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GlennWest

cutting a roof rafter?

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Got to looking at my Teton brochure.T Joist rafters every 16". Well my purposed Mini Split Cassette is 22.5" square. So I will have to cut thru a rafter. Any suggestions from the carpenters here. If it was steel I would know what to do. lol My thoughts are cutting a rough 2' square opening and box it in with 1X12 board. I will need to trim it down some but 1X10 will come up short. Roof is 6", unit is 10.5" tall. Do not have a unit in hand but instructions I have read it seems the unit protrudes down 1/2". So I need an 1X11" board. I can attach box to one rafter and cut thru next and attach rafter to box. Do you think 2X be better? Or you have better suggestion. Planning on screwing it all, no nails.

Edited by GlennWest

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Hmmmm I thought someone would know. Maybe try you tube or other info for regular home construction. I would think the technique is transferable. 

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Are your rafters arched? Could you get a plywood rafter screwed and glued to the side of the uncut rafter? 3/4 cdx plywood would probably be my Choice both screwed and glued to make your box. Your biggest problem is that the two adjoining Rafters will have to carry the load of the one you cut. So any extra support you can give them is good. Are the rafters them selves 6 inches or does it have a dropped sealing?

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No dropped ceiling. From looking at the brochure on my Teton, they are solid board cut at an arc, flat on bottom. I will be able to attach the box to both outside rafters of the box. Had not considered plywood. Thinking about it, is better choice. Lots of campers has 24" spread on rafters. Only way to double the rafter is to tear roof up so I can. Don't want to do that if I can accomplish this without it. I can put it in two pieces. Will have a 2' opening in center of roof. 

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As Lance said, how are you going to re-distribute the load from the cut rafter? You've also got to calculate in, the weight of the people on the roof installing, and eventually working on, the mini-split. Remember, it's not only the actual weight, but the kinetic energy of that actual weight bouncing on the roof while traveling the wonderfully smooth highways.

Cutting the hole and constructing the box is easy, compensating for the kinetic energy is not so simple. That might require the services of a structural engineer.

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As others are saying, I don't see anyway of doing this short of tearing into the roof or ceiling to put cross joists from the joists on both sides of the one your cutting. There is no other way  I know of to support the load.

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Well, the box will connect the cut rafter making it rigid again. Yes, it is weaker. The rafters on each side connect to box. So 3 rafters connected together with a box. The box needs to be built strong. Teton put some fairly strong rafters up there. 

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On 5/19/2019 at 10:30 AM, GlennWest said:

Got to looking at my Teton brochure.T Joist rafters every 16". Well my purposed Mini Split Cassette is 22.5" square. So I will have to cut thru a rafter.

Would centering the box on a rafter, that needs to be cut anyway, and framing it using the rafters on each side be stronger? The 6.5" would be split to 3.25" on each side then.

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Stronger??? I think it will be weaker but not enough to hurt. But that box is going to be ridged. I think all will be fine. Just wanted others opinions. I haven't done a lot of wood work. 

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I was thinking that by centering the box that it would be closer to the adjacent rafters to incorporate them into the framing for the box. In this way two uncut rafters help support and add rigidity to the box vs what you mentioned above in fastening to one rafter, cut the next, and then frame. It seems that the 6.5" reach could be weaker than building the frame off of the adjacent uncut rafters that are 3.25" away on each side.

Edited by rm.w/aview

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How do you plan to address the kinetic energy weight problem? That link has the equation necessary to calculate this factor, but the speed of vertical movement is an unknown.  You will note kinetic energy increases by a factor of four for a specific vertical speed.  This is required to insure you do not stop for the night to find the minisplit on the floor, along with some of the roof. An EWAG will not suffice.

Edited by Ray,IN

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I  not able to address the kinetics. Beyond my education. But I also don't believe the sinerio you described will play out either.  That unit weighs roughly 10#. My ceiling fan weighs close to that. My existing mini split rides on a wall which is nothing strength wise. Horizontal studs. Had to put plywood on back just to have something to screw too. Actually look for it to bounce off but it stays put. That rafter won't be weak. Maybe not as strong. Got me curious though. Might study up on kinetics this weekend. Thanks everyone. Gave me some good pointers. 

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what about the power for the unit, it says 208- 230 single phase power, most RV's take only two 110 volt circuits at any power panel that is what you will get I believe, not sure what your generator puts out, but I thought usually there is never 208 to the RV,  just asking a question

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Gleen please educate me all of the mine splits I have looked into only neaded a small hole for the pipes to go through and had a power unit on the outside  and the cooling unit on the inside. What kind is yours?

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The wall mount unit is just like you said. Hole for the freon lines, signal wires, drain tube. That is what I have now. I would add another to bedroom if a had a place to put one. The wall unit blows air one direction. The casette unit blows air in 4 directons. It mounts in the ceiling.Also it has a pump for the drain. One outside unit will have 2 sets of ports for freon lines and signal wires. 

 

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19 hours ago, GlennWest said:

It runs on two 110v circuits. I have plenty of space in my box for the breaker.

I realize that, but that, but you will have to add a box to capture the circuits off of a breaker to feed it, bacause there is no place to have 208 in my motorhome, yours might be different in your rv,  anyway it sounds good, 

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1 hour ago, hamrs_62 said:

I realize that, but that, but you will have to add a box to capture the circuits off of a breaker to feed it, bacause there is no place to have 208 in my motorhome, yours might be different in your rv,  anyway it sounds good, 

50 amp RV service has two 120 volt leads.  This is identical to most residential systems and there is nominally 240 volts supplied when both leads are used.  The breaker box is setup so that a double breaker can capture and supply this 240 volts.  Most RV's don't use the 240 volts but properly wired 50 amp service does provide it.  30 amp RV service only supplies 120 volts.  In most cases the correct breaker is all that is needed for 50 amp service.

Edited by Randyretired

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On 5/22/2019 at 5:19 AM, GlennWest said:

I  not able to address the kinetics. Beyond my education. But I also don't believe the sinerio you described will play out either.  That unit weighs roughly 10#. My ceiling fan weighs close to that. My existing mini split rides on a wall which is nothing strength wise. Horizontal studs. Had to put plywood on back just to have something to screw too. Actually look for it to bounce off but it stays put. That rafter won't be weak. Maybe not as strong. Got me curious though. Might study up on kinetics this weekend. Thanks everyone. Gave me some good pointers. 

My bad Glenn, I thought I read in your initial post this thing weight 100#, I'm not dyslectic only old.

Randy I think the 50A main breaker panel in older RV's have redesigned bus bars that prevent the application of a 240VAC breaker. Of course a new panel may be installed whenever desired. Perhaps one of the sparkys can supply correct information.

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