What makes your metal roofed building rain?

[Joe_K]

I ordered a building from PPB a couple months ago, they just delivered materials yesterday and are set to drill and start construction on Tuesday. I ordered a 36x60, 1' eaves all the way around, vented soffit, ridge vent, gutters, doors & windows, 6" concrete, ceiling liner and R38 blown in insulation. I intend on finishing my walls and running electric in the future (not this year). I figured if the ceiling was done already it would save me a headache and I could empty out storage as soon as the concrete is complete. Future plans are to turn it into a light home shop, maybe install a lift, store my tractors and my '67 C10 and install a 240V industrial hanging heater. I would only turn the heater on when working in it to take the chill off, the rest of the time it will be unconditioned.

I live in SE Pennsylvania, and have many friends and coworkers that went a similar route. Ceiling liner, blown in insulation, vented attic space. Nobody had anything done to the underside of the roof steel. I keep reading here and other places about their roof raining? Does this happen in a different climate zone then mine? I thought the purpose of a vented attic space was to prevent this? My friends and coworkers claim it's not happening to theirs and I shouldn't worry. I should mention I've already spent my budget for this building, I don't want to add any expense last minute.

Thanks,

Joe

 

[Big Bad Dad]

Vented soffitt and ridge should have you taken care of. As long as the baffles were installed to make sure air could flow up from the soffitts, past the insulation, and exit the ridge vent..

 

[Joe_K]

Vented soffitt and ridge should have you taken care of. As long as the baffles were installed to make sure air could flow up from the soffitts, past the insulation, and exit the ridge vent..

I saw baffles in the pile of material they delivered, and I specified that when I ordered the building. Thanks for the insight, I hate to spend all this money and have a problem.

 

[Leaflessshadetree]

My roof tin has a felt like coating to absorb the moisture just for this purpose. I have ventilation Soffit and ridge but I don't have a ceiling or any insulation yet.
On cool mornings it sometimes feels a little damp.

 

[brownbagg]

humidity

 

[Jeepster04]

Condensation forms on a surface that is below the dew point. Obviously the more moisture thats in the air the more it will build up on the surface thats below the dew point. When the sun first pops up the air temp quickly goes up and the metal lags behind for a bit.

Ive got to assume the majority of the buildings that 'rain' are open and/or dont have a ceiling. When you close everything in like youre wanting to, the attic holds a bit of heat over night thus keeping the underside of the metal above the dew point. The air around the underside of the metal also doesnt quickly warm up like the air outside does.

Im sure there are certain situations that would allow the underside to rain even if the attic space is closed off like you're wanting to.

People add the bubble wrap/thin styrofoam board/buy metal with the felt backing to keep the warm moist air from coming into contact with the underside of the metal and condensing.

 

[larry_g]

Jeepster is right on, warm humid air hitting a cold surface. Just like fogging the mirror in the bathroom when you shower. To prevent this in the bathroom you have a vent that pulls the hot humid air out. Same in a building.

One other item to understand is that a lot of building materials dry out over time and put moisture in the building. This is normal you just have to deal with it. Keep it vented and air moving and you'll have few to no problems. For me I have to be careful in the spring where we have cold nights and warm days. The shop will cool down over night and in the morning when I open the big doors the warm air will rush in and condense on the cold metal in the building. I usually keep the big door closed till later in the day to avoid the problem.

lg
no neat sig line

 

[Joe_K]

Sounds like I should be OK, thanks for the info.

 

[matt_i]

The metal building that rained on me happened when it was warm, muggy, humid outside. Then a rain started which was much cooler, the metal roof went cold instantly from the rain, the humidity was trapped inside, and drips started.

I've never owned such a building. My guess is that the blown insulation collects and absorbs the finite number of drips and then it bakes out later on and you don't see the effects for the relatively few times this happens.

 

[Firebrick43]

By insulating the huge mass of earth and concrete from the roof by way of an insulated ceiling you will eliminate the problem.

Make sure you put down a 15 mil plastic sheeting between the concrete and stone. Insure good drainage by either an elevated pad or tile and at least 6" of compacted stone under the slab.

 

[mcbane]

There are conditions even with a really well vented metal roof on an unheated structure where it will rain. I have seen this happen when there is a persistent snow pack on the roof in late spring and then the weather warms up suddenly. When the 60-70 degree air hits the metal you get lots of condensation because the snow is keeping the metal at 32 degrees.

One solution seems to be putting insulation immediately below the roof metal and then using a vapor barrier to keep any moisture migrating through the insulation and reaching the cold metal. I have no idea how the detailing works to use that insulation approach while venting the roof.