With all the questions we receive on felt options, we thought we would let you know what the real and intended purpose of felt underlayment is without all the sales techniques companies use just to sell you a new roof.
Here’s the real purpose of the felt paper. As warm moist air rises, it passes through the sheetrock ceiling in your home and up through the attic. The felt paper allows the moist air to pass through, but if some of the moisture condenses on the underside of the shingle, the felt paper keeps that condensed moisture from touching the wood. If the condensed moisture were to be in contact with the wood, over time the wood would be damaged.
There’s never enough condensed moisture on the underside of the shinlges for the moisture to flow down the roof (between the shingles and underlayment). It’s simply a good idea to keep the moisture from touching the wood.
The other reason for the felt underlay is to provide a simple barrier between the asphalt in the shingles and the wood sheathing. The sap in the wood isn’t good for the shingles. If a shingle roof is being installed over an existing shingle roof, no underlayment is needed, because the first roof keeps the new shingles from laying on the wood sheathing.
15 lb felt paper is slightly better than the 30 lb., for underlayment. If the felt paper was going to be exposed to any wear and tear there could be a benefit in using 30 lb felt instead of 15 lb felt. But the felt paper never wears out. The felt is completely protected from wear and tear.
The only reason some companies prefer 30 lb felt is that sometimes, some roofing contractors may cover the roof with the asphalt felt and then, due to weather or simply bad planning, they’ll leave the job like that for several weeks. You’ve probably seen a house around your town where the work seemed to stop and the black felt paper was covering the roof. In that case, the felt paper is doing a good job, but it’s not the job the felt underlay was designed for.
The other reason many roofing contractors sell 30 lb felt is that it sounds better. It sounds twice as good. The reality is that air needs to pass through the felt paper and so the 30 lb felt makes that slightly more difficult. If the 15 lb felt paper and the 30 lb felt paper were the exact same price, I would choose the 15 lb felt paper for my house. Unless I planned on leaving the job half done for 3 weeks.
On the low pitch roofs, under 4:12 but over 2:12, a double underlayment is required. Not double thickness, like going from 15 lb to 30 lb. But two actual layers. It also doesn’t mean a complete covering of one layer of underlayment, and then another layer of underlayment over the first one. It means that after one row of felt paper is layed down, the next row needs to start at the halfway point on that first row, not at the top edge of the first row. If you do the whole roof in underlayment, on the low pitch sections you’ll be seeing rows that look about 18″ wide, whereas the rows on the regular pitch sections will look about 34″ wide.
Call ARROWHEAD for all your roofing needs.