I’m helping a friend finish a roofing job on his unattached garage. The garage is almost flat (maybe a 12/1 pitch) and has a concrete block parapet on the 2 rake edges. He applied rolled roofing horizontally and ran it up to the top edge of the parapet. I’m going to help flash the parapet.
We plan to fashion a simple cap out of aluminum roll flashing to straddle the top of the parapet, but I’m concerned about the eventual corrosive reaction between the concrete block and the aluminum. Since we don’t have a brake we want to stick with aluminum, which I assume will be much easier to bend than galvanized or stainless steel (plus it’s cheaper). I’m thinking of 2 possible approaches to isolate the cap from the block to avoid the corrosion effect – either capping the parapet first with ½” ply and affixing the aluminum cap to the ply, or bedding the cap in a layer of roofing cement directly over the block. Are either of these workable solutions? Each parapet runs 20 feet long and is 8 inches wide across the top – I haven’t done the math to see which would be cheaper, but it seems that the roofing cement would be simpler (but sloppier).
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Any input is appreciated. Thanks!
Rob
Replies
How are you bending the aluminum? Is the rolled roofing half lapped and tarred? Does the rolled roofing come up the parapet and over the top of the block? Is there a treated plate on top of the block? Is there a cant strip at the roof and wall juncture? I would core the block every 4 feet and put in anchors, then a treated 2X8. I would not use aluminum unless I could accurately bend a drip edge on it. Why don't you check some local roofing supply houses to see what they stock for pre-made cap.
I was going to bend the aluminum using a pair of 2X8's as a poor-man's brake - perhaps I'm naive but I think I can fashion a functional (if crude) drip edge this way. The rolled roofing is lapped and tarred. It comes up the parapet wall but not over the top of the cap. there is currently no cap on the parapet - wood or otherwise. It's just exposed block.
if you don't care how it looks your plan might work... but for it to look right you need a hem and a slight kick...on the edge if it's thin alum...
but... any one man sheet metal shop could knock out what you want in galv.. even in some of the lightest stock i think you'd be better off... in 10ft runs you could get away with block'n nailers into your blocks 5ft oc (30" oc would be better) and use gasketed roof screws to hold it in place...
p
I don't care how it looks - it's my buddy's garage!
Actually, I think I'm going to look into the prefab option at a sheet metal shop. On your installation instructions you lost me a bit on the terminology. By "block n nailers" do you mean fastening wood (PT?) into the block and screwing the cap into the wood? So the cap is essentially floating above the block? Or are you fastening the cap directly to the block parapet?
yes fasten the wood to the block.... actually in the block i only say this because... you should be able to get away with the best use of materials.... most sheet metal comes 48 x 120... so if you were real good you might get away with 8" top with 2" legs... but that would be real close.... but 8" top with 4" legs would be easy... better make that 8.5" top and 3.75" legs... i like legs over bent so they kinda "clip" on, with legs this long you could skip the wood and use a few tap cons thru the legs into the blocks on both sides...
p
Look at the roofing section and find the galvanized drip edge. They have 6" X 1" drip edge, just lap them with tar in between.
Thanks for the responses. Here's what we did - first I called a sheet metal shop - $250 for a simple 3 sided bend (24 ga 4"X10"X4"X42'), $300 with a drip edge lip bent also. Works for me, but more than my buddy wanted to pay.
So instead we went to HD and bought a roll of 20"X50' galvanized steel flashing for $40. cut to appropriate lengths, used a moron's brake (a few lengths of 3/4 ply and a 3# dead-blow hammer) to bend / bang the sheet out of it. Ended up with what we were after, but I value the time and effort spent getting to that point as at least $250 (plus the material was thinner than what the shop would have used). Anyway, he's happy and I have a larger balance in the favor bank.
It got too dark to install, so that's the weekend plan (of course there was a torrential downpour this morning). Any benefit to afixing it to a PT nailer on the block vs. just using tapcons to attach it directly to the block? Also, do I need to bend a drip-edge lip on the bottom legs of the cap? If so, do I bend it in or out?
Thanks!
Wow - just checked a steel thickness gauge chart - I guess the rolled galvanized from HD was A LOT thinner than the 24 ga. the fabricator was going to use - like about 50% thinner. I think the HD roll said 0.010" thickness.
Silly DIY'ers!
Shameless bump. Any thoughts on the relative benefits of attaching the cap to a PT the nailer vs. directly to the block?
Rob
You are not ready yet for a cap flashing.
Where each roll of 90# runs over the cant strip to start going up the wall, will eventually break the seal there. You still need to run another ply with the rake wall dropping down over the cant to the base roof.
I wouild rather fasten a 2x8 to top of wLL AND HAVE SOMETHING TO NAIL IT ALL OFF TO.
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