Construction site curiosity

March 5, 2008 11:44 AM

What's the purpose of painting concrete/cement block black and what are those flaps above the windows for?

I pass by the building site for an addition to a large, multi-storied school. It looks like eventually the building will have a brick exterior to match the rest of the school. Right now, the building is concrete block with no windows. At some point, flaps (tyvek?) were added above the window frames and the concrete/cement block was painted black. Why?

posted by nnk to grab bag (11 comments total)

The black is probably sealer, and the tyvek flaps around the windows are probably part of the layup process for the flashing and everything that goes around a window.
posted by Netzapper at 11:46 AM on March 5

Waterproofing.
posted by aramaic at 11:59 AM on March 5

nthing waterproofing. The flaps are self-adhesive waterproof membrane (one brand is called Jiffy Seal), and the black paint is probably some bituminous waterproofing substance (we like to call it "pookie).

Tyvek is a building wrap used in lieu of the more traditional building paper, generally because it's tougher or whatever. I've never heard of it being used for waterproofing on a block building, but they might have developed a product along those lines.
posted by LionIndex at 12:08 PM on March 5

why not tyvek all around the window frame?
posted by nnk at 12:09 PM on March 5

LionIndex - the flaps might not be tyvek -- and they are just at the top of the window.
posted by nnk at 12:10 PM on March 5

ooh. I should've read first. So it's not tyvek. still why only at the top?
posted by nnk at 12:11 PM on March 5

still why only at the top?

That's the direction the rain comes from. The other edges have less direct exposure and don't allow water to pool.
posted by Jeff Howard at 12:31 PM on March 5

The top "flap" is laid on underneath the black "tar" (asphaltic waterproofing) to direct water away at the top of an opening. For the bottom, the "flap" is placed on top of the waterproofing during window installation. This directs any water towards the outside. Think of how shingles are done. Similar concept: direct the flow of water outward.
posted by mightshould at 1:09 PM on March 5

still why only at the top?

They'll lap the flaps under whatever they eventually put over the block to create a kind of shingling effect that takes water to the outside of the wall. They're just loose right now because they haven't installed the finish yet. On a wood frame building that would get stucco, the flaps would go underneath the building paper (or tyvek) that goes over the rest of the exterior wall, so that water seeping through the stucco and running down the building paper would stay outside of the building envelope. If your building ends up being a block/brick cavity wall building (where there's an air space between the block and brick), the lower end of the flap will eventually sit under the brick in a steel angle lintel or something similar. Building paper will eventually come over the top of the flap, to lead water onto the flap, then under the brick and out of the building.

They'll eventually do something similar at the bottom of the window, but in that case, you want the waterproofing to go on the outside of the finish, so that any rain falling onto the windowsill drips right off the exterior. If the building in question does end up having a brick veneer, they'll flash (apply a waterproofing membrane of some sort to) the sill after installing the brick, with a drip edge just outside the face of the brick. If the flashing is copper or some other material nice enough to stand on its own, they might just leave it like that, but they might also install some sort of sill coping, made out of bricks or stone or concrete or whatever. The coping would still have flashing underneath, since masonry is basically porous.
posted by LionIndex at 1:21 PM on March 5

See detail C on this page. Where the detail points out "copper flashing", just substitute "black flappy stuff" in your head.
posted by LionIndex at 1:32 PM on March 5

That's really cool. Thanks.

I have other questions, but I am going to leave it at that.
posted by nnk at 2:21 PM on March 5