Is Roofing Felt Necessary on a Shed?

Are you building a shed but not sure how to finish the roof? You’ve decided to use shingles, and putting a layer of roofing felt underneath the shingles can add years to the life of the roof. At a minimum, you want your shed to be a protective shelter from outside elements. While felt is an added cost to what is supposed to be a cheap project, it can also save you from future repairs. 

So, is roofing felt necessary on a shed? Roofing felt is necessary on a shed, as it not only protects the interior from water damage but also extends the life of your shingles. Felt is asphalt-infused synthetic or wood composite paper that repels water. It provides a smooth, unbroken surface for the application of shingles. 

In this article, we’ll examine the reasons why you should include roofing underlayment under the shingles of your new shed.

Is Roofing Felt Necessary on a Shed?

Shingles on your shed are the first line of defense against the elements. Underneath the shingles is OSB sheathing. This is a type of engineered particleboard made specifically for covering the roof of a shed or dwelling. 

Standard asphalt shingles do an excellent job keeping the weather out. However, moisture has a way of finding its way into a structure. Particularly strong windstorms or ice can help moisture get under shingles via nail holes or other gaps.

If you don’t have any material between the shingles and OSB sheathing, then you are more likely to have water in your shed. OSB is water repellent, but only in small amounts. Roof leaks, on the other hand, tend to start small and slowly build over time. Often you won’t notice a leak until it starts dripping on the ground, or your face.

Water will find its way into the OSB joints, and you will have a saturated interior and potentially damage whatever you’ve stored in the shed. Installing roofing felt will give you an added moisture barrier at a very low cost. 

When you build a shed, your primary goal should be to create a dry space. Whether you are storing your new riding mower or just want a place to put your gardening supplies, you don’t want moisture ruining your stuff.

Roofing felt goes over your OSB sheathing. It comes in large rolls, and you simply roll it on. It is designed to overlap itself, much like shingles on a roof. This way, any moisture that gets beneath the shingles has a clear path to the bottom and back outside.

Another reason to get roofing felt is to cover the joints made by your sheathing. OSB board exposed to moisture tends to swell at the edges. The edges then push up on the shingles, and over time, cracks them. This compromises the ability of the shingle to keep your roof dry. Putting a layer of felt over the OSB will help keep OSB joints dry, and prevent cracking.

Since roofing felt is cheap, and sheds are small, you can add more than one layer of roofing felt for added protection. 

However, is roofing felt waterproof? No, but it is water repellant. A slow leak in a shingle will allow the underlayment to wick the moisture away, out of the shed, and prevent it from soaking into the OSB. However, large leaks or roofing felt that is exposed by storms will not keep water out of a shed. 

Types of Roofing Felt

The two main types are roofing felt 15 and 30. The numbers refer to the actual weight of the felt. 15-pound roofing felt costs around $20 for just over 420 square feet. Felt is typically 3 feet wide, so that means a roll is approximately 144 feet long.

30-pound roofing felt costs more, but also offers more protection. Many roofers use 30 to cover their roofs, as it offers better traction when they apply shingles. It also is less likely to tear and get damaged during a wind storm. However, a roll of 30 felt costs about double the price of the 15, or $20 for a 72-foot roll.

If you are building a shed that isn’t the size of a house, which most of us are, then a single roll of 30 felt may be enough. Or, you can purchase one roll of 15 felt and add two layers. One roll of 15 felt is more than enough for most sheds, unless it is unusually large. 

Typically, roofs with a lower slope need overlapping underlayment. This means that, for a 36-inch width, the underlay should overlap the row below it by a minimum of 19 inches. Slopes with steeper pitches can get away with overlapping by 6 or even 4 inches, depending on manufacturer recommendations. The lower the slope, the more felt you will need.

Roofing Felt Alternatives

Synthetic roof underlayment does not use asphalt, which is commonly referred to as bitumen in the underlayment industry. Instead, the synthetic roofing membrane is made out of polypropylene or similar material. It is usually completely waterproof and provides an excellent work surface for applying shingles.

However, many synthetics do not self-seal around nails. Other underlays will seal themselves when a nail is driven flush into the material, maintaining the moisture barrier. As well, many shingle manufacturers recommend asphalt, not synthetic, underlay when installing their products. Using synthetics may void shingle warranties.

Rubberized asphalt underlay, called polymer-modified bitumen, is just asphalt roofing felt with a polymer coating. This underlay often has an adhesive strip to assist in attaching it to the sheathing. A benefit of this type of membrane is that the rubberized surface makes the paper much stronger than standard felt paper. 

A roll of standard, synthetic underlayment costs about $80 for 1000 square feet. This is comparable to a roll of 30 felt paper. Premium waterproof ice and rain guard, also a synthetic, is much more expensive. Costing around $120 for 230 square feet. 

Unless you are storing your vintage collection of rare sports cars in your shed, a premium synthetic underlayment is probably overkill. 30 felt is more than enough to keep your shed dry.

How to Install Roofing Felt Paper

Installing roofing felt is not difficult. You’re biggest concern will be trying not to tear it as you install the shingles.

Just as you would with shingles, your first layer of underlayment will be at the bottom.

Step 1: Roll out the first layer of felt, starting on the left and rolling right. Make sure you nail one corner with plastic capped, galvanized felt nails. Line up the left edge along the side of the roof and nail the other corner. 

Step 2: Nail a maximum of every 8 inches along the edges. Your asphalt-coated felt will self-seal the nail holes, so in theory, your nail holes won’t allow moisture to penetrate those areas. 

Step 3: Make sure your felt goes over and above the drip edge on the lower edge of the roof.

Do not nail too close to the top of the felt, as your next layer will overlap and you will nail through the overlap to fasten both rows at the same time. 

Step 4: Install the next layer of felt, being sure to overlap up to 4 inches over the previous layer. If you run out of felt and have to start a new roll on the same row, make sure you overlap by at least 6 inches.

Step 5: Nail through the overlap to secure both pieces of felt. Repeat until the roof is complete.

Keeping your rows of felt level is important, as you want to have even coverage over the entirety of your shed roof. Most felt underlayment has built-in chalk lines to assist in layering your rows. If it doesn’t, use a chalk line to line up all subsequent rows of felt after the first.

Another slightly cheaper alternative to nails is stainless steel staples. You can use a manual staple gun, or as I did in my last project when I was building a Lean-To shed, use a cordless crown stapler.

When is Roofing Underlayment Not Necessary?

Roofing felt is always recommended, but is not actually required by all shingle manufacturers. For steep-pitched roofs, felt is often not required. The steeper the pitch, the less likely moisture is to sit and seep into gaps and cracks in the shingles.

Extremely dry environments are also one area where felt may not be necessary. However, if there is even a slight chance for storms or high winds, then taking the added precaution of fortifying a shed with felt is the smart move. 

Final Thoughts

When investing in a shed, a roll of tar paper is a small cost that can prevent costly damage. The ease of installation and affordability of felt makes it a must-have for your next shed roof installation. 

I hope this article will help you make a more informed decision about installing roofing felt on your shed. Please share this article if you’ve found it helpful, and feel free to add any comments you might have about this subject below.