Types of Flat Roofs
Updated: Jan 1, 2020
When people discuss the types of flat roof systems they are normally referring to the top layer of a system, also known as the membrane. Although flat roof systems are also composed of the deck (wood, metal/steel, concrete), and insulation, for the purpose of this article when we discuss flat roofing types we will be referring to the membranes.
There are 2 main categories of flat roofing systems. in the first category are systems that are protected by a single layer of membrane, also known as single ply systems. In the second category there are systems protected by multiple layers of membrane known as multi ply systems (Asphalt roofing systems).
SINGLE PLY ROOFING
Single ply roofing consists of 3 main types, but remember all the systems in this category are only one layer.
1. EPDM - (Ethylene Propylene Diene Monomer)
EPDM is a rubber membrane made out of material similar to that of a bicycle a tire tube. It usually comes in black, but they are starting to also produce it in white. EPDM is applied either mechanically, which means using fasteners and plates, or fully adhered, which means using adhesives. The seams are taped together with a special double sided tape.
2. TPO - Thermoplastic Polyolefin
TPO is also a rubber membrane however it feels a lot less pliable than EPDM. TPO comes in white and has better resistance to grease than EPDM, so if you have retsaurants releasing grease out of vents you may consider it. TPO can be installed mechanically with fasteners and plates or fully adhered with glues/adhesives. The seams of a TPO membranes are heat welded with a specialized machine.
3. PVC - polyvinyl chloride
PVC is also a thermoplastic membrane similar to TPO. PVC is the most expensive of the 3 single ply systems. PVC comes in white and can be installed mechanically or fully adhered. Similar to TPO the seams on a PVC roof are heat welded.
Single ply system pros
1. Because the rolls can come in sizes up to 30ft wide it is easy to minimize seams on a roof system. Seams can potentially open causing leaks so the less seams the better.
2. Because the rolls come in such large sizes it is easier to cover more roof footage more quickly resulting in a cheaper price (only if your roof is large enough to make it viable).
3. Some single ply systems have superior resistance to damage from grease or chemicals so if you have a restaurant or more in your building with grease vents on the roof it may be worth considering single ply.
4. Single ply membranes are naturally U.V. stable so they do not need to be covered in gravel or coated in reflective coatings to protect them from the sunlight.
1. Because single ply systems only have one thin layer of protection they are fairly susceptible to damage from punctures (ex hail damage, a woman walking in heels). If your roof gets lots of foot traffic from tenants, or from maintenance crews for rooftop machinery, or is in an area that gets hail you may not want to install a single ply because of the risk of leaks caused by punctures.
2. It is more difficult and expensive to recover an old single ply system than it is to recover an asphalt roof system. so if you are considering recovering (going over your roof with a new layer) your roof in 10-30 years when your new roof eventually fails, instead of ripping it off down to the deck and starting from scratch you may want to consider using an asphalt system.
3. Single ply membranes are not compatible with the classic roofing repair materials (flashing cement, tar, Karnak, roofing cement). If roofing cement is used on a single ply system the solvents in it will slowly eat through the membrane making the situation worse and more expensive to fix. The problem is that because single ply membranes are relatively new to the market many roofer are not aware that they are incompatible with classic repair materials and it is common for a roofer who is not familiar with how to repair these types of roofs to attempt to use roof cement to fix the leak making the situation worse. And it is not only roofers who make this mistake, HVAC installers, plumbers and electricians who make holes in the roof to run their necessary pipes and wires through the roof are all at risk of trying to seal around their holes with the wrong material.
4. single ply membranes come from 45- 90 mils thick. modified bitumen roofs come from 120-300 mils thick.
ASPHALT ROOFING SYSTEMS
Asphalt roofing consist of 2 main systems.
1.BUR (built up roofing) AKA tar and gravel roofs
Of all the roofing systems spoken about in this article built up roofing has been around the longest (our 100 years). It has a good track record and is time tested. Built up roofing consists of multiple layers (plys) of felt adhered together using hot asphalt. Usually between 3-5 plies. The BUR is installed by melting cubes of asphalt. Then the liquid asphalt is mopped onto the roof and the felts are rolled into place. Usually gravel would be embedded into the top layer of asphalt to protect the roof from harmful UV rays, alternatively aluminum coatings can be used to reflect the UV rays. More recently there has been the advent of cold process BUR. These are the same as traditional BUR but instead of hot melted asphalt being used, cold adhesives containing asphalt dissolved in solvents or asphalt emulsions are used.
1. BUR systems are durable because of their multiple layers and so they stand up excellent to foot traffic and wear and tear.
2. Because any given spot on the roof will be covered with multiple layers it reduces the chance of a leak occurring due to workmanship error. (if 4 plys are being installed, the workers will have 4 chances to seal the roof so even if they do not seal a spot properly on the first layer they will have 3 more layers to get it right). Single ply roofs do not offer this luxury, any small mistake and a leak is inevitable
3. Installing a BUR requires lots of equipment and experience. A kettle is needed at ground level to melt the asphalt cubes into liquid, overheating or underrating the asphalt will cause issues in the roof. A pump is required to get the asphalt from the kettle up to the roof. Experienced workers are needed to work with the asphalt as not to injure themselves and to be safe. Because of this usually only reputable companies with lots of experience offer this type of roof system (installing a BUR roof significantly reduces the chance that your roofer is a fly by night company.).
4. BUR is the oldest flat roof system still installed today (invented over 100 years ago). It is time tested and proven to an effective flat roofing option.
1. BUR roofs are dangerous to install due to the hot asphalt that is used to adhere the plys. Minor burns by asphalt among workers is quite common. (this does not apply to the new cold process BUR that uses cold asphalt in place of hot).
2. Hot asphalt creates a strong smell, there is a chance that the building occupants or the neighbors will complain about it.
3. BUR roofs need protection from U.V. light, without protection the roof will degrade prematurely. The 2 options for protection are
1. cover the BUR in gravel or pebbles, this prevents the sunlight from directly hitting the BUR. However if repairs or a new roof are necessary it is difficult to see what needs to be done under the gravel or pebbles. This creates an extremely laborious situation where the gravel or pebbled need to be removed to see the damaged areas and to repair them.
2. Coat the BUR in a reflective coating most common is aluminum paint. However these coatings do not last forever and every 3-5 years(for aluminum) need to be reapplied which adds an additional expense (though an aluminum coating is usually a minor expense).
4. Although BUR is widely accepted as a highly effective and long lasting roof system it is also the oldest flat roof still installed today. As such it may be considered old and outdated compared to the newer roofing systems. Because of this it can be hard to find a roofing company that still offers BUR as an option (many companies do not do this type of install).
2. MODIFIED BITUMEN
Bitumen is an ingredient mixed with asphalt. Someone discovered that bitumen could be modified by mixing it with rubber or plastic and that this would increase the elasticity of the asphalt. They then began creating 3ft wide rolls of modified asphalt (mixed with rubber or plastic), with a polyester or fiberglass mesh reinforcement embedded. These rolls became known as modified bitumen. There are 2 types of modified bitumen rolls.
APP = these are the rolls made out of asphalt that was mixed with plastic
SBS = these are the rolls made of asphalt that was mixed with rubber.
Modified bitumen can be installed in three ways
1. TORCHING = first the rolls are placed in position on the roof. Then a propane tank and torch are used to heat up the roll and melt the bottom layer of asphalt. While the torcher is heating up the roll he slowly unrolls it by either pushing it with his foot or using a metal cane. This results in heat welded seams that have a bead of molten asphalt oozing out. The benefit of torching is that this is the cheapest method of installation. The negatives are that using an open flame on a building is dangerous and has resulted in fires, and that if the installer is not a professional it is very easy for him to accidentally underheat or overheat the roll which will result in early failure of the roof system. Usually 2 or three plys of modified bitumen are installed, a base sheet/ply a mid sheet and a cap sheet. When only 2 layers are used it is just a base and a cap, and rarely only one layer will be installed, just a cap sheet, to save money.
1. This system is cheap because adhesives are not used to attach the membrane and the price of propane is comparatively inexpensive. Also the equipment needs to install this system is very minimal.
2. Because the equipment needed is very basic and inexpensive this is a very popular system for new roofing companies who do not have lots of money to spend on tools. Almost all companies offer this roof system as an option.
3.This system can be installed in any temperature.
4.. Like all modified bitumen systems this system is durable and not easily punctured
5.. When installed with more than one ply this system is redundant reducing the likelihood of a leak.
1. This system is dangerous because it requires an open flame to be pointed at your building. There have been many occurrences of fires as a result. Using an experienced contractor who follows safety protocol and trains his employees to do the same will greatly reduce the risk. Also in some states using this system on a roof with a wood deck is illegal. in other states if your roof deck is wood then a fire barrier is required in between the wood and the torch such as fireproof insulation.
2. Because the price of this system is so cheap and the equipment is so basic many fly by night/shady companies install this system without the correct knowledge, training or insurance. When you inevitably get a leak as a result of using an inexperienced company and try to call them back to make good on their warranty, most of the time they are no where to be found.
3. Most roofing companies insurance specifically exclude torching from their coverage. in some cases the owner of the company is not aware of this and in other cases they are aware but they choose to mislead you. If you choose a torching system be sure to specifically check the roofers insurance to make sure it includes torching (do not just rely on their honesty).
4. This system requires a very skilled crew to install it correctly. If a roofer is inexperienced, instead of fully heating/melting the rolls and delivering you a fully adhered system that has no air pockets or channels for water to travel, they may only spot heat the rolls with will give you an end product that is only adhered in certain spots but can allow water and air to travel under it.
5. Like all modified bitumen systems this system has lots of seams which can potentially open and cause leaks
2. COLD PROCESS = This type of installation relies on cold asphaltic adhesives to adhere the modified bitumen rolls to the roof. The adhesive is spread using a squeegee and the rolls are unrolled onto of it.
This is more expensive than torching because there is the additional cost of the adhesive. However this is a safer more reliable installation.
1. Because this system is installed by spreading glue it is easy to visually check that there are no spots missing glue before they roll out the sheets. This means that it is easier to instal this system correctly.
2. It is a safe system to install.
3. Like all modified bitumen systems this system is durable and not easily punctured
4. When installed with more than one ply this system is redundant reducing the likelihood of a leak.
1. Because this system has the added expense of glue it is more expensive than a torch system.
2. Because this system is installed with black adhesive that is sticky it can be very messy and the glue can quickly spread on feet all over the roof. If you purchase a white roof it is not uncommon for it to have black footsteps all over it. I suggest that when doing a cold applied roof it is better either to choose black so that the glue blends in or to coat the roof in aluminum paint after the glue dries to cover it up.
3. This system can only be installed when the weather is above 40 degrees Fahrenheit otherwise the adhesives will not perform properly.
4.Like all modified bitumen systems this system has lots of seams which can potentially open and cause leaks
3. PEEL AND STICK / SELF ADHERED = as its name implies, peel and stick modified bitumen has a layer of adhesive on the back of the roll. The protective layer of plastic is peeled off and the roll is adhered to the roof using pressure from a weighted roller. Peel and stick modified bitumen roofing is relatively new and doesn't have a long track record.
1. Because this system is peel and stick it is pretty easy to install correctly.
2. It is safe to install
3. It is pretty cheap to install
4.Like all modified bitumen systems this system is durable and not easily punctured.
5.When installed with more than one ply this system is redundant reducing the likelihood of a leak.
1. This system can only be installed when the weather is above 40 degrees Fahrenheit otherwise the sticky part on the back of the sheet will not perform.
2. This system is not good for roofs that are flat or close to it. When installing a peel and stick it is preferable that the roof has a good pitch towards the drainage areas. the reason being that peel and stick seams are not as strong as torched seams or as glued seams.
3. This is a relatively new system and has not been time tested. Some companies have had issues with the adhesive not working correctly.
4. Like all modified bitumen systems this system has lots of seams which can potentially open and cause leaks
The best and most expensive modified bitumen systems have three plys of material installed, a base sheet/ply a mid sheet and a cap sheet. When only 2 layers are used it is just a base and a cap, and rarely only one layer will be installed, just a cap sheet, to save money.
Now if you are reading this article in an attempt to decide what system is best for your building I have good news and bad news. the good news is that all of the above systems can last long, 10-20 years, and protect your building from water. The bad news is that all of the above systems can also fail prematurely and be a waste of money. How can this be?? In the industry we have a saying, "its not the type of roof systems that you purchase but rather the people that you hire to installl the system."
However taking all that into account I will list some tips on certain situations where it may be better to choose on system over another.
1. If your roof is large in size (over 25,000 sqft )with few penetrations on the roof, such as hvacs, curbs, conduits, vents, stacks, pipes (ex warehouses, shopping centers), generally a single ply will be a better choice. The reason is because single plys can come in wider rolls, up to 30 feet wide, and so there will be less seams on your roof, lowering your chance of a leak (seams opening up are a common cause of leaks). Also, because the rolls are so big it will be easier to cover more ground in a day of work and this will result in a cheaper price.
2. If you have lots of penetrations on your roof you are usually better off with a modified bitumen system (usually in cities). The reason is because the rolls are only 3 feet wide it is easier to maneuver around all the penetrations when laying out the rolls.
3. If your roof gets lots of foot traffic you are better off with an asphaltic roof system because they are more durable.
4. If your building is in a city usually modified bitumen is the way to go (even if your building is greater than 25,000 soft). the reason is that usually in cities roofs have lots of penetrations favoring small easily maneuverable rolls of material. Also cities usually have lots of foot traffic on the roofs of buildings from tenants and maintenance men and modified bitumen is more durable than single ply. Also in cities buildings are usually constantly getting new tenants who create new penetrations in the roof whether from plumbing or from kitchen exhausts and modified bitumen is easier to repair after installing new penetrations.
5. If your roof is wood deck is wood you should consider cold applied modified bitumen for safety reasons.
6. If your roof deck is metal or concrete you should consider torch down modified bitumen for its cost savings. (however make sure the company is reputable and in business for a long time under the same company name, not constantly changing the company name to avoid lawsuits).
With all this in mind the most important thing to remember is that "its not the type of roof systems that you purchase but rather the people that you hire to installl the system.". When all is said and done you want a company who is experienced installing the product that you purchase. (Don't hire a modified bitumen roofer to install a single ply system or vice versa.) and who is going to backup their work by honoring their warranties.
If possible it is always better to get a manufacturer warranty on a job, this means a warranty from the company who makes the roofing material not from the roofer himself, and I'm talking about a labor and material warranty no dollar limit, not just a material warranty because material warranties are worthless, (what good is getting free material if you need to pay to have it installed). The reason being that manufacturer are much more likely to be in business in 10 years from now and are also more likely to answer the phone and honor the claim.
Here at Roofing Team Inc we are capable and experienced installing certified roofs with warranties from all of the main roofing manufacturers. Including GAF, Firestone, Soprema, Mulehide, and more.