Built Up Flat Roof Installation Process

Is it time to replace your flat roof? Do you have sections that are leaking on your flat roof? We have you covered! In most cases that do not have severe wood damage, waterproofing will be the most cost effective and longest lasting option available. If you have ruled out waterproofing your roof, then replacing your flat roof will be the only other option. Listed below is a detailed overview of the built up flat roof installation process for South Florida written for you by Steven Hardy, the owner of Atlantic Coast Contractors. A built up flat roof installation is an intricate process and must be done by a professional, licensed, and competent crew. At Atlantic Coast Contractors, we strive to install every roof as if we were installing it on our very own home.  Roof installation methods can vary from contractor to contractor, and the installation method can determine whether you end up with a 5 year flat roof or a 20 year flat roof!

Most built up flat roofs will consist of a base sheet, 1-2 fiberglass mid ply layers, and a modified cap sheet. There are also many instances which a flat roof wood deck will have a .5 in 12 pitch or less or will have a sag in the roof deck, both of which can cause standing water. Standing water on a flat roof is the most detrimental situation you can have on a flat roof. This will greatly decrease the life of your roof. A flat roof that does not have standing water will last 15-20 years. However, a flat roof that has standing water will typically only last 5-10 years and sometimes even less. This is why it is so crucial that you pick the right contractor who is comfortable and experienced in installing a tapered ISO insulation flat roof system. This will ensure that your flat roof will shed water instead of holding it. A tapered ISO built up flat roof system is not the easiest flat roof system to install, and Atlantic Coast Contractors has installed many. We can offer you many different options for your roof, and will be able to offer this extensive system if it is necessary. This system is detailed out in section #6 of this page. Should you have any questions regarding this installation process, please do not hesitate to contact us and we will be happy to answer any questions that you may have.

#1 (Start)

The permit will be pulled by Atlantic Coast Contractors at the appropriate building department. We will take over the entire permit process and you as the owner can sit back and rest assured that you are in good hands!

#2 (Permitting and Scheduling)

Once the permit is issued, we will schedule a convenient start date with you and prepare our material order.

#3 (The Tear Off)

The next stage is the tear off phase in which the entire roof covering will be removed and the roof deck will be inspected. We will bring our 20-30 yard dump trucks to job site and remove all of the debris and trash, typically in one day. Many roofing companies will use a roll on/dropped dumpster. This dropped dumpster can be unsightly and most likely will damage your driveway or driveway finish. And in addition to the eyesore it can complicate your home parking situation. Atlantic Coast Contractors will never leave a dumpster sitting in your driveway for days on end! During the tear off we will remove the entire roof covering and inspect the roof deck. Any rotted roof deck boards, rotted fascia boards, or rotted trusses will be removed and new wood will be re-installed. Typically, our roof replacement contracts will include a set amount of wood replacement so you do not have to worry about a hefty bill issued to you for common wood replacement during this process. We come prepared with tarps, visqueen plastic, and protective materials just in case we have any unexpected weather.

#4 (Re-Nailing The Roof Deck)

Now that we have your roof down to the roof deck, it is time to start putting your roof back together! Before we do this there is a crucial step that cannot be neglected. We will re-nail your roof deck using 8d 2-1/2″ ring shank nails spaced 6″ on the edge and 6” in the field or 2 nails per board for tongue and groove roof decks. This is a very important step! Not only will this bring your current roof deck into compliance with the current building codes, but will also get you an additional credit applied to your windstorm insurance policy if it was not previously nailed to code! This nailing pattern will meet or exceed insurance company standards. And most homes built prior to August 31st 1994 will not meet these nailing requirements.

#5 (Installing The Roof Base Sheet)

Now it’s time to start installing the felt layers. Many contractors will offer a peel and stick SAV or SAP flat roof system. We strongly advise you to avoid these roof systems as the life expectancy is much lower than a tried and true hot mopped system. This may be offered due to a lack of access to a hot tar kettle. The first step is to install an 75 lb. felt to the entire roof deck. This 75 lb. felt will be nailed per code with galvanized steel tin caps and 1-1/4″ ring shank nails. These tin caps will be spaced 4″ on the overlap seams and have 3 rows in the field that are spaced 6″ on center. All sloped roof transitions will have a 16″ galvanized metal installed to them, and will be nailed per code on the outside edges. Any deteriorated flashing will then be replaced using a 4”x5” “L” flashing metal and a counter flashing, either with a sealant bead or one prepared for a stucco stop installation. A 3”x3” white galvanized drip edge metal (or special order color drip edge) will be installed along the entire perimeter of your home and nailed every 4 inches per code. All vents and lead plumbing stacks will be replaced with new ones as we will never cut corners and re-use the old ones. In addition to the tear off and 75 lb felt installation, we will heat up our kettle and start installing fiberglass mid ply layers to expedite the time frame that your roof project will take. Most city building departments will allow us to call in for our tin cap inspection and our mop in progress inspection all in the same day. We will also take extensive photo documentation of each step. Doing this will typically reduce the installation time frame by 1-2 days. Now it’s time for our work to be inspected by the city building department official. They will come and complete the tin cap and 75 lb felt installation inspections, and the hot tar mopping procedure that we are using. Once these inspections are completed, we will have the green light to finish your roof, and will have one final inspection left for your roof project.

*Note* – We will spare no expense for your roofing project. There are 2 types of hot tar that are available for purchase from local roofing supply distributors. The first is a standard roofing tar keg that is melted down in the kettle and applied to attach the layers of your flat roof. This is the cheapest and most commonly used tar keg. However, for all of our hot mopped roofing projects we will buy the (*No smell*) roofing tar kegs. Although, they do cost a little bit more, we have questioned our customers and found out that the smell of the roofing tar in the home can stick around for a few days and be quite bothersome. While the installation method for this roofing tar is the same, the smell is greatly reduced. We care greatly about our customers comfort and always purchase the roofing tar that does not smell as bad as the standard roofing tar kegs. The smell of tar is a common part of a flat roof replacement done properly. We cannot make any guarantees that there will be no tar smell in the home during installation, but take every effort possible to minimize or negate this inconvenience.

#6 Do I need a tapered ISO system for my flat roof?

Now this is a very important question with a very simple answer. If you have standing water on your flat roof the answer is unquestionably YES! As mentioned above, a flat roof with standing water can last 5 years (or 75%) less than a roof that does not. This is a huge difference in what you are getting (or the bang for your buck). Atlantic Coast Contractors offers these systems and many more with expert installation and the work quality surpassed by none. The subsection below describes the tapered ISO installation process if it is applicable for your roof replacement.


For this roof option there are many layers of the roof that will be seamlessly mopped together using (*No Smell*) roofing tar as one solid roof system. The tapered ISO will be schematically ordered piece by piece to create a sloping flat roof.  The first step is to install the 75 lb base sheet as detailed out in section #5 of this page. This installation method will vary from the steps of installing the fiberglass mid plies as they will come later. All layers of the roofing system will be attached using roofing tar and will have no mechanical fasteners outside of the 75 lb. base sheet. The next step is to install the tapered ISO board. The boards will be strategically installed across the roof to slope it, either from the center on a freestanding flat roof or to taper away from a sloped roof transition on a connected flat roof. These tapered ISO board will be attached using hot roofing tar. The next step is to install a venting layer. We at Atlantic Coast Contractors like to use a strato-vent layer rather than a fire board layer as it is less absorbent. It is best to not have absorbent roof layers on any roofing system, not only for water intrusion, but also for moisture levels and humidity levels in the air. After the strato-vent layer is installed, 2 fiberglass mid plies will be installed over the top of the strato-vent. To achieve 2 layers of fiberglass mid plies, they are overlapped approximately 24-28″ to achieve a double layer. The final layer of the flat roof system is a fire rated 90-120 lb modified cap sheet.

#7 (Cap Sheet Installation)

Now that we have passed our tin cap and mop in progress inspection with the city, it is time to start installing the modified cap sheet to your roof! This fire rated modified cap sheet will be installed using (*No Smell*) hot roofing tar and will be overlapped 4″. All of the excess edges that hang over the perimeter of the roof will be trimmed precisely using a razor blade. This outer edge will have a very strong bond as the drip edge metal in which it is attached to, will have been primed by asphalt spray primer. We are now ready to call in for our final inspection through the city building department.

All of our jobs are treated as if we were installing this flat roof system on our own home. There is no compromise in quality of work, and all jobs are personally inspected, worked on, and overseen by the owner Steven Hardy.

#8 (Job Completion)

Now your flat roof is all finished up! We will have our final inspection from the city building department within 1-2 business days of completion and your permit will be closed out with the applicable building department. You will be watertight and will have a strong and reliable roof that is safe for upcoming hurricane seasons! Now you can tell your friends and family about how great of a job we did and take advantage of our referral program! For any replacement that you refer to us, once it is completed we will give you a crisp $100 bill!

I hope this detailed explanation helped answer some questions you may have had about the flat roof installation process. If you have any question about the process or would like to get a quote on a new roof please give us a call and we will be happy to come out to your home or business and give you a free estimate!

If you would like to review built up roofing materials please feel free to visit the GAF website by clicking the logo below.

GAF Shingles - Atlantic Coast Contractors