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Question DetailsAsked on 12/12/2016

I have condensation under my old tin roof should I remove it or go over top?

I have a mobile home with a tin roof. I am putting on a epdm rubber membrane roof. There is condensation under the old tin roof because the previous owner removed the barrier when replacing insulation. My question is do I have to remove the old tin roof or will the insulation under the membrane on top of the old tin help stop the condensation. Thanks for any help in advance.

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2 Answers

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Here is a prior similar question with response FYI -


http://answers.angieslist.com/getting...


Basically, if the condensation is from leakage water getting through the tin roof and condensing underneath, a new waterproof roof should solve that problem.


If you mean tin roof over plywood sheathing, yuou ned to dry that out and check for sheathing rot - so I would say remove and discard the metal roofing in that case.


If the condensation is on exposed underside of the metal roof from a cold roof condensing moisture from warmer, moist air below, the answer is in two parts:


1) if you have an unheated attic, so the condensation is forming on the cold underside of the roof, there will be little change after putting a new roof on, because in cold weather the top and underside of the roof will both be near equally cold unless you are losing a LOT of heat from the living spaces to the attic so it is quite warm - in which case that would be a source of significxant moisture, so you would still probably get a lot of condensation in cold weather and likely attic framing mold or rot.


2) if the tin roof is your ceiling (inside conditioned space, perhaps underlain by a liner for appearance sake) then an insulated roof, provided to has good enough insulation to yield an under-surface temperature of say 50-60 or above, should greatly reduce if not eliminate the problem - so would need substantial insulation on the roof. Also, near the eaves the metal will still be real cold if it is cold outside, so generally removing the metal first is recommended in your sort of case.


A couple of questions - and ones to discuss with the roofer proposing membrane roof and insulation - is if you have a metal roof now (assuming you mean without plywood underlayment) then how does he propose to fasten the roofing down ? Adhesive to metal is generally not recommended as a long-term solution for roofs, and most membrane over insulation systems presume a wood sheathing to fasten into, so ask how he proposes doing that. Not undoable, but most metal roof fastening systems cannot reach through thick insulation into the metal roof, and most membrane roof systems also assume tying into a sheathing under the insulation, so that needs to be addressed - especially in high-wind areas, where a poorly fastened down insulation and/or membrane might try a Mary Poppins and just fly away in the next storm.


Another point is how he is going to get a smooth, uniform appearance - since most metal roofs are corrugated or raised-seam, that would make for a real uneven roof and poor fastening conditions unless he puts in closely spaced battens to provide a uniform bearing surface. To do that is OK if done right - though it is recommended that the batten fasteners are sealed where they go through the metal so if water gets in there you do not get a lot of leaks. Also, because moisture is likely to condense in that space, it is important to use treated wood for the battens. Also, since you are talking insulated roofing, it is important that the void space between insulation board and metal is not open to the outside air or you will get airflow and cooling/heating at the metal interface to near outside air temp that will destroy the value of the insulation.


If he can't satisfactorily answer those questions, then you might need a different contractor - or he may say the metal roof has to come off - which if it is the only roofing layer, would probably mean you would need new plywood sheathing installed to fasten the new roofing to - which will add anywhere from commonly $3-5/SF to the installed cost.


My recommnedation - if you intend to live there basically forever, and assuming the money is there to do the job right, I would reroof from scratch - take the metal roofing off. This assumes he is talking board insulation - if talking sprayed-on closed cell urethane foam, then with proper surface prep that can go over metal roofing fine - but the issue still exists of how he intends to tie the membrane down so it cannot blow free, because urethane foam does not have much fastener holding power, especially the lower density foam-in-place foam. it is also VERY hard to get that prodcut real smooth and flat, so a membrane roof over that would likely look a bit shabby. Used on industrial buildings and tanks where close-up appearance is not real important - but for a mobile home usually it is only used that way if being sprayed with a membrane, not a sheet membrane (whcih has much longer life, BTW). So the appearance factor might lead you back to having to remove the metal roofing anyway.

Answered 1 year ago by LCD

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Answered 1 year ago by Member Services




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