Should You Roof Over Wet Sheathing?

There is a popular saying, “April Showers bring May Flowers”. Roofing during the rainy season can be difficult because it can bring delays to our schedules and sleepless nights in the re-roofing business. However, it can also be a rewarding experience if you are prepared by using a synthetic underlayment designed for use in wind, rain and generally bad weather conditions, as it can expedite the installation process. Not losing days of work because of a slight chance of rain, means more revenue for the company and less time lost for the employees.

Meeting the general contractor’s new construction deadlines can be difficult during wet weather, as applying synthetic underlayment or roofing to wet damp sheathing is a real concern. Commonly asked amongst roofing contractors is, “It has been raining all week and today the roofers show up. They are roofing over the wet sheathing. Shouldn’t they wait for it to dry out?” The answer is always “Yes” and these are the reasons why:

  • TAPArapped moisture on low sloped sheathing can cause steam pockets and bubbles in the roofing system
  • Wet roof sheathing can cause slips and falls
  • Synthetic underlayment is a vapor barrier and will trap moisture against the roof deck if installed when it is wet
  • Wet sheathing may cause mold and mildew

For contractors these issues mean headaches for the business. Often the question is, “How wet, is too wet to roof?” For new roof construction, if the Plywood or Oriented Strand Board (OSB) has the American Plywood Association (APA) symbol, it is designed or approved for normal exposure to weather and rain. The roof deck can be roofed as soon as the surface dries.

CircleNote: In many jurisdictions, local codes prohibit installing underlayment and shingles on wet surfaces. It is up to the contractor to understand the codes and the manufacturer’s specifications and have company policies in place to deal with wet sheathing. Since the water may not damage the sheathing what are the risks? The biggest problem that can occur is the expansion of the wood when it gets saturated.  This is sometimes called “Buckling”.  Wood is hygroscopic, which means that wood is like a sponge and will gain or lose moisture from the air or rain based upon the conditions of the surrounding environment.  Proper spacing is critical for the roof deck, so that it can expand when it gets wet. It is even more important on low sloped roofs since water cannot move downwards as quickly and is trapped causing big issues later. Therefore, it is essential low sloped roofs are dry before installing primary roofing materials.

The American Plywood Association (APA) primary function are to provide product certification and testing, applied research, market support and development of wood products like OSB and plywood, just to mention a few. They recommend “using a #10 penny box nail as a spacer as a way to gap the 1/8 inch needed for panel expansion.” Please take a minute to watch this great video where Mr. Clark of APA explains this expansion issue in detail.

Wet Sheathing Can Cause Problems

To avoid problems it is a good idea not to start a roofing project where the roof sheathing is wet. Mark Tupas from InterWrap, says “The drying process is entirely subjective, in order to eliminate second guessing we normally recommend waiting to install underlayment until the roof deck is dry to the touch; better yet, a moisture meter could be used and should read 5% or less.

A moisture meter is inexpensive and may be a useful tool in new construction. With re-roofing projects, it is important to not remove any more roofing that can be covered up to the same day with underlayment. It is recommended to have tarps ready in case you get a surprise shower.

Warfield House 


To increase efficiency, some roofers will have their roofing materials and supplies delivered to the roof top. Compared to #30 oil saturated felt, Titanium Underlayments are up to 11 times lighter, come in longer rolls and are 48” wide compared to 36”. Therefore with Titanium, you will need less rolls per roof, this makes the pre-load package lighter and easier to manage. Having at least the underlayment on-site allows the contractor to dry-in the building before the primary roofing can be installed.

One roll of Titanium can cover up to the same as 5 rolls of #30 felt. That can mean a lot less trips up the ladder for a roofer and significant store space reductions.

Unlike smooth surfaced underlayments, Titanium products possess a revolutionary patented slip resistant Sure-Foot® nodular walking surface technology that allows for easier steep slope walk-ability, even in wet conditions.


All data and information appearing on this site is anecdotal in nature, reflecting the contributor’s personal experience and not fact-based research and, therefore, to be used for information purposes only. Read full disclaimer