You may notice that a tar and gravel roof is installed on many low-slope or flat roof commercial buildings.
Contractors and builders use gravel on flat roofing for two primary reasons; the first is to protect the underlying layer of roofing materials on built-up roofs (BURs), and the second is to secure or weigh down the roofing material, as is the case with single-ply flat roofs.
What Are Tar & Gravel Roofs or Built-Up Roofs?
Built-up roofs or BURs (also called a tar and gravel roof) is a well-established and popular style of flat roof that is made up of alternating layers of roofing felt or fiberglass and asphalt that is applied with heat, which combines to make a long-lasting sturdy roofing system.
The gravel embedded on the top coat of asphalt (the flood coat) protects the underlying layers from the elements, including ultraviolet rays and natural threats like roof hail damage or heavy rains.
Various types of BURs require a different type and weight of gravel depending on the underlying layers, the structure of the building, and the climate of the location.
Even though the asphalt waterproofs the roof (highly important for flat roofs where water and moisture can build up instead of draining off as it does on a sloped roof), the gravel layer extends the life of the roof.
Note that the “gravel” is usually some sort of aggregate material like pea gravel, slag, or mineral granules that are pea-sized and about a quarter-inch thick, and serve the purpose of satisfying the Class A surface burning designation.
Why Install A Tar & Gravel Roofing System?
The benefits of a tar and gravel roof are numerous.
They tend to have a lifespan of around 20 to 25 years depending on the location and the structure of the rest of the property (barring any major natural disasters of course).
The weatherproofing aspects of the gravel embedded in the roof can prevent degradation like cracking or blistering that leads to leaks or other failures over time.
Types of Gravel and Tarred Roofs
There are two main types of BURs or tar and gravel roofing systems, modified bitumen roofs which are a mix of asphalt and rubber with a layer of gravel on top, and ballast roofing, which is a different style of larger gravel applied on top of a single-ply roofing system.
Modified Bitumen Built-Up Roofing
Bitumen roofing systems are built up when modifiers are added to the typical roofing asphalt, replacing plasticizers removed during the distillation process.
It is then reinforced with fiberglass or polyester fiber matting in order to increase its durability and strength.
The two most common types of modified bitumen roofing are SBS (styrene-butadiene-styrene) and APP (atactic polypropylene).
SBS improves the flexibility of the roofing membrane, giving it stronger expansion and contraction capabilities, while APP improves the ability of the roofing system to stay strong as it ages.
Each type is popular in different climates and applications – an experienced roofing contractor in your region will be able to advise you as to the right choice for your property.
The reasons to choose a modified bitumen roof are many.
The gravel holds and releases heat and helps to improve water evaporation, keeping both heat and moisture away from the underlying layers.
The looser gravel helps to provide a better grip for foot traffic when your team has to access the roof or contractors are performing maintenance and repairs on your bitumen roofing system.
Note that the gravel on modified bitumen roofing systems must be kept fairly even on the roof at all times, and some of the gravel must remain loose to prevent damage or punctures from foot traffic.
Ballast Tar & Gravel Roofing
On the other hand, ballast roofing might be the right choice for your commercial property if you live somewhere with a ready source of ballast (rocks/gravel of the appropriate size) and you prefer the aesthetic look and texture of the larger stones.
Paving stones can also be added to create walkways, which add value to your property.
Ballast is commonly used with roofs made of tar-and-gravel roof alternatives, including ethylene propylene diene monomer (EPDM), polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO) single-ply flat roofs.
It’s fire resistance (ballast roofing offers Class-A fire resistance without the need for gypsum underlayments or expensive fire-retardant chemicals), sustainability, and ease of repair (since the ballast stones aren’t actually embedded in the membrane, they can be easily removed and the membrane repaired as necessary), and overall energy efficiency make ballast roofing systems a popular choice in many areas.
The Maintenance Requirements of a Tar and Gravel Roof
How long a roof can last while properly maintained tar and gravel roofs can last several decades.
A roof can be strengthened or further built up by adding more layers to the roof like additional fiberglass or foam to increase its insulation; what’s more, applying a new acrylic or elastomeric “cool roof” coating can help to further weatherproof your flat roofing system.
Tar & Gravel Roof Repair
Repairing tar and gravel is of course best left to the professionals, and since a properly maintained BUR roofing system can last for decades, chances are you will have to repair your roofing system at some point during its existence.
Leaks and other damage to tar and gravel flat roof are repaired by patching the roofing membranes, and a temporary roof leak repair may be required before a roofing contractor can create a more permanent fix.
That said, if you’re evaluating an existing tar and gravel roof on a commercial property that you are considering purchasing, note that many leaks may indicate a significant amount of past leaks and weak areas, which are undesirable for obvious reasons.
You can and should always consult with your public adjuster/building inspector and local contractors before acquiring a new property.
After all, if the patches were not done correctly by experienced professionals familiar with these types of roofing systems, these areas are more likely to leak again, especially if your region experiences large amounts of rain or natural disasters like hurricanes, floods, and hailstorms.
Most experts tend to say that if that if more than 25% of a tar and gravel roof is covered in patches, then the roof needs to be entirely replaced.
What’s more, if you are considering buying a property with this type of flat roofing, walk on the roof yourself to see how it feels under your feet.
Things like loose areas, depressions, cracking, discoloration, blistering, and other deteriorated surfaces are all indications that this roof will likely need to be replaced sooner rather than later, so it is up to you if you think it’s worth the investment.
When You Should Not Choose A Tar & Gravel Roof
While tar and gravel roofing or built-up roofing systems have many positive aspects, if you live in an area or the roof is in a location where it will receive strong sun exposure, hot tar roofs will likely damage the roof’s membrane if it is left uncovered or unevenly covered by the gravel.
It is also not typically recommended for areas that receive large amounts of snow or rain, since extreme precipitation can cause significant damage as this roof type is more prone to leaks, particularly if the underlying layers and flashing were not installed properly to begin with.
This is because flat roofing is prone to something referred to as “ponding”, which is when water pools and stays on a flat roofing surface for more than forty-eight hours, forming a “pond” that places increased weight on the roof.
Ponding can change the roof’s overall structure, causing depressions and affect the drainage slope which causes the ponding to continue, increasing the likelihood of more damage.
Ongoing pooling or ponding can result in vegetation and mosses as well as mildew growing on the roof surface, which can lock in moisture and cause further deterioration.
An adequate drainage system can prevent this problem, but it is still not always recommended to install a tar and gravel roofing system in an area that receives high levels of rain or snow.
As always, check with local experts like your public adjusters when seeking this kind of advice.
Protecting Your Investment in A Built-Up Roofing System
It all starts with proper documentation.
No matter which type of roofing system you choose for your commercial property (or the one that you happen to end up with based on the previous owner’s choices), you should document the condition of the roof from the outset, and regularly update that documentation.
It’s as easy as going over everything with a smartphone and taking pictures and video, and uploading those images and video files to several secure locations (think an external hard drive and the cloud).
This is because if a natural disaster or significant damage of some kind happens, you have proof of the prior condition of your built-up or tar and gravel roof, which will make your upcoming insurance claim much easier to process.
You will thank yourself – and your public adjuster will thank you too – later.
Getting a new roof installed is a major investment, especially in the case of tar and gravel flat roofing systems which may require significant maintenance or tar roof repair over the course of their existence, which can extend for several decades.
Which is why it is recommended you work with a professional roofing contractor and a public adjuster in advance of your claim, getting all your questions answered before you start to repair or rebuild your roofing system.
Can you recommend any contractors in SF Bay Area? We had a tar and gravel roof over a sunroom replaced approx 3 years ago. After this Winter’s rains, the roof leaks. Apparently no flashing was installed where roof met the 2 vertical walls.
Thanks for any recommendations/referrals you may hav. The home is in Oakland Ca.
Peg, Thank you for reaching out. Unfortunately, we cannot specifically recommend a contractor. You want to make sure that your contractor is a member of the NRCA and is in good standing with them. I would recommend contacting the NRCA (National Roofing Contractors Association) and asking them for a referral. They have their finger on the pulse of the top tier commercial contractors across the country. Good luck!
We bought a brand new home and just had our first major rainstorm ( Albuquerque, NM gets flash floods) and handfuls of gravel fell down with water from our flat roof. Is it normal that the pebbles just wash off? Thank you for your time.
We had a new tar and gravel roof installed about 2 summers ago. I’ve noticed lately there has been more than usual (gravel) that falls from the roof into my sidewalk in front and back of the house. Could this be caused by wind? Also, the roof has a small pitch along the house but nothing too dramatic. We live I. anchorage Alaska.
I am the assistant chief of lake shore volunteer fire company in pasadena md i have contacted 4 different roof companys and i am being told that companies no longer do tar and stone roofs they all want to do a membrane type are these companies just trying to make more money or has the industry gone away from them ?????
Roger, I live in Bellingham WA and am finding the same thing here. Our existing tar and gravel roof is now 40 years old and has been remarkably low maintenance, so would like to go with the same thing but so far haven’t been able to find anyone to do it.
areas prone to high winds/hurricanes/tornadoes do not allow gravel roofs as the gravel has a tendency to fly around and do more damage to surrounding structures. So, no, it is a code thing and not a money thing
James :hi I have a low slope role roof type shingle , always having trouble with wind ripping the shingle off the roof every year some times 2 or 3 times and it’s getting costly .. my question is ? can I spread hot tar and rocks to add weight to the roofing material ?
Roger, I live in Bellingham WA and am finding the same thing here. Our existing tar and gravel roof is now 40 years old and has been remarkably resilient and low maintenance, so would like to replace it with the same thing, but so far haven’t been able to find anyone to do it. I suspect there may be some ‘planned obsolescence’ at work here.
I have a very small leak in my tar and gravel roof that’s about 25 years old. How much will it cost to repair it or do I need to replace it? I live in an four bedroom Eichler home that has a tar and gravel roof. Please let me know who the contractors are that can repair or replace my roof.
If you have an Eichler, you should get a TPO roof. T&G is no good for Eichlers.
Hello, an “Eichler” describes a structural design of a house that you would see built in the late 1950’s and 1960’s. They typically have a flat roof or a roof with a very slight pitch and big windows. The T&G reference is “tar and gravel”. This day and age, I would agree that a TPO roof would be a good option for this home design as the TPO is lightweight for the roof structure and the white color of TPO is reflective and would reduce cooling costs.
How to clean moss and mildew from a tar and gravel roof?
I have a project that requires resurfacing of an existing aggregate surfaced BUR. Do you know where I can find some guidelines for this process that will help the compose a Specification Section for my Project Manual? I looked in my NRCA guides and could not find recommendations for a large roof area, only spot repairs.
Thanks for this great article Tar and Gravel roofing I believe has a nice finish. Has its disadvantages but it is not a bad use in some parts of the world.
Tar and gravel roofing I can relate to this had some experience. Thanks for sharing this.
Is it possible to add a third layer to a tar and gravel roof that has already had two layers put on top of the original 100 year old roof?
Hi Tom, adding a third layer to a tar and gravel roof could be considered risky due to the weight of the roof itself. A 100 year old structure would probably need structural framing reinforcing to have a 3rd layer added. It is always smart to have a structural engineer assess the situation and write a report. Aside from the weight, the gravel would need to be scraped off and discarded, so the newer technologies out there such as SPF foam and TPO offer solutions that could be a better way to go. If you ever lay a new roof over an existing roof, you must make sure there is not any moisture under the existing roof.
Ok stupid question but it’s worth a shot I do not have a flat or low sloping roof can I still do tar and gravel it I love the way it look
Hi Lori, a tar and gravel roof can only be installed on a “flat” or “low slope” roof. If you tried to apply this roofing system on a roof with a pitch, the liquid applied tar would follow gravity and run down hill before it was able to “set up”.
Need new rock roof
Need a new rock roof
Yep .same here. I’m in Bay area. Got quoted for a 1100sq.ft roof of $21,000. OMG !!! $21,000. That’s a Tile roof. Single story, straight Ranch Style. $21000 !!! Somehow, I think that’s outrageous. Am I right? BTW, I have a Tar&Gravel but nobody wants to do it. They say that would be even more if I find someone.
Is it possible to remove dark pea gravel and replace with white gravel for increased reflectance? If so, what size pea gravel (I found 3/8″ locally) and how deep? Currently, the roof is sound and has no leaks. Thank you.
Is it advisable to install gutter on flat roof ? because loose gravel might come down.
Thank you for this informative article on tar and gravel roofs! I appreciate the details on both modified bitumen and ballast roofs and the advice on properly maintaining and repairing this style of roof.