Greetings.
We’re in the process of adding a screen porch on to the back of the house. Very rough sketch below. The overall width of the screened-in area is 20′ and the overhang on the left side is 6 feet wide. I’m wondering about the possibility of using scissors trusses to gain a little more ceiling height. With common trusses we’ll have 8 feet but a little more would be nice to open it up.
In my research on scissors trusses it appears they are normally sized so that they sit on the top of the walls where the top and bottom chords join. (Is it called a bottom chord on a truss of this design?) Is it a complex thing to modify the truss design to allow for the overhang? I realize the left wall would be taller and this might look strange on the inside. But as far as the trusses are concerned, would it be as simple as adding a vertical post between the chords directly above the wall?
Is there an alternative truss design that allows the wide overhang on one side and the ceiling one gets with scissors trusses?
Thanks.
Replies
BossHog would be the one to answer this, but I think I've seen modified scissor trusses similar to this before. Obviously they would be custom, though.
If you could have support posts on the overhang end it would be more "normal".
(Actually, your design is very similar to our deck, where we have a "room" under half the roof, and the other half is open. This is stick-built, and since the total ridge length is short and there's a (light) ridge beam no rafter ties are needed other than at the ends. We do, however, have an X-shaped cross-brace inside, to keep the roof from racking.)
Thanks Dan. That is helpful.
I should have searched other sorts of trusses before posting. I came across a "cathedral" truss that looks essentially like what I'd want although the drawings I've seen also show it being supported out near the end instead of cantilevered. I wonder what it would take to modify that sort of a truss for cantilevering the overhang. I suppose we could run a support post up at the corner of the landing (stairs will run down toward the "camera") but we don't really want a post to the ground at the near corner.
There are a couple of thoughts that come to mind.
Probably the easiest thing to do would be to order what I would call a "cathedral truss". It would have a 6' flat BC (bottom chord) that would extend out over the porch. (Actually 6' plus the width of the wall) Then the remaining 20' would have a sloped ceiling in it.
If you want to go with a scissor truss, the truss plant can put a flat BC on the part that extends out over the porch. (Again, actually 6' plus the width of the wall) But you're correct that the peak would be off center in the room.
I don't know what pitch you're thinking about, but the usual rule of thumb is that the inside pitch is half the outside pitch. You can push that a bit in a smaller truss like this. (i.e. you could probably get 6/12 over 4/12)
If these descriptions don't make sense let me know and I'll try to post some sketches.
Thank you. You've made perfect sense. I'll have to investigate the cathedral truss option.
Is there anything special to consider because of the cantilevered overhang?
Is there anything special to consider because of the cantilevered overhang?
The only thing I can think of is wind uplift. With a wide open area that large you could have one heck of a lot of it.
The 6' cantilever is no big deal, as far as the truss design is concerned.
Wind uplift is a good thing to consider. Hmmm....
Dan, you're right. It does sort of look like the pitch is different on opposite sides of the ride. That must be an optical illusion because they were drawn at the same pitch and the ridge is centered over the span of the roof and not the deck.
I would like to put painted bead board panels on the ceiling and I was thinking I'd leave a gap at the peak as a vent. Then use a continuous ridge vent on top. I'd hide the gap inside with a piece of 1x4 or 1x6 stuff suspended about an inch or so below the bead board. This ought to let hot air escape out the top.
I'm thinking a common truss will be used on the end and it will be sided to match the house so that raised ceiling will be a good place for hot air to collect without venting.
The plan is to install a ceiling fan and I think the cathedral ceiling would be nice for getting a bit of clearance for that.
I was also figuring on a flat "ceiling" on the overhang with a couple of can lights to illuminate the stairs and the landing.