So I Have Shiplap Planks For My
Roof Deck – What Now?

September 1, 2022

What Many Roofers Won’t Tell
You About Shiplap!

Originally, “shiplap” was used for roof decking due to the watertight nature of its design. The overlapping wooden boards of a shiplap deck formed a perfect seal with their tongue and groove structure, kind of like modern flooring.

It was a great system when paired with the soft, flexible, felt-backed shingles of the day, but modern shingles aren’t nearly as flexible.

Since Seattle’s temperatures (and humidity) can jump from one extreme to another, the thin slats of shiplap decking move quite a bit. Modern asphalt shingles are not meant to move around, so this can cause nail pops and leaks.

So instead of the intended wind and waterproof barrier, you have an unstable surface covered with inflexible shingles. A roof like that will fail in half the time it should.

Why Do Some Seattle Roofers Say Shiplap Is OK?

Most Seattle roofers are all about getting that sale, and nothing else. So, when they see that you have shiplap decking, they tell you not to worry.


Because giving you the right answer – that shiplap decking is a problem with asphalt shingles – will make their bid go up, and they hate that more than anything. Their game is one of numbers; a race to the bottom. But we refuse to play that game. We guarantee our work for life, we won’t compromise quality to just tell you what you want to hear.

So when we come across shiplap decking, we cover it with modern decking materials that meets current building codes so we can ensure that your asphalt shingle roof will last it’s intended life.

The Many Problems With Shiplap Decking

Nailing modern shingles over old-school shiplap decking is never a good idea. It may seem like a handy cost-cutting solution, but you end up with a roof that simply won’t last.

Here are just a few of the problems you can experience if you don’t cover shiplap with modern decking materials.

Swiss Cheese Decking

Shiplap Knocked Out Knotholes Seattle SmartroofOver time, the movement from the boards expanding and contracting as temperatures change will make the knots in the boards fall out, causing what we like to call “swiss cheese decking.”

As you can see to the left, many knotholes have fallen out of the shiplap boards, leaving space for outdoor temperatures to enter the attic space without hindrance.





Shiplap Nails Blowing Through Gaps Seattle RoofsmartSo when the new shingles are installed, the nails can shoot right through the open knotholes without the installer noticing. The result is a roof that leaks from day one.

Look at the nails in this image. They have gone straight through a crack between the shiplap planks. Water will be able to travel directly from the shingles and into your home.

Nail Pops, Loose Shingles, And Leaks

When you nail through a modern shingle into shiplap decking, you are setting your nails up for failure. The movement of the shiplap beneath the rigid shingle acts as a slow-motion prybar on the nails. Eventually, they will work themselves out leaving you with nail pops.

These raised nails then allow your shingles to loosen, creating gaps in your roof’s protective layer. So instead of directing rainwater to your gutters, it’s directed straight into your decking to cause leaks throughout your roof system.

Shiplap Popped Nails Under Asphalt Shingle Seattle Roofsmart

Trouble above, popped nails, leads to:

Shiplap Watermarks From Popped Nails Seattle Roofsmart

Trouble below, water damage from leaks

Decreased Energy Efficiency

With all the missing knots and nails that have wiggled loose, you can also have problems with your roofing system’s integrity. 

Gaps and missing knots in the shiplap roof decking will allow airflow through the roof’s surface, causing decreased energy efficiency in winter and summer. This can prematurely degrade the quality and performance of your roofing membrane, costing you extra money on heating and cooling. It can also accelerate condensation and saturation of the wood from underneath. 

Code Violations

Local building code specifies that shiplap or plank roof decking must be covered with modern decking materials when working with asphalt shingles. Considering all the problems shiplap and wide boards can cause, this is no surprise. Humanity has been learning as we go, new codes are the additive result of this.

Solidly Sheathed

Building Code for Sheathing

Manufacturer’s Warranty Could Be Voided


The picture above is from IKO’s Limited Warranty on their asphalt shingles. As you can see, it clearly states that installing asphalt shingles over shiplap is not recommended.

If the manufacturer finds out your shingles are mailed to shiplap, your warranty is voided on the spot. 

Your Best Solutions For Dealing
With Shiplap Decking

When we go to a home that has shiplap decking, there are only two acceptable solutions: cover the decking with modern materials like plywood, or convert to a metal roof.

While a bad surface for shingles, shiplap is a great surface for installing modern deck sheeting. And you don’t need to rip up or cover shiplap decking unless there are areas with rot or mold. With the larger panels in place, the movement in the shiplap is negated.

With metal roofing, you don’t even need to add modern decking.

Shiplap Gorgeous Metal Roof Installation Seattle Roofsmart

With massive, flexible panels, standing-seam metal roofs are able to ignore the movement of the shiplap layer. While a little bit more costly over the price of new decking and a shingle roof, metal roofing means you’ll never have to mess with it ever again. 

If you need help with a roofing project of any kind in the Seattle area and want it done by installers who know the rules, contact RoofSmart to schedule your free consultation.

  1.  https://up.codes/viewer/washington/irc-2015/chapter/9/roof-assemblies#R905.2.1
  2.  https://www.iko.com/wp-content/uploads/sites/6/2014/05/IKO-Industries-Unified-Warranty-Booklet-EN.pdf


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