Tar & Gravel vs. Modified Bitumen

In this case, the tar & gravel BUR is superior to the modified bitumen roof. The modified bitumen roof is substantially cheaper (and lower quality). This should not be a surprise... If you're willing to pay for better quality, the superior flat or low slope roof replacement solution is an SPF foam roof. There's many reasons behind this and we have many articles on this forum discussing it. For example, here's one article which compares tar & gravel or modified bitumen to sprayed foam: Foam Roofing vs Tar & Gravel.

Pictured: an example of a modified bitumen roof:

Question: Tar & Gravel vs. Modified Bitumen ($21k or $11k)

I have a few bids on my flat roof in Palo Alto, California (Bay Area) and I am a bit confused because of a huge price discrepancy. Here are the best two bids that I have received so far. I think part of my confusion comes from the fact that the cheaper roof comes with a better guarantee (7 years for the Modified Bitumen vs 5 years for the Tar & Gravel). Can you please look this over and let me know which is the better deal?

Below: a Built Up Roof (BUR), also referred to as Tar and Gravel:


Bid #1 Tar & gravel $21K : 3-ply class A, 1 layer Malarkey UDL modified base sheet and three layers#eleven Type 4 fiberglass ply sheets hot mopped together. All walls are two ply with the Malarkey base sheet and a single layer of Certainteed Poly SMS sheet (also hot mopped together). The roof will then be surfaced with a coating of hot asphalt and new gravel. This roof comes with: a five year guarantee, and the business is in good standing on the BBB Honor Roll. (This deal also includes drains, flashing (galvanized), and chimney set)

Bid #2 Modified Bitumen $11K: Mop-on Modified Bitumen Roof System, 1 ply Glas Base sheet 28lbs (nailed or spot Mop) per 100s/f area, asphalt solid mopping 25lbs 100s/f and 1 ply Flintlastic modified bitumen 100lbs (Granule one side of Membrane). This roof comes with a seven year guarantee, and I have checked around locally, and I have confirmed this is a reputable firm. (This part is the same as bid#1, this deal also includes drains, flashing (galvanized), and chimney set)

Below: Modified Bitumen Roof three dimensional cut-away:

Answer: Tar & Gravel Is Superior to Modified Bitumen

This should come as no surprise--you get what you pay for. The tar & gravel roof is more expensive but, properly installed it can last 40 years with proper maintenance. The modified bitumen will probably only last 13-20 years, and that assumes proper maintenance and installation. Expect your flashing to deteriorate first somewhere around the twenty year mark (this will require some maintenance). As long as you don't allow the substrate roof material to deteriorate, the Tar & Gravel roof could continue to last twenty more years (about 40 years total, as mentioned above).

Pictured: a Tar and Gravel also known as Built Up Roof (BUR) three dimensional cut-away view:

Maintenance Is Key To Making A Flat Roof Last Longer

If you want to get the full potential from your roof, be sure to do regular maintenance; I recommend you inspect the roof every 6 months, and check for problems such as clogged drains, deteriorating areas, uplift on sections, ponding, or holes, etc. Be sure to nip the problems in the bud early on, or they can quickly grow into serious matters which will force you to replace the entire roof.

Price Discrepancy between Tar & Gravel vs Modified Bitumen

The BUR (tar & gravel) roof will last about 40 years and the Modified Bitumen (mod bit) can be expected to last 15-20 years. The pricing difference really is rooted in the difference in materials costs. The BUR roof costs more, and is higher quality material. It lasts 2-3 times longer as well. Now, as you know, the value of the dollar has been slowing falling due to the rise of inflation (and perhaps with the rise of the Euro too). Paying more now is an investment that will pay off over the long run. Discount the present value of those dollars (finance term there), and considering you'll replace the mod bit roof a few times on the way (and the price will be higher too), and the BUR roof is clearly the best deal.

Maintenance of Tar & Gravel & Modified Bitumen

I agree--maintenance is very important if you want to maximize the life of your roof, and any good contractor will echo that. Your twenty year modified bitumen roof will be broken in just ten years (half that time) if you completely neglect it. Many contractors offer extra guarantee or maintenance packages that can allow you to extend the life of your roof. Unless you're going to do the maintenance yourself, it could actually save you money in the long run (depending on the cost of the package). Be sure the company you're buying the roof from is AT LEAST twice as old as the warranty or maintenance package they are offering. You would have to think me a fool to buy a 10 year guarantee package from a 5 year old, or even 10 year old company. Now a ten year old company offering a 5 year warranty? That's something I could believe...

Foam Roofing: The Superior Flat Roof Solution

With all that in mind, why not just by an SPF Foam roof. They last the life of the building, insulate and they use cool roof coatings. They might seem expensive, but you'll break even much sooner than you think (vs the cost of other roof systems). Just consider you'll cut your energy bills anywhere from 50-80%. Check out this post which compares a Mod Bit roof vs an SPF foam roof. Both of the roofs were hit by Hurrican Hugo--and take a wild guess at which roof lasted through the gale force winds? Modified Bitumen vs SPF Foam Roofing

Below: closed cell Foam Roofing systems insulate, correct low areas, protect from leaks, and last the life of a building when properly installed:


Learn more about the best flat roof solution here: Foam Roofing