Flat Roof - Chimney Flashing Question

[Gulf]

Dan,

Mike brings out some good points about metal roof condensation. We call it "sweating" down here. But first::

I had planned to lay the corrugated sheets right over the framing of the roof (rafters every foot).

I just wanted to note that there must be horizontal lathing installed to which the metal roof is attached. You probalbly knew that, but I have to be sure. The sheathing that Mike describes would take care of that.

Down here, insulation under the metal roof is installed to limit the sweating. I was wondering if you are going with metal or wood framing?

[Danno7]

Hi Joe and thanks for your message. I am planning metal framing with corrugated metal roofing panels. I am not sure I understand about the lathing. Is this the same type of lathe that we can use for stucco and scratch coats, and just lay it over the framing, under the corrugated metal? Second question, will either method (lathing or durock) work to help against the condensation under the roof? I did not know about the lathing so thanks for mentioning that.

Dan

[Gulf]

Carpentry terms vary due to location. Your area may call it "metal roof battens". Metal roof panels seams run vertical. Lathing "battens" run horizontal. They tie the rafters together. In some cases they allow the room for insulation and ventilation needed to limit condensation.

[Danno7]

So, based on these great suggestions, I am thinking that this would be ideal: Durock over steel roof rafters; waterproof roofing membrane on top of that; 1 x 4 wooden battens laid across, spaced 16 in. or so; lastly corrugated metal panels on top with . All necessary flashing, foam closure strips, roofing screws, etc. Really like the idea of gutters to divert the water. Any thoughts greatly appreciated and thanks again!

Dan

[SableSprings]

Dan, roofing plan looks doable...but...Remember that you are going to need to tie the durarock/batten "sandwich" to your metal rafters and the roofing panels will need to be attached to the sandwich. I would use exterior plywood (5/8") instead of durarock simply because you can screw the roofing panels down into the plywood. You don't need lots of roofing screws to hold down each sheet, but you do want them securely fastened to the base roof material. Durarock is a good material, but I think you might have issues getting the metal roofing attached with your #19 post idea (IMHO)...I'm also a little leary of walking/working on Durarock. I used it inside to do a shower stall and I just wouldn't be comfortable using it for a larger roofing project. Since yours is only 5' x 6', the Durarock should be fine as long as you can secure that roofing down. I just haven't worked with metal joists and don't know what kind of options you have to tie down the roofing.

Also be aware that gutters work well for most of the runoff water, but if you have ice/snow slide off the roof, it can catch the gutter and take it off. Gutter covers can help not only to keep leaves/needles out of the gutters, but help keep the "snow slide" from catching them. Again, in snow prone areas, hardware/roofing supply stores sell small "fins" that help keep the snow from sliding off so easily. Usually you install several rows of these across the lower half of your roof. Just some thoughts that you might consider...I assume you are not putting electricity out to your enclosure. If you had electricity, there are heating wires sold to melt the snow along the bottom half of roofs...keeping the snow slide minimized and keeping ice from forming along the gutter line. You might go pick the brains of somebody that has done roofing work in your area or neighbors that might have experienced some of these issues (and dealt with them ). Even though your roof area will be relatively small, possible "snow slide" still should be incorporated in your planning.

[Danno7]

Hi Mike,

Do you think I could get away with roofing felt on top of the plywood? I didn’t have much luck at the local HD with the membrane (they only have rolls for $200 ). Thanks again.

Dan

[SableSprings]

Yes, having to buy a whole roll of the membrane was the issue I had as well. I used the roofing felt/tar paper and it works just fine. I don't see why it wouldn't work in your situation. The stuff I used seemed a bit thin and would rip/tear a little too easily (at the staples), so getting a little heavier duty felt if possible is worth it. Since the tar paper I used was from 9 years ago, I suspect what they sell now and in your area is of better quality. I still think it would be worthwhile checking with some local builders or roofers to see if they have some scrap membrane material...since you really don't need that much.

[Danno7]

OK great! only $25/roll and I can use it on the sides when I do the stucco next year. Gonna get to framing roof this weekend!

Dan

[SableSprings]

Dan, just checking (with your "heavy" rainfall) that you are making sure you have enough overhang in front to shelter you when working the oven. It's not fun to stand/work in the rain. Also a bummer to not have a bit of covered work space to prep a pizza or to set down a finished one. If you can do a slightly flatter roof, you can extend farther out on the sides to cover you and your prep table...just another random thought

[JRPizza]

I also used heavy felt paper between my metal roof and the plywood below, and have only had one problem. I had thought the insulating properties of the felt paper in combination with the 3/4 inch plywood would be enough to prevent condensation on the underside of my roof, but when everything is cold soaked and we get a blast of warmer wet weather here in the Northwest, we still get quite a bit of moisture on the lower surface. Over the past few years we have gotten some mildew spotting on our stained surface and will eventually need to do something to clean it up.