Weird Roof - with Pics!

10 Replies

Hi BP nation! 

Would like to ask for some advice on a deal in the works - 12 unit multi-family in the Tokyo area in Japan.

The building is a reinforced concrete (RC) structure, which is common here due to earthquake codes. What is a bit strange, and I hadn't seen until now, is the roof. Instead of the more typical flat concrete roof with waterproofing sealant, this one is built with concrete slab laid on a slight incline, with what look like asphalt/composite shingles over top, and the shingles painted over with a waterproof coating!

Pictures attached (Right click -> view image; for a larger version). Note the flat concrete roofs on some of the buildings in the background, which are alot more typical in Japan. Also the building right next door with a very similar roof construction - newly replaced. The age of both buildings is similar, so I suppose that should tell me something :)

The seller will "redo" the roof as part of the deal, but that really just means re-apply the waterproof coating. Existing shingles stay in place as-is. I think it's mostly a way for them to cover their a$5 (seller must warranty some items such as roof leaks and plumbing for 2 years after sale, by law), and I'm not sure that it really meaningfully extends the life of the roof. (The whole exercise costs around $3k USD).

Just wondering if anyone might have come across this type of roof, and could comment on what I should expect from it in the future. It does seem to physically be in decent shape besides some flashing which will be spot fixed as well. It does look like the shingles are absorbing some moisture around the seams..

Not super worried about the replacement cost if it's needed in the future, since the total area is under 2000sqft on a building that grosses $130k/yr. (The aging hydraulic elevator is a much bigger concern... but that's a different story!) 

But if anyone has any experience or tips, would greatly appreciate it!

Cheers

-Dmitri

I wouldn't think it would be a problem. If the roof is made of concrete I don't think it would decay as easily as wood no matter what you decide to put over top of it.

@Dmitri L.

Is there a single layer membrane (either sheet or rolled/painted on) underneath the shingles to seal the concrete?   

It's a new roof to me as well, but my  suspicion is the shingles are for aesthetic purposes with something underneath sealing the concrete and the clear coat over top to prolong the life of the shingles.

Originally posted by @Roy N. :

@Dmitri L.

Is there a single layer membrane (either sheet or rolled/painted on) underneath the shingles to seal the concrete?   

It's a new roof to me as well, but my  suspicion is the shingles are for aesthetic purposes with something underneath sealing the concrete and the clear coat over top to prolong the life of the shingles.

Ah, you may be totally right, I hadn't thought of that. It's difficult to see underneath, they are on there pretty tight, but I'll try to find out next time I'm at the property. There is insulation on the underside of the concrete roof slab (confirmed from inside one of the units - so given that, I would not be surprised if they also sprayed the top of it, or laid down a membrane, for waterproofing before putting down the shingles.

Originally posted by @Jassem A. :

I wouldn't think it would be a problem. If the roof is made of concrete I don't think it would decay as easily as wood no matter what you decide to put over top of it.

Actually, you'd be surprised! Fortunately we don't get very deep freezes here in Tokyo like they get further North, but - raw concrete does soak up quite a bit of water, so I'm betting it would eventually get through. If nothing else, through seams in the individual slab sections.

One of my pet peeves is actually another technique that is pretty common here for the exterior walls of buildings, as well as concrete fences - leaving the raw concrete exposed to the elements! It looks great when the building is brand new, but without a good power wash every year or so, the aesthetics deteriorate quickly - dust will accumulate on horizontal surfaces, then get washed down the face by rain - leaving ugly dark stains down the side. I'll try to find some pics

Obviously it won't crumble overnight or even within 1-2 years like a wood structure might if mistreated.. But concrete needs lovin too

It looks like a perimeter gutter system to roof drains.  Concrete will crack due to shrinkage, which leads to leaks.  Are the shingles asphalt?  What material creates the roof pitch? Wood 2x and plywood? Metal stud framing? Does the owner have 'as-built' plans?  Only way to truly know is doing some demo, but thats not going to happen. Some coating can last quite awhile.

Originally posted by @Jim Adrian :

It looks like a perimeter gutter system to roof drains.  Concrete will crack due to shrinkage, which leads to leaks.  Are the shingles asphalt?  What material creates the roof pitch? Wood 2x and plywood? Metal stud framing? Does the owner have 'as-built' plans?  Only way to truly know is doing some demo, but thats not going to happen. Some coating can last quite awhile.

 Yes that's right. The drains tie in to public sewer.

No wood or metal framing, the structure is fully RC throughout. Looking at the underside of the roof from the top floor units, it looks like the roof slab lays on top of structural concrete interior walls that separate the units.

I do have the building plans, will double check if they have anything listed there as far as materials but I didn't think so.

Btw if you're an architect you should definitely consider japan - the amount of building that goes on here is insane. Something like the highest ratio of architects per capita population, in the world. Lots of demand

Cheers

Dmitri

Originally posted by NA Beard:
Originally posted by @Jim Adrian:

... Concrete will crack due to shrinkage, which leads to leaks. 

So every building in NYC is cracked?  :giggle:

 Yes lots of crack houses.

Let's not get started on shrinkage

Originally posted by NA Beard:
Originally posted by @Jim Adrian:

... Concrete will crack due to shrinkage, which leads to leaks. 

So every building in NYC is cracked?  :giggle:

 LOL yep thats what gives them character! 

There are ways to waterproof concrete like xypex that can be added to concrete in the plant or a topical mortar applied after concrete is cured.  I've seen water move thru an 18'' thick concrete wall with hairline cracks (hydrostatic pressure) and the owner was not happy!  Benetine and asphaltic top coats ot epdm membrane products are commonly used.  

Originally posted by @Dmitri L. :
Originally posted by @Jim Adrian:

It looks like a perimeter gutter system to roof drains.  Concrete will crack due to shrinkage, which leads to leaks.  Are the shingles asphalt?  What material creates the roof pitch? Wood 2x and plywood? Metal stud framing? Does the owner have 'as-built' plans?  Only way to truly know is doing some demo, but thats not going to happen. Some coating can last quite awhile.

 Yes that's right. The drains tie in to public sewer.

No wood or metal framing, the structure is fully RC throughout. Looking at the underside of the roof from the top floor units, it looks like the roof slab lays on top of structural concrete interior walls that separate the units.

I do have the building plans, will double check if they have anything listed there as far as materials but I didn't think so.

Btw if you're an architect you should definitely consider japan - the amount of building that goes on here is insane. Something like the highest ratio of architects per capita population, in the world. Lots of demand

Cheers

Dmitri

 I let my firm handle international work.  We have offices in china. Too much risk when you are an outsider and dont know the culture over there. I have done work in Honduras thru a DoD military contract.