Tar paper facts and tips

September 07, 2012|By Tim Carter, Tribune Media Services | Ask The Builder
  • Tar paper has a proven track record of success. It will keep this shed dry for hundreds of years if the shed is maintained.
Tar paper has a proven track record of success. It will keep this shed dry for hundreds of years if the shed is maintained. (Tim Carter)

DEAR TIM: I'm building a room addition, and as I was doing the demolition of my outside wall I discovered tar paper was in the wall. I thought tar paper was just used on roofs. Why did they use tar paper in the wall? Should I use it on the walls of my room addition? What about all the new fancy products that I see that were developed to take the place of tar paper? It's all very confusing. -- Rob R., Lebanon, Ohio

DEAR ROB: I'd wager that hundreds of thousands of houses here in the U.S., and possibly millions across the world, are still standing because smart builders of old used tar paper as a weather barrier. You're correct that it's been used under roofing for years to help keep wood dry. It does the same thing on walls if given the chance.

I've also had the pleasure to tear apart many an old exterior wall. On just about every one that was wood-framed, I came across tar paper under the exterior siding material. In almost all situations the tar paper had become brittle, but it still worked. You could put water on the black paper, the water beaded up and didn't penetrate to the wood behind.

I don't know what builders of old used before tar paper, but I can only assume that once crude oil was discovered and then refined for its many current uses, builders immediately saw the benefits of a waterproof product made from oil that could be applied in sheets.

Tar paper used to be made by soaking a porous paper made from cotton rag scraps with thinned liquid asphalt. Asphalt, of course, is one of the final products that results from refining crude oil. The tar paper comes in different weights. The most common weights are 15-pound and 30-pound. The 30-pound tar paper is heavier and has much more asphalt in it.

Much of today's tar paper is made using multiple fiberglass mats, as the cotton rag industry has declined significantly over the past few decades. This is why fiberglass shingles were introduced. There simply was a shortage of cotton scrap to make the mats that are the foundation of shingles.

Using tar paper to protect wood sheathing and wood framing members on houses, room additions or outdoor sheds is a fantastic idea. This time-tested product is affordable, it's easy to work with and it's readily available.

The trick is to make sure you install it so it overlaps correctly. You want each piece to overlap the one below. Feathers on birds, fur on animals and shingles on a roof use this same method to stop water penetration.

It's all about shedding water. There are pre-printed lines on the tar paper that help show you where to end the overlap. Usually 2 inches is plenty on a horizontal seam. If you have a vertical seam where one piece ends and another starts, make the overlap at least 6 inches.

Another great installation tip is to make sure the first strip of tar paper is installed so it overlaps the top of the foundation at least an inch. You want any water that does get behind the siding to run down and never be allowed to get near any wood. Many homeowners and builders fail to create this mission-critical overlap at the foundation.

The newer weather barriers made from synthetic fabrics are great products, and I've used them as well as tar paper. Some of the new products come in tall rolls that allow you to cover a typical one-story house with only one vertical overlap seam. You surely can't do that with tar paper as it usually only comes in rolls 3 feet wide.

Tar paper will take more labor to install than the newer wider synthetic weather barriers, but if you're doing the work, it costs you just your time. You just need to do the math to see what material will save you money.


Some of the newer weather barriers have great drainage channels built into them. These channels help direct water quickly down and away from the exterior siding material. They also promote quick drying allowing air to get behind any siding. This is a good thing.

Tar paper does not offer this. Siding applied directly over tar paper creates a sandwich effect and can trap water between the siding material and the tar paper. If you want vertical drainage with tar paper, you have to add treated lumber strips on top of the tar paper. This is a time-intensive process and requires all sorts of skill.

I don't add vertical drainage strips on my jobs when using tar paper because I take the time to install the siding so that water will have a very tough time getting to the tar paper. Just use the best practices when installing siding and you'll get the same results.

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