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cgrfish 05-29-2012 10:40 PM

Double wide roof construction / ventilation issue
 
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I recently purchased a older double wide mobile as a fixer upper for a rental. I knew when I purchased it that it had roof issues, and I was able to get it for a price I think will make it worthwhile.

The roof has significant waviness to it on one side of the mobile, and not nearly as pronounced on the other side of home. Having heard horror stories of 1x2 trusses on mobiles, I was very anxious to find out what the structure looked like to make a plan. This weekend I opened a small section of the roof so I could try to minimize the number of surprises when I start the project.

Here is what I found: The mobile has vaulted ceilings, and it has no trusses - it is single 2x6 rafters spanning 14ft 4 inches, spaced on 24 inch centers. The pitch is 3/12. The roof sheeting attaches to the top of the 2x6, and the sheetrock attaches to the bottom.... The baffling point is that I can see significant sag and wave in the roof, but when you look inside the rooms, it is not evident, nor does the sheetrock show significant damage or cracking....

One other wierd point - at the peak, the sheeting has a 4 inch strip of sheeting at the peak, and then the sheeting from the roof overlap's that, leaving a very small gap near the peak that the sheeting doesnt contact the rafter. My only guess is this was an attempt to create air movement.

My initial plan is to marry a new 2x6 allong side each sagging rafter to provide a solid base for the new sheeting (I plan to replace it all as it feels soft). Anyone have any suggestions on a different way to approach this?

My biggest concern is ventilation. I suspect the sagging and soft sheeting may be from poor ventilation. Each rafter section basically creates a sealed box with no ventilation. Currently their are 7 standard vents on each side of the roof, but their are 35 rafter, so alot of the roof is not ventilated effectively.

I was leaning towards a ridge vent spanning the entire structure, but where the two halve of the home joins creates a solid ridge of about 3 1/2 to 4 inches in width, and thus the ridge vent would need to be upwards of 6 inches wide to be effective. Is that possible - some people say you cant ridge vent a double wide, and others say it works...

If a ridge vent wont work, does anyone have any suggestions on a different approach for ventilation?

Also, I'd really like to add some insulation when I have the sheeting off, but I'm concerned it will just hinder the air flow even more - how much space do I need to leave for airflow?

Thanks in advance for all your input.

gregzoll 05-29-2012 11:00 PM

The 4 inch of strip sheathing is added when the two halves are placed together. Did you ever get a home inspection before you purchased the home? Normally, you enter from inside the structure, not from the outside, when inspecting the sheathing.

cgrfish 05-29-2012 11:06 PM

No home inspection - it never would have passed - cash deal as nobody would finance it... So I know it has problems, but the price was really good... There is nowhere inside to access the attic / trusses / sheeting - without tearing out ceiling... I know the roof has to be replaced, so I made an inspection hole to see what I can learn, and then put a temporary cap over it until I replace the roof next month...

Is the 4 inch sheeting strip standard on mobile's? does it have a purpose?

GBrackins 05-30-2012 02:42 AM

you could always build a traditional roof over it, if it's staying where it is. or move it to it's final resting place and build a roof that will overlay the mobile home. where is it located (town, state) see your local building official and see what the requirements are for your area for such a project. if the inside ceiling is fine leave it alone

bill01 05-30-2012 07:37 AM

I bet it is only the sheathing that is sagging. The most likely used the code minimum chipboard which always feels a little soft to me. I would suspect water damage is causing the sags. You will need to remove a section of the sagging sheathing and see if their is water damage.

Be very careful reading the ventilation recommendations most are designed for northern areas where they are worried about ice dams or condensation on the under side of the sheathing. In hot areas there is much debate if you need to vent roofs since it is mostly a shingle life issue. If you are in a snow area I would bet that is from snow weight and melt with a low slope. Venting and insulation may help.

What I have seen done on a couple of mobile homes was to lay furring over the existing roof and then put new sheathing and shingles on that. Still you maybe better off money wise just replacing the sheathing and shingles and just assume you will have to do it again in 10-15 years. You could use a water membrane over the whole roof if you really want to prevent damage but it is expensive. If its like a mobile home I used to live in once they get past a certain age they just start to fall apart.

Better hope you are not in a city with lots of permits. Mobile homes are built to a different standard, once you start modifying them you will be subject to the local building codes which in some cases you will be better off bulldozing it and building from scratch. Then again most places that allow mobile homes in the first place tend to have more relaxed code requirements

hand drive 05-30-2012 10:21 PM

Quote:

Originally Posted by cgrfish (Post 931893)
I recently purchased a older double wide mobile as a fixer upper for a rental. I knew when I purchased it that it had roof issues, and I was able to get it for a price I think will make it worthwhile.

The roof has significant waviness to it on one side of the mobile, and not nearly as pronounced on the other side of home. Having heard horror stories of 1x2 trusses on mobiles, I was very anxious to find out what the structure looked like to make a plan. This weekend I opened a small section of the roof so I could try to minimize the number of surprises when I start the project.

Here is what I found: The mobile has vaulted ceilings, and it has no trusses - it is single 2x6 rafters spanning 14ft 4 inches, spaced on 24 inch centers. The pitch is 3/12. The roof sheeting attaches to the top of the 2x6, and the sheetrock attaches to the bottom.... The baffling point is that I can see significant sag and wave in the roof, but when you look inside the rooms, it is not evident, nor does the sheetrock show significant damage or cracking....

One other wierd point - at the peak, the sheeting has a 4 inch strip of sheeting at the peak, and then the sheeting from the roof overlap's that, leaving a very small gap near the peak that the sheeting doesnt contact the rafter. My only guess is this was an attempt to create air movement.

My initial plan is to marry a new 2x6 allong side each sagging rafter to provide a solid base for the new sheeting (I plan to replace it all as it feels soft). Anyone have any suggestions on a different way to approach this?

My biggest concern is ventilation. I suspect the sagging and soft sheeting may be from poor ventilation. Each rafter section basically creates a sealed box with no ventilation. Currently their are 7 standard vents on each side of the roof, but their are 35 rafter, so alot of the roof is not ventilated effectively.

I was leaning towards a ridge vent spanning the entire structure, but where the two halve of the home joins creates a solid ridge of about 3 1/2 to 4 inches in width, and thus the ridge vent would need to be upwards of 6 inches wide to be effective. Is that possible - some people say you cant ridge vent a double wide, and others say it works...

If a ridge vent wont work, does anyone have any suggestions on a different approach for ventilation?

Also, I'd really like to add some insulation when I have the sheeting off, but I'm concerned it will just hinder the air flow even more - how much space do I need to leave for airflow?

Thanks in advance for all your input.


It looks like you are planning to go ahead and take the old sheathing off while roofing it. A way to fix it correctly for what you are looking for. tear old roofing and sheathing off, go in house and jack up ceiling(rafters) in the middle of the span. go back outside and sister 2x10 rafters next to the newly straightened 2x6 and nail together. insulate with baffle board stapled at the top of the rafter edge to vent against the sheathing. you will need proper soffit ( overhang) ventilation for the baffle board to work. If you need to add an overhang it is a good time to add any overhang blocking while the roof sheathing is off.
Sheathing, after framing the new roof rafters sheath the roof and if using 1/2" plywood then plywood clips are needed because the rafters are 24" centers.

When the shingles go on a 4 inch lap is preferable to a 5" lap because the pitch of roof is 3/12. it takes a few more shingles but really helps for roofs with pitches less than 4/12 to decrease shingle lap size. And they make an extra large ridge vent. They have been needed at my jobs where the roof ridge is a double lvl and the roof still needed venting.


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