Home>Discussions>EXTERIORS>Do I need to put ply over shiplap before installing new composition shingles
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ET0520
Do I need to put ply over shiplap before installing new composition shingles
ET0520

I am replacing my composite roof with a new composite roof shingle because we have had so much traffic related to various chimney repair and maintenance, accessory vent installs and modifications, etc... that it is showing signs of definite wear and I don't want to wait for the leaks to start. So, recently I was told by one of the many sales associates providing an estimate that I will need to have the existing shiplap roof boards covered with 1/2" ply before applying the felt and composition shingle or the shingle manufacturer will not warranty the job. To support their position, they read an excerpt from an installers guidelines book that was provided by the shingle manufacturer that made it sound like it needs to be done for a proper installation and said that the warranty would be voided if not done that way. The case the sales rep was making was that the shiplap shrinks and grows at far more different rates than the ply and without using ply overlay increases the likelihood that the roofing job would not last for the guaranteed lifetime period. The sales associate said that every shingle manufacturer requires that ply be installed over shiplap or the warranty will be voided and that the competition is taking a risk to our detriment that a fully warrantied and quality job will be the outcome. Has anyone else heard of this, or am I being misled?

A. Spruce
Re: Do I need to put ply over shiplap before installing new composition shingles
A. Spruce

I would recommend that you go to the manufacturer's website of the roofing material you want to use and see what they have to say about the proper installation of their product. I did a quick Google of your question and came up with another forum discussion, the responses split about 50/50 on whether or not a plywood overlay is necessary. One of the responders posted a manufacturer's recommendation that spelled out that gaps in the shiplap under 1/4" were fine, gaps in excess of 1/2" and/or missing knots would require an overlay of 1/2" plywood.

The concern is that movement of the boards will cause ripples in the paper and shingles, resulting in leaks and poor appearance. Additionally, gaps will reduce nail bite, due to nails going though the gaps instead of into the wood, resulting in shingles that are not secured properly AND likely leakage points due to holes in the roof materials.

IMHO, a plywood overlay is very cheap insurance for the best possible installation, roof life, and warranty of the new roof. Would I necessarily go with the one and only estimator that demanded it be done, no, but I would solicit revised bids from your other estimators to include a plywood overlay in their bid so that you can compare all bids based on the same information.

Things to watch out for:
1 - OSB and plywood are two different things. I would recommend plywood because it is more dimensionally stable than OSB.
2 - Thickness of the sheathing, some say 3/8" is sufficient, some say 1/2". IMHO, 1/2" is the way to go, ultimately, in this instance, it likely doesn't matter, unless specified by the roofing manufacturer.

Fencepost
Re: Do I need to put ply over shiplap before installing new composition shingles
Fencepost

Prior to the use of sheet goods on roofs, the two comon ways of sheeting were shiplap and skip sheeting.

Skip sheeting is commonly used with wood shingles and generally consists of 1x6 boards lain horizontally with about a 4-6" gap between each board. This gap helps keep the underside of the shingles dry, preventing rot and extending the life of the roof. Of course this will require sheeting with plywood prior to installing asphalt shingles.

Shiplap, as you're aware, is a more "solid" form of construction. Expansion and contraction across the width of the boards could result in movement of the shingles, which could break the adhesive bond between shingles or tear the shingles where they are attached.

Before adding plywood, you may want to consult with a structural engineer to ensure that your roof structure can withstand the additional weight of the plywood.

Prior to installing asphalt shingles over plywood, you may want to consult with the shingle manufacturer. They will be a more authoritative source than a sales rep, who may not be familiar with common historical construction techniques.

From the Malarkey Roofing Products installation guide for 3-tab roofing:

Quote:

Roof Deck: The surface to receive the new roofing should be in good shape and constructed of a minimum 3⁄8" (10 mm) thick, exterior-grade plywood, 7⁄16" (11 mm) thick oriented strand board (OSB), or seasoned lumber, nominally 1" (25 mm) thick. Boards should be positioned tight to each other and securely nailed to framing members. Deteriorated or rotted boards should be replaced. For excessively resinous areas and loose knots, cover with sheet metal patches.

Malarkey strongly recommends installing sheathing over wood board decks. Problems with the performance of your roofing system, such as leaks and buckling, increase if installed directly over wood board decks. Failure to use properly conditioned deck materials can result in deck movement which can damage the roof covering. Deck movement may void your warranty.

dj1
Re: Do I need to put ply over shiplap before installing new composition shingles
dj1

In my area, GAF is one common shingle choice, and their instructions are very similar to Malrkey Roofing:

"Use minimum 3/8" (10mm) plywood or OSB decking as recommended by APA-The Engineered Wood Assn. Wood decks must
be well-seasoned and supported having a maximum 1/8" (3mm) spacing, using minimum nominal 1"(25mm) thick lumber, a maximum 6"
(152mm) width, having adequate nail-holding capacity and a smooth surface. Do NOT fasten shingles directly to insulation or insulated deck
unless authorized in writing by GAF. Roof decks and existing surfacing material must be dry prior to application of shingles."

Notice that they call for 1/8" spacing.

Except that LA Department of building and safety will not accept 3/8" thick plywood, but 1/2" or better, depending on the rafter spacing.

My suggestion: when you have different sub contractors claiming contradicting arguments, you ought to go to your department of building and get their building code version. Your building dept has the final say.

A. Spruce
Re: Do I need to put ply over shiplap before installing new composition shingles
A. Spruce
dj1 wrote:

My suggestion: when you have different sub contractors claiming contradicting arguments, you ought to go to your department of building and get their building code version. Your building dept has the final say.

True in ALL cases, local jurisdictional rule has the supreme say in ALL things, however, manufacturers strive to meet or beat jurisdictional rules, so whatever your locality mandates should also be in line with the manufacturer's recommendations. There are times when local jurisdictions can be pickier, but this tends to be a rarity to the normal accepted ways of doing things.

keith3267
Re: Do I need to put ply over shiplap before installing new composition shingles
keith3267

Good advice above. You seem to be the type that is proactive, replacing a worn roof before it is a problem. I assume from this that you are also the type that wants it done right and you want it to last.

I would not recommend 3/8 plywood for the sheathing by itself, but if the ship lap is in good shape and it is not sagging between each rafter, then 3/8" plywood will be fine as long as the local building inspector agrees. If you have to remove the ship lap, then 1/2" minimum.

If you live in a tornado zone, then consider the following. Temporarily remove one or two courses of ship lap at the bottom of the roof and the top of the exterior wall to install tornado/hurricane straps between the rafters and the wall studs. Use straps between the rafters at the peak as well but that can be done from inside the attic. Then replace the ship lap and have the plywood screwed down, through the ship lap and into the rafters at least 1.75 inches. Use stainless steel screws or ceramic coated deck screws.

Use ice block around the edges of the roof and at least 30# felt under the shingles. Then use the best shingles you can get like the 50 year shingles. With the plywood and the shiplap, your roofer can use longer roofing nails to hold the shingles better, 1.75" instead of 1.25" nails. Use ice block over the whole roof if the pitch is less than 5/12.

Many of these extras don't cost much to add now, but can add many more years of life to the roof. Good ROI (return on investment).

ET0520
Re: Do I need to put ply over shiplap before installing new composition shingles
ET0520

I appreciate all the helpful suggestions and information. I like the idea of going to the manufacturer's site and the local building code office. Thanks again.

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