Help Me Design Carport?

[jyl]

I am thinking about building (DIY) a carport.

A sketch of the area and some possible post locations is below.

It would be a simple structure. I'm envisioning a very lightweight roof of clear (tinted) corrugated plastic, slightly angled so that rain drains off the side farthest from the house. I don't plan to run the roof all the way to the house, thinking it will be about the same width as the garage (appx 12'). The roof length would be appx 20', from the garage to post "C". There may be some decorative element on the end of the roof facing the street. to conceal the corrugated, but nothing significant.

I'm hoping to use 4' x 4' posts. They could be fastened to the concrete driveway surface, or concreted into the soil next to the driveway. I prefer the former, since that soil is probably my neighbor's property.

I envision a beam (continuous, or sections) tying posts A, B and C together and to corner of the garage closest to post A. And, at 90 degrees, I envision a beam connecting post A to the house, a beam connecting post B to the house, and a beam connecting posts C and D to the house. Those beams are not shown in the sketch. I figure that having the structure tied to the house and garage in two axes will give me the necessary rigidity, so I won't have to triangulate or use thicker posts. On top of those beams, I can put a light framework for the corrugated.

And it is those beams that have me stalled. Posts A, B, and C are each appx 17' from the house. I can build up (laminate) a beam that long, but I wonder if it will sag.

Further, while the roof will ordinarily carry no load other than itself and some rain, it will be exposed to wind, and occasional snow.

The highest recorded wind gusts ever in Portland have been 70-80 mph (that's a 100 year storm around here) and in winter 30+ mph is not uncommon. I'm thinking in the 70 mph scenario the corrugated plastic would simply rip off before the wood structure would be damaged. Make sense?

As for snow, it is very infrequent here, like once every several years, but it does happen. We had a week of 12" snow last winter, and I suppose 18" is not inconceivable someday. I've read snow weighs 8 lb/cu ft so 12' x 20' x 8 = 1920 lbs.

What do you guys suggest I do about the beams? I could use two additional posts opposite from posts A and B, but would like to avoid it if possible. I can see them getting in the way.

Any other suggestions, thoughts? I will probably build one carport in my life and this is it, so I am by definition a newbie.

[Racerbvd]

When I asked for help on a garage, I got slammed by by one who does that type of work, for asking for free help, funny, the same person is always asking for free Porsche advise.. Good luck..

[Heel n Toe]

You mentioned that putting posts A, B, and C into the ground might be putting them on your neighbor's property. Before you go any further with this design, you need to check to see if it's okay for it to be sloped towards (thereby sending rain runoff) onto his property... it is probably a city building code thing. It is here.

Of course, you could most likely solve that problem with a gutter.

[HardDrive]

Ummmm.....you sound like you have lots of can do spirit, but I think the City of Portland might have something to say about your plans. Even if you have cool neighbors who wouldn't say anything, prolly want to get everything kosher with the local despots. Be a damn shame to put time and effort into it, only to have the city force you to remove it.

[HardDrive]

I'm not trying to be snarky, but I think you should track down an architect. Even if you want to build it yourself, the architect can ensure that you have a safe structure, and can help you navigate the city building codes. Your wife and kids are going to be walking under this thing......

[RoninLB]

around here it's not a permanent structure unless it has nails in it

if only screws it's temporary

don't know it that may help?

[jyl]

I can put in a gutter. That part of his property is a 4' wide planted strip along the side of his house, the strip is invisible from his house and I maintain it for him anyway - prune the roses, water the plants, pull the weeds, lay down the mulch, etc. So he won't have a problem with some runoff, but you are right, a gutter is the way to go - if only to avoid drowning the plants that I am tending.

As for the City of Portland, well, I am hoping to avoid inviting them to my house, because my treehouse was built, shall we say, without the benefit of municipal services. And I am a big believer in screws over nails.

Good point on the architect and safety. I have some close friends who are architects and will run this by them. But, as usual, I want to try to figure it out for myself first. Of course I'm perfectly willing to cheat by asking PPOT . . .

[Porsche-O-Phile]

I doubt you'd need an architect's or structural engineer's stamp on drawings for something that simple. That sounds like it's a very basic, over-the-counter type permit that (at most) would require some basic calculations to show that the framing members are large enough for anticipated loads (including snow loads for your area which may apply - check your codes). In any case, it's something that shouldn't take very long to put together to the satisfaction of the building department official taking your application.

If I were doing the design (I'm not) one thing I'd add to your schematic is some sort of provision for lateral loading - either shear panels or cross-bracing, or showing moment connections that can withstand anticipated lateral forces (select the correct Simpson connector). I'd recommend footings as well (thickened slab in the areas where the posts come down should suffice for something this light).

Another consideration you alluded to above was water - show a minimum of 1/4" per 1'-0" slope on the roof to drain - either sheet drain or with gutter & downspout. Some municipalities require you to show that downspout discharge will ultimately end up in a catch basin or other collector on YOUR property (i.e. you can't have it discharge and sheet drain onto an adjacent property or public right-of-way, you need your own drain and to show how this will be accomplished). Again, simple phone call type stuff.

Other than that, this is pretty straightforward. You've got the right idea with the factors to be considered (live & dead loads, wind, water & snow loads).

One other thing I'd consider - posts "C" and "D" (especially "D") look rather vulnerable to getting "clipped" by someone with a car if they're not paying attention. I'd suggest a bollard or some other mechanism of protection (stone wall, berm, whatever) so that if someone comes home tired some night and accidentally bonks a column, they don't collapse the structure and kill someone.

[jyl]

Not sure what to do about protecting posts C and D from an errant car. A big bollard isn't going to be very DIY for me. I am thinking about multiple posts there, the first ones being non-structural and essentially sacrificial. Or I could set post C in the planted strip, where it is protected by the "curb" separating strip from driveway. Similar for post D, there is a curb around the adjacent planter (brown patch in the pic). Gets into lot line issues though, maybe I need to research exactly where the boundary is. My neighbor won't care but the next owner of his house might.

[island911]

Originally Posted by jyl

...

As for the City of Portland, well, I am hoping to avoid inviting them to my house, because my treehouse was built, shall we say, without the benefit of municipal services. . . .

Wait wait, I thought you were pro-govt. ...maybe just for others.

Well anyway, POP gives some good advice.

I would look to the garage for some shear (lateral) support. ...reinforcing the garage walls and/or ceiling as needed. Or maybe run an arbor structure over to the house (use it to help keep the roof from being a push over.

You could also make a "shed roof" sloping from the house. (would cover your entry, as a bonus)

I wouldn't rely on the plastic just ripping off in a bad wind.

[Porsche-O-Phile]

If you don't want a bollard to protect the column you can always do an encasement (concrete or metal), or you can do a concrete post up to about 3'-0" or so with the wood post on top of that. I doubt it would come up with the building dep't (they're strictly reviewing fire/life safety for code compliance) but it's a best practices thing that you might want to consider...

Every garage door I've ever seen eventually gets doinked, same goes for poles, building corners, etc. A warehouse I did some work on once had forklifts running all over the place - one of those guys actually put the metal fork of a forklift THROUGH the web of a wide-flange steel column (!) Point is, stuff eventually gets hit. It's a good idea to protect it.

I have a basic bollard detail - just a 6" dia. steel pipe filled with concrete that goes down into an augered hole in the ground about 3'-0". I can dig the detail out and send it to you if you want. A pipe bollard might look like crap in a residential application though, but it's a starting point...

Or maybe I'm just being paranoid and overdesigning it... I've been known to do that on occasion too.

[porsche4life]

I would go for tinted plastic or tin. That clear crap gets sun yellowed really quick and really doesn't keep the uv off of your vehicle

[TerryH]

Love the carport. Garage is further back and is so more functional without any cars in it.

[jyl]

Oh well, more posts for more peace of mind. Tied to garage in two places, to house in three. Off to do some calculations and dimensions.

I plan to make it a bit more decorative than this, but its too nice a day to be drawing latticework in Sketchup :-)



Solved the post vs car issue too. Two honking big ceramic pots, staked into the earth, overlapping the driveway by enough to shield the posts, filled w/ stone and earth and pretty plants. Kind of like a breakable bollard.

[stomachmonkey]

Can you make it work without B and E?

I fond posts like those are almost always in the way when you are doing stuff, washing, waxing wrenching etc...

[jyl]

Need to do the calcs. I'd sure like to do without at least "E". But the distance from "F" to "D" is nearly 19' and the distance from "B" to the house is appx 17'. That seems like a pretty long run to be unsupported. I'm imagining 18" of wet PNW snow at 10 lbs/cu.ft. plus 60 mph winds, and my Porsche underneath.

[island911]

What type of roof?

One big center ridge beam out to an 'A' truss (each end) would rid those middle supports, easy squeezy.

[jyl]

island, I'm sure you're right. But alas a truss is beyond my DIY abilities, so I'll have to live with more posts than I'd ideally like.

(One of these days I'll have the garage rebuilt to a two-story, sort of a carriage house thing. Maybe I'll have a professional build a proper carport then.)

The roof will be tinted (smoked gray) corrugated plastic. Light, cheap, easily repaired, lets light in while blocking rain. Flat with just enough of an angle for drainage.

[jyl]

[911pcars]

John,
Would a glue-lam or even a steel beam on either side be strong enough to eliminate posts B/E? Steel posts with a welded saddle on top would provide added strength and collision protection from your 911 I also imagine maintenance with steel better than wood around ground level. Powder coating, at least the posts, provides very long term protection.

Potential snow loads might be a bit much for thin, corrugated plastic laid flat. Thus your idea for add'l support is a good idea. A pitched roof can shed more snow, but it also changes the design. A slightly pitched rock or composition roof with a low-profile skylight could provide the weatherproofing you want along with some illumination underneath.

This design would look right at home in a So. Cal. environment. I'm looking into building something like this in the future.

Good luck,
Sherwood