Parts of a Flat Roof: Key Components and Their Functions

A proper roof slope allows for ample drainage of rain and snowmelt, but a flat roof relies on multiple layers and careful engineering to keep water out of your structure. Understanding the parts of a flat roof is a good first step toward having a meaningful conversation with your roofer, and it can also help you identify issues and get the proper repairs when the time comes.

What Are the Different Parts of a Flat Roof and How Do They Function?

There are a few key parts of a flat roof that allow you to avoid leaks and water damage even without a standard roof slope. Let’s take a look at the different components and discuss which are necessary and which are optional but helpful for flat roof lifespan and overall health.

1. Structural Deck

The structural deck of a flat roof is the bottom-most layer of roof decking, normally made of plywood. A roofer will use mechanical fasteners to attach this directly to the rafters below, and it provides a level base to install the rest of the layers.

2. Vapor Barrier

Next is the vapor barrier, which is technically optional, but it’s a popular addition to flat roofs. It prevents moisture from entering your building, and it can help mitigate water damage to the building materials below in the case of a small leak.

3. Insulation Layer

An insulation layer goes on top of the vapor barrier (or the structural deck if you forgo the vapor barrier). As you might guess, this improves the energy efficiency of your building. In more extreme climates, your roofer will choose polyiso insulation or rigid foam board with a high R-value, but spray foam and extruded polystyrene insulation are also common options.

4. Cover Board

A cover board is another layer of roof decking that your roofer lays down over the insulation layer. This is another optional layer, and it’s more common for creating a perfectly flat, clean base over spray foam insulation.

5. Base Sheet

The base sheet of your flat roof goes over your insulation or cover board, creating the first waterproof layer for protection. Your roofer will either adhere or fasten this fiberglass or reinforced polyester layer to the layer beneath and then seal it with asphalt.

6. Waterproof Membrane

The waterproof membrane layer is the most crucial part of your flat roof, as it provides the bulk of the protection from water and moisture. It’s the layer you’ll need to repair the most often, as issues will quickly lead to obvious roof leaks. There are a few options for creating this waterproof membrane.

• Bitumen: The most common waterproof membrane option, a modified bitumen asphalt roof is made of roofing felt and asphalt laid down in layers, each adhered to the last. This system creates what is called a built-up roof (BUR).

• Modified bitumen asphalt: This is similar to a BUR, but it uses fiberglass of polyester instead of felt and asphalt modified with polymers for better performance.

• PVC: Polyvinyl chloride (PVC) roofs use a single-ply waterproof membrane that your roofer adheres to the layer below. Your professional will seal seams between sheets with a waterproof adhesive.

• Thermoplastic polyolefin (TPO): TPO is another type of single-ply waterproof membrane. It’s normally white and more reflective, so it’s popular in hot, sunny climates for increased energy efficiency.

• Ethylene propylene diene monomer (EPDM): EPDM is a single-ply membrane made out of synthetic rubber. Your roofer can either use mechanical fasteners or adhere the membrane to the layer below for better waterproofing capabilities.

7. Aggregate Surfacing

Aggregate surfacing is an optional layer on a flat roof, but it’s popular with both bitumen and built-up waterproof membranes. It’s made up of loose gravel that your roofer lays over the waterproof membrane. It adds UV protection and wind protection, and it can help reduce the risk of foot traffic or blown debris damaging your waterproof layer.

8. Roof Flashing

Roof flashing is a necessary component of your flat roof that prevents water from getting through the seams between building materials. Your roofer will use base flashing around the base of the perimeter wall and cap flashing over the top to direct water down over the base flashing and onto the waterproof membrane.

9. Drainage System

Finally, you have the drainage system, which removes any pooling water from your roof. Removing water reduces the risk of roof leaks, and it helps avoid structural damage from the added weight of water. Drainage systems can clog, rendering them ineffective, so you’ll need ongoing maintenance to keep the system functioning properly.

How to Maintain the Different Parts of a Flat Roof

Flat roofs require more maintenance than sloped roofs simply because they drain water less efficiently. The different layers each come with varying levels of required maintenance.

• Structural deck: You’ll rarely need repairs to this layer unless you’ve had an ongoing roof leak that has led to rot. Full replacement is more common than repairs because you need to remove the top layers to access it. Flat roof replacement costs an average of $7,500.

• Vapor barrier: Issues with this lower layer mean you have major damage, so you’ll likely need a full replacement, totaling $7,500, on average.

• Insulation layer: The insulation layer is unlikely to need repairs, as it’s waterproof. Issues here mean you have major damage, and a full replacement is a better option.

• Cover board: The cover board, if you have this layer, can rot from ongoing exposure to water and moisture, so damage here is an obvious sign you need a roof replacement.

• Base sheet: The base sheet on your flat roof can sustain damage from excessive foot traffic or blown debris, and issues will result in roof leaks. Repairs cost an average of $1,200 and will require repairs to the waterproof membrane above it, as well.

• Waterproof membrane: The waterproof membrane is where you’ll need repairs most commonly, as it’s the first line of defense against water and moisture. It will also see the most wear and tear from foot traffic, debris, and UV radiation. Expect to carry out repairs every five to ten years. You can extend the life of this layer by recoating it every two to four years and carrying out roof inspections annually to identify problems early. Repairs cost an average of $1,200, recoating costs an average of $1,450, and roof inspections cost $250, on average.

• Aggregate surfacing: The aggregate layer of your flat roof won’t need repairs, as it’s just loose gravel.

• Roof flashing: Roof flashing is made out of aluminum, so you won’t need repairs unless there’s obvious damage from rust or impact from blown debris.

• Drainage system: Drainage systems are another area where you’ll need routine maintenance to keep the drains clean and functioning properly. This service costs an average of $200, and you should expect an inspection and cleaning annually. Repairs can cost up to $1,000, but they’re uncommon.

Cost to DIY Flat Roof Repair vs. Hiring a Pro

The average cost to have a professional repair your flat roof is $1,200, which can include repairs to the waterproof membrane and patching small leaks. Unfortunately, layered waterproof membranes like asphalt and felt paper or modified bitumen asphalt and fiberglass require specialized tools and experience for the best results, so you should hire a roofer for the best results when it comes to flat roof repair. The same goes for single-ply waterproof membranes like EPDM, TPO, and PVC.

It’s possible to recoat your flat roof yourself—doing so will cost you an average of $1 per square foot for the elastomeric coating. You can apply this with a paint roller or a paint sprayer, which will cost an additional $20 to $400. If you apply with a roller, you can save between $500 and $1,000 doing the work yourself rather than hiring a pro, and you can enjoy those savings every two to four years.

Another relatively easy DIY task for flat roofs is keeping your drainage system free from debris. Depending on tree coverage in your area, you should plan on cleaning out the drains at least once a year and doing the work yourself will save you an average of $200 each time. You can also do a cleaning every six months, which would cost nothing compared to the $400 annually you’d pay for professional service.