Adding a porch to existing deck.

[robbnj]

I'm planning to put a roof over my deck (eventually to be 2 or 3 season porch), but can't find any DIY info. on correct structure design.
Can anyone help (or point me in the right direction)?
The roof will be 16'x16' with a topside height from 7'9" down to 6'9". I plan to use 2x4x16' beams (stringers?) but am wondering if I have to use a cross beam support to avoid sagging. We get a lot of snow here, but I am pretty dilligent about keeping it low on the roof.
I know the pitch is shallow, but I don't want to tear into the existing roof structure unless absolutely necessary, and it doesn't make sense to make the roof smaller than the exisitng porch.
Any help is greatly appreciated.

Rob
DIM(do-it-myself)BULB

[Under_Miner]

OK, I'm assuming that your roof snow load will be about 52.2 psf.

And using No. 2 or better S-P-F 2x4 roof supports the spans would be as follows:

12" o.c. =>7'-6"
16" o.c. =>6'-10"
24" o.c. =>6'-0"

In each situation you you would require at least 2 beams. So spacing the 2x4's at 24" would save you the most money. From there you would have to size your beams accordingly, however, your best option may be to look into sizing your roof rafters from 2x4's to perhaps 2x8's or even 2x10's.

The code also doesn't factor in the removal of any snow, so shoveling it off won't help you any.;)

You will also have to make absolutely certain that the deck supports are adequate for supporting a roof snow load. Check with your local authority for this, they can be of great help.

[suemarkp]

I don't know of any span table that would allow a 2x4 rafter to span 16 feet. The span distance is actually the true horizontal distance and not the angled distance, so if you're using an inclinded 16' 2x4, the span must be less than 16'.

However, I think you'll want a 2x8 for this distance, or perhaps the 2x4 would work if you can support it mid span. If you can get the span down to 14' and your snow load is low (20 PSF or less), you could maybe use a 2x6. But you'll need a beam in the middle of the room to go with a 2x4 or heavily load 2x6unless you go with a truss design instead.

A key design factor is to know what the local building code is in your area for snow loading. Another factor is if you'll be putting sheet rock on the underside of those rafters. If so, they need to be a size or two larger so they are stiff enough to not crack the plaster. Once you know that and the unsupported span, picking a timber is relatively straight forward.

[robbnj]

Thank you for the help!
So, it sounds like I'll need extra support since 2x6 is the widest rafter I can use. Since the cross-rafter will be 16' long, should I plan to add at least one column (or more) to ''support the support'' in the center?
The deck is built on BIG lumber (2x10's spaced at 16") so I don't think the load is a problem.
Finally, I planned on using 4x4's to support the structure (at least 10 of them). Would these work?
The

Rob

[Under_Miner]

Thank you for the help!
So, it sounds like I'll need extra support since 2x6 is the widest rafter I can use. Since the cross-rafter will be 16' long, should I plan to add at least one column (or more) to ''support the support'' in the center?
The deck is built on BIG lumber (2x10's spaced at 16") so I don't think the load is a problem.
Finally, I planned on using 4x4's to support the structure (at least 10 of them). Would these work?
The

Rob

Any support that you plan on having in the centre of the main supporting beam would have to be transfered right down to the ground due to the fact that you would be creating a point-load, which a floor is not designed to carry. Again, go down to city hall and get yourself a permit, the building department there get's paid to give advice on what is acceptable given the ground type and snow load. Any building official/inspector that is unwilling to help, or at least advise on what the minimums are, is an uneducated bum and should be fired!