of architectural composition shingles will fail prematurely when installed on a shiplap roof deck. But they will fail often enough that we do not believe it is worth the risk of having to tear off a roof 3 years after installation.
Conversations like these are never easy to undergo. An overlay can easily increase the price of your roof by 50%. As a homeowner, the choice is ultimately up to you whether or not you hire someone who is willing to roof directly on shiplap.
At Moser Roofing, we had to learn the hard way when 3-Tab Shingles were being phased out. Because we have been around awhile, we know it's not worth the risk. We will, under no circumstances, install architectural composition shingles directly over shiplap. Many manufacturers will not even warranty their shingles if installed directly over shiplap, while others have extremely stringent criteria concerning the condition of the shiplap in order to qualify for a warranty.
If you know that your house is over 50 years old, it is wise to make sure your roofing contractor has checked whether or not you have a shiplap deck. If they are willing to roof your house without overlaying the shiplap, we strongly recommend working with someone else, or at least getting a second opinion.
We do our best to find out as much as we can about the roof before the job begins, and we encourage you as a homeowner to do the same. Insist that your contractor be as thorough as is reasonable. The worst way to find out you have shiplap is when your shingles are being torn off.
Here is a great video from Roof Life of Oregon briefly going over the topic:
If you have questions, or would like one of our estimators to check your roof out, feel free to Contact us. We provide free attic and roof inspections, as well as free no pressure estimates if you are curious about the price of a new roof.
Comments