Flat roof technology has come a long way over the last century from bitumen-based flat roofing to plastic polymer products, and there are a number of options to choose from today.
There are seven main ways to cover a flat roof, using a variety of time-served and new materials. Whether you are renewing a flat roof or building one new, it is good to understand which one would be the best choice for your project.
7 Types of Roofing Materials for Flat Roof
Below, we’ll go over seven different materials that you can use for a flat roof.
Built-up Flat Roof
This is the flat roof that you will find on most older buildings. It is a multi-layer tar and paper waterproofing system with a gravel topping. The gravel contributes to impact and fire resistance, while the multiple layers below act as a redundancy strategy. Remove one layer, and there’s another below it still doing the job.
The materials used in built-up flat roofing have changed over the years with tar and paper being replaced with tougher fiberglass sheeting, but the basic principle remains the same. As far as budget goes it is the least expensive system to use, but not really recommended as a DIY job, as hot tar can be hazardous.
Tar and gravel can also add weight to a roof structure, so it is worth factoring this in when designing the frame of the building, whether it is a renovation or a new build. Be aware that wind and rain will, over time, dislodge the gravel and send it into the gutters, so it is worthwhile to check these regularly.
Finding leaks on a bitumen-based built-up flat roof is difficult if there are no obvious signs, as the water can track anywhere between the multiple layers. Oftentimes it is simplest to strip off the top layer and re-do the entire flat roof.
Modified Bitumen Roof
The good news is that you can use a modified bitumen single-ply flat roofing system to replace your aged built-up roof, once the deck has been cleaned up. It comes in two different iterations, a hot adhesive, which will require a blow-torch to stick down to the roof, and a cold adhesive which simply rolls out, and tamps down as you go.
A single-ply product doesn’t have the built-in redundancy of a built-up flat roof, but it has a tough-wearing coat that is impact and fire-resistant and is safe to walk on during installation. It is also a lighter product, requiring no additional structural support.
A modified bitumen roof covering will be slightly more expensive than a built-up roof, but you won’t be paying for installation, so you will save money if doing the work yourself. Both systems are expected to last at least 15 to 20 years.
Single Ply EPDM
Rubber roofing is a favorite roofing product for large-scale commercial projects, but it has also become popular at a domestic level and can be installed by seasoned DIY enthusiasts. Some say it’s like covering your roof with a bicycle innertube, and that it is just as easy to repair.
However, an EPDM roof will outlast a bitumen-based system, with professionally installed roofs guaranteed for up to 50 years. It has excellent resistance to UV and temperature extremes and is recommended for roofs that regularly experience sub-zero conditions.
If you are fitting an EPDM roof over an existing structure that has already had a bitumen-based system installed then it is essential that all traces of the bitumen, grease, adhesives, and other debris or associated materials are cleaned off entirely.
If in doubt, it is better to over-board or re-board the roof deck before fixing EPDM as it relies upon a solid clean adhesive bond between the underside of the material and the substrate. It also comes in a selection of widths, so your installation could be carried out without having to add seams.
This is one of the most durable, UV and impact-resistant flat roofing systems on the market. It is a complex roof to install, but well within the range of experienced DIY enthusiasts. It’s a great choice if you are looking to use the flat roof for access to solar panels, as a bonus outside space, or as part of a fire escape route.
A GRP roof will typically last a good 30 years before it might need renovation or replacement. It is a seamless installation but can eventually crack over time in exposed areas where the roof either changes direction or meets an upstand, however, it is a simple system to patch, and leaks are easy to discover.
Make sure you have checked the weather forecast before installing a GRP roof, as the materials do not set well in colder weather, so it’s not a great system to fit in the winter when temperatures are close to, or below, 40°F. Also, when installing GRP, it is best not to do it in the summer when the roof substrate temperature gets close to 85°F.
If it starts to rain during the installation, then it’s best to cover the roof and wait for the weather to dry up before continuing. Once complete, the GRP roof can be inspected once a year when cleaning out the gutters, or carrying out other regular maintenance.
Metal roofing is ideal for use on a low-slope roof, which is what a flat roof is in reality. Galvanized steel sheet roofing has gained popularity for its easy installation, curb appeal, and durability. It can be fitted down to a pitch of 3/12 and can cover any existing roof structure.
The best type, for aesthetic reasons, is the standing seam system, which has no visible fixings, with each panel interlocking on the joint. The screw-down system is recommended for back-of-house or garage structures.
Metal roofing is popular for its resistance to fire and impact damage, and it is very simple to install with many manufacturers able to supply cut-to-measure panels if you contact them directly. Some metal panels come coated in a granular finish, and pre-molded to resemble shingles, so your project can blend in well with the neighborhood.
Hybrid Green Roof
Hybrid green roofing is a finish that requires serious consideration by your structural engineer, as it will be heavier than other roofing systems. It should not be used as the first line of defense against the elements, but rather as a protective layer over the top of a roof finish.
As such, it should extend the life of many of the roof finishes already mentioned by decades. The first step is to cover the entire flat roof area with an impermeable membrane such as a butyl pond liner. This will retain water and also act as a root barrier, stopping plants from invading the roof space.
This option uses drought-resistant plants such as succulents and mosses that will absorb and retain moisture. The growing medium is left shallow at about 2” with a drainage layer of about 3” below it in order to deter unwanted self-seeding plants.
It is good to ensure that there is safe access for weeding purposes, and is a superb way to improve views from the upper stories of a property, helping your roof to blend in with the surrounding environment, or establishing much-needed biodiversity in a town.
SPF Spray-on Roofing
Spray foam is one of those products that has been used extensively as an internal insulation material, but it has also been formulated as an external roofing system. It is, by far, the most expensive way to cover your flat roof per board foot, but it does come with some very positive points.
For a renovator, the first big plus is that you do not have to scrape up every last piece of existing roofing material. As long as the roof is dry, and grease or oil-free, spray foam will have no difficulty adhering to any substrate, be it shingle, asphalt, wood, metal, or concrete.
Should your flat roof area be made up of more than one level, angle, or substrate, then spray foam requires a minimum level of preparation. It will expand into every nook and cranny, ensuring an airtight and waterproof seal, including around penetrations such as vents.
A protective silicone layer tops off the spray foam with embedded gravel to make it impact and UV-resistant. With 3” of spray foam you will also add another R-21 of insulation to the roof.
Although DIY spray foam has become more mainstream for internal applications, it is recommended that a professional team is employed for external work. Expect to be quoted between $7 and $12 per foot.
What Is the Best Roofing Material for a Flat Roof?
The least expensive flat roofing systems are bitumen based and need no great level of expertise to apply. They are probably the best fit for projects that sit in milder regions and are least exposed to extreme climates.
Mid-range options are the liquid GRP and EPDM systems that require a much higher degree of preparation and attention to detail. They are probably better suited to new construction projects rather than renovations where they will be installed directly onto a clean and dry OSB or plyboard substrate.
A metal roof is a great-looking way to finish any flat or low-slope roof but would be best avoided if your project is in a maritime or humid region where excess moisture or salt-laden spray might shorten its lifespan.
A hybrid green roof is also a great option as it combines the added insulating aggregate layer and the bio-diverse mitigation of planting too. It will add to the cost of the roof but local codes could point the way to tax credits, making it more affordable.
At the top of the list cost-wise is spray foam, however, if your roofing budget calculations include foam board insulation, then this might just offset the final sticker price.
The best type of roof finish for your project depends upon three things: project budget, location, and your DIY abilities.
If you are a seasoned DIY enthusiast, then many of these roofing systems will appeal. Fortunately, there is a diverse array of systems and manufacturers so it should not be difficult for you to find one that suits your situation.
In case you are looking to cover a non-flat roof too, check our article about the best types of roofing materials.