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Low Slope Roofing

Low slope roofs are roofs that are too low for shingles or tiles. Shingles or tiles are not sealed and depend on water running downhill but in low slope roofs it is too flat for water to drain easily so a sealed type of roof must be done. Although all roofs (and walls and ceilings) are flat most people call low slope roofs flat roof so we will stick by that.

This section is meant to show you what goes into a flat roof so that you will be able to know if a job was done properly. You would still need to look for a roofing company to get the work done properly. You can do that under Finding a Roofing Company. 

Types of roofs
There are two types of residential roofs. The first and older type is the Built Up Roof (BUR) which is done with hot asphalt and layers of paper. The second type is the newer rubber roof. The rubber roofs are not true rubber which does exist in commercial roofing. It is technically called Modified Bitumen roofing. It has gotten the nickname of “rubber” and the public only knows it as that. Therefore, to keep the public from being confused I will go along with that nomenclature here and use the terms “hot or hot asphalt” roofs and “rubber” roofs.

BUR roofs
Also just called “hot” roofs these are applied in layers from 1 ply up to 4 plies. This means that there are 1 to 4 layers of paper bound together with hot asphalt. They are not placed directly over each other but are set back slightly every time another layer is placed. Therefore if you were to cut a hole in the roof, no matter where, you should always find the same number of plies. For a 3 ply roof you would find 3 layers of paper and 4 layers of asphalt (see diagram below). The vertical lines with arrows represent vertical cuts in the paper.

As the diagram shows, no matter where a vertical cut is made you will always cut through 3 layers of paper. The space in between the layers is filled with asphalt. In a 2 ply roof the layers do not overlap as far and any vertical cuts would only go through 2 layers of paper and 3 layers of asphalt. Since each row of paper is 36'' wide the overlapping becomes 24'' for a 3 ply and 18'' for a 2 ply.  

Rubber roofs  
The term "Rubber" roof is just a nickname. The correct name is "Modified". It is Chemically Modified Asphalt.

Rubber roofs are not as complex as BUR roofs. They can only be installed as a one ply system. They either go on correctly or incorrectly. The entire system is built into the one layer. Each layer overlaps the one before by 4”. These seams are welded together. The material is long lasting and will hold up better to the elements than BUR roofs. Some brands last longer than others. I currently only use Certainteed brand of modified. I won't risk using just any brand that might I find out years later, and many roofs later, that it didn't hold up as well. The main problem I have seen with these roofs are installation problems. People that are inexperienced with rubber roofs do not properly weld the seams together. This type of system brings a lot of “nobodies" to roofing since they don't need special equipment but you do need good technique. An improperly installed rubber roof is more prone to leaking than an improperly installed BUR roof.

Roof failures
There are 3 main things that can cause  water to get through.  

Obstacles: When an obstacle is encountered the paper must be cut around it and sealed. Obstacles range from chimneys to vents to drain boxes. In essence there is a hole in the roof where an obstacle occurs and the roof must be kept sealed around it. In time, if the roof is not checked, over the years these can open up.  

Old Age: Eventually the roof will rot away. It can not last forever. The life span of a BUR roof will depend on how many plies there are, the drainage, etc. The fewer the plies the less that would have to rot before water could come in. The life span of a good BUR or rubber roof should be about 10 to 15 years if taken care of. 

Open Seams: It is possible for a seam between 2 layers to open although this would be the least likely of the 3 except for rubber roofs. If a seam were to open on a BUR roof water could ride between the sheets until it gets to the deck underneath. Since the layers are overlapped in an uphill direction water would have to travel uphill between the layers to get through. So even if a seam were to open the water would still not get through. But, when snow lays on a roof it prevents the water from draining properly and this can cause water to travel uphill. As far as BUR roof, the more plies there are the more overlapping there is and the farther water would have to travel  to get through. But on a rubber roof, if a seam opens water does not have to travel far to reach the old roof. If the seams on a rubber roof are not welded properly there is a greater chance of water penetration. 

So what's best?
The best roof is a rubber roof. They will hold up better and do not require any coatings over the life span of the roof. As far as BUR roofs a 3 ply is considered the best BUR. Years ago roofs were done in 4 ply but the paper used then was asphalt paper. Today, fiberglass paper is used and a 3 ply fiberglass roof is considered equivalent to a 4 ply asphalt paper roof.  

A BUR roof requires periodic coatings to keep it in good shape but rubber roofs do not. Also, if you skip or miss your coatings then the guarantee expires. Rubber roofs cost more up front but you will save in the long run by not having to coat them. Over the life of the roof a rubber roof comes out cheaper. But many people like to aluminum coat their rubber roof to get a reflective surface to repel heat in the summer time. This will keep the house cooler and air-conditioning bills lower. It will also protect the rubber roof. The rubber roof is guaranteed to last 10 years but with aluminum coatings to repel the UV rays it will last longer. Ten years is the minimum life of a rubber roof. It is guaranteed to last at least that long.

Roof vents
Most roofs do not have adequate ventilation. It is a good idea, but not essential, to install a vent or two to air out the crawlspace under the roof. There are two reasons for this. First, it would keep the roof cooler which would cut down on air-conditioning bills. On the other hand if you install a vent you would be loosing heat in the winter. But some people overwork their air-conditioner more than their heater. But the more important reason is to prevent moisture damage. In the summertime there are many humid days. When you cool down humid air you are left with cooler air and moisture that is forced out of the air. This is what causes dew in the morning. This moisture is then deposited on the wood underneath and eventually causes the wood to rot. A good 12'' rotating vent is recommended for each 500 square feet of area.

There are other things that need to be checked when the roof is done. These things can cause leaks and are part of roofing but technically are not the roof. You may get only a roof and these items may be left out. Drain boxes should always be replaced unless they are new or copper. The drain boxes are the most vulnerable part of the roof. They cost very little to put a new one in when the roof is being installed but to replace it later, if it rots, would be costly and that part of the roof would have to be cut out and patched. Also, does the skylight glass need to be resealed or does the skylight need to be painted? Both of these need to be done every couple of years on older skylights. Do the barge boards need to be covered? These are the boards that run along the edge of your roof. If you have them covered with aluminum they will not rot and you will never have to paint them again.

If you have an antenna on the roof think about removing it, especially if you have cable. If you have an antenna attached to the roof this is only a hole in the roof where the base is and if you have guy wires on the antenna then each of these also are holes in the roof that have to be kept sealed. These are weak spots and become vulnerable. If you must have an antenna see if you can mount it on the chimney.



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