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If the deck is sloped (has separate rafters) then the joists go on top of the beam next to the rafter tails. If it's a flat roof the joists can either sit on top or hang off the beam. If they sit on top you need a rim joist to tie them all together.
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you are being helped by Dave Hic Expert, you have done well to connect yor new ceiling/roof joists to the top of your existing plate, not particularly easy. The question here is that you are constructing a deck... so at the beam you have two options. you can go either way because it is a deck, cover and im presuming its a solid deck but that diesbt mater in this case. if you go on top of the beam you must make sure you have the minimum required height of 7'out at the bottom of the beam, presuming this is an iopen deck and not a room. this choice will limit the width of your deck by at least two feet because your beam will be at the joist farthest span when you terminate the joists into it, however if you go on top of the beam you can cantaleaver as much as 3' i believe. depending on you joist size and spacing. And the type of wood you are using. because of the nature of the construction comming off the plate yo have 8'height to begin with. this will leave you adequate distance for sloap . you will want a minimum of 1/4 "per foot for flat roof but make sure when going by the soppan charts you considder fiber and bending of you type of lumber because all lumber has a certain amount of flexation under load. do not build your roof with just the minimum sloap or you wo=ill end up with ponding water in thge center of yoiur deck roof.There is something called deflection and that is the anount of negative arch a roof is calculated to have for a certain distance.
Figure in a deflection of .5 which is twice the minimum sloap of .25 which will ensure you do noy have water standing on the roof. if you cantaleaver you will have to figure in the .5 deflection to the entoire width of the roof all the way to the end of the cantaleavering joists. but it is the same theory .
Im speaking in minute extreems , you will likly have a pitch of 1/2 in 12 or 1 in 12 so nothing to worry about except if youplan to put shingles on it you must have a minimum of 2 in 12 pitch and two layers of 15 lb astm felt min.
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if you bare plabnning to make this into a room latter on, only because you called the joists ceiling joists. you would be better off going on top just as the rafters are on top of the plate so you will have the advantage of and eave later on.
i believe you can go as low as 6.5 feet under the beam