How many ceiling joists can I remove for skylights?

[Mack454]

I was planning on installing 2 skylights next to each other in my kitchen. The house is a ranch with a low pitch roof. The skylights are 28"x46", and I would like to center them in the ceiling. If this is to be done, I would need to remove 46" plus header thicknesses of the middle of 3 ceiling joists in a row. Is this too many ceiling joists to cut in a row? The other alternative was to space the skylights out a few inches each way from the center. This would require only 2 joist to be cut, with the center one remaining. I would rather cut all 3 to really open up the ceiling. Any suggestions/advice would be greatly appreciated. Thx

 

[dik]

It depends on the existing framing, the spans involved and the location of the skylight. If it's close to a bearing wall, it may not be a problem (joist hangers can normally be readily post-applied). If it's around mid-span as I would guess from being in the middle of the ceiling, it may be more problematic from both load and deflection.

 

[Mack454]

The kitchen is only about 11'-12' wide between the exterior wall and interior (what I believe to be a loading bearing) wall.

 

[dik]

What are the joist sizes, spacing, species and grade of lumber and the actual distance face to face of stud? Are you cutting 2 joists? or 1?

 

[Mack454]

They are 2"x8" ceiling joists spaced 16" O.C. The spacing is 135" from face to face. I was planning on cutting 3 joists in a row. If I go with a 44" skylight instead of 2 28" lights, I can reduce that to only cutting 2 joists right next to each other.

 

[dik]

Forgot to ask... what sort of snow loading or wind loading do you have in your area? Are the joists ceiling joists in addition to roof rafters (joists?)? or are the 2x8@16 the only supporting structure? Without doing the sums, it could be close... the members may require reinforcing... I'll crank out some numbers tonight and see what loading can be supported.

 

[Mack454]

I have ceiling and roof joists. Snow loads? I live in New Jersey. We had a pretty nasty winter this past season

 

[whyun]

if ceiling joists are indeed ceiling joists separate from the roof joists, what you are proposing can be done by cutting joists and "sistering" additional joists next to the two immediately adjacent uncut joists. By sistering, I mean add a new continuous piece next to an existing connected using screws or nails. If flexure/deflection is the controlling factor in design, sister joist need not be extended to each end of the joist support.

As dik stated, it is a situation that depends on a lot of factors. It doesn't sound like an impossible task but I strongly recommend having a structural engineer's input.

 

[Mack454]

OK. Thanks guys for the advice

 

[SteveBausch]

The load doesn't disappear, it finds a path. If you keep track of the strengths/paths that were removed, and replace that capacity you are a good ways towards solving the challenge.

Keep track of the connection strengths, also. Nails are good for shear, not for tension.

A skylight situation shouldn't be hard to comprehend.

You might want to study buildings in construction, see how their skylights are framed.