Advice for attached carport plans?

[scoutabout]

Hey folks -- I want to build a carport and attach it to this old garage to store the boat under in the winter. (The building is too short to put the boat inside).

Three questions:

1. I've got the basic power tools but haven't built anything more complicated than a deck before. Are there any plans or suggestions for design anyone would recommend? I'm looking for simple, simple, simple. I'm thinking three vertical posts in concrete holding the roof up, open on three sides with maybe plywood screwed on the sides for the winter to keep snow from drifting in. In the summer I'd take all the plywood down and just have the carport shell standing and park ol Fergie under it.

2. Suggestions for attaching it to the building? As you can see the eaves come down considerably on the sides. Would you overlap the carport roof on top? Underneath? Cut the rafters and eaves off just outside the garage wall on that side and go flush against the building with the carport roof?

3. Snow load. This place can get 3 feet of wet snow on the roof easy. Is there some way to tell if the original building is up to supporting one half of a roof that will be about 10 by 20 feet? Other than waiting for it to fall down, of course...The garage is 2 x 4 studs and sheeting. Pretty light construction.

All advice appreciated!

 

[WizeOne]

Re: Advice for attached carport plans?

You haven't given us much to go by. Your picture is not adequate to give us a feeling of height and you've given no dimensions.

What is the height of the end of the eaves from the ground? What is the height of the top of the wall to the ground?

You haven't mentioned how wide you would hope to build the cover.

All that being said, from what little I can see, you might want to cut the eaves off, flush with the outside wall, lag a plate across the newly exposed rafter tails, then screw the appropriate hangers onto the new plate, slip your new carport rafters into them an run them out to a supporting wall with a suitable header on it.

It also depends on how wide you want it. You probably won't have much slope so you'll probably want to use 2 x 6's every 16" to carry that load. You also haven't said if you want a solid outside wall or to leave it open. That would make a difference how you might construct the outer support wall, ie 2 x 4's or 4 x 4 posts.

 

[fishingman220]

Re: Advice for attached carport plans?

i would use 2x6 for the roof, lay ontop of the trusses in the existing roof, then just over lap the shingles. run as much pitch on the roof as possable. from the very outside to the garage run a large microlam then a post on each end by the garage to hold up that snow, the 2x4 in the exst walls will not like it witout more support. hope i could be of more help, let me know if you need me to explain more. i can send you pics of mine, it is very similar. my garage is 30x24 and the carport is 30x27

 

[scoutabout]

Re: Advice for attached carport plans?

Thanks guys - yes, Wizeone, admittedly the pic isn't great but it's the only one I've got at the moment and the building itself it 300 km away. I did mention the new roof would be about 10 x 20 and the structure open on three sides.

I'm going to estimate the height of the garage wall is 8 feet with the eaves dropping that to 7 at the lower edge.

I don't think I can go much wider than 10 owing to some trees and base of a hill on the other side hidden by the boat. Certainly there won't be much room in there, height or width but as long as I can nudge the boat under on its trailer that's all I need.

Fishingman220 -- I'd love to see some pics of yours and get a longer description. PM sent.

 

[WizeOne]

Re: Advice for attached carport plans?

.....from the very outside to the garage run a large microlam then a post on each end by the garage to hold up that snow, .....

Could you explain the first part of this sentence a little better? Do you mean to run a beam across, at the garage wall, and support it with posts right along the wall? If so, I would assume several posts along the length of the carport, up against the garage wall.

 

[fishingman220]

Re: Advice for attached carport plans?

this is a pic of mine, i will get a better one when i get home from work today.

 

[scoutabout]

Re: Advice for attached carport plans?

That's it!~ I need to build something just like that, only much narrower. It looks to me like the carport overlaps the other roof? Detailed pics and/or description on how you did that would be great. And does water get under the edge.

I was wondering about the microlam comment myself. I do see you have supports close to the garage itself.

Those outer posts look pretty hefty. How far into the ground do they go?

 

[Bob_VT]

Re: Advice for attached carport plans?

Considering the snow load.... I would have a good pitch (we never enjoy raking roofs), metal roof and post and beam construction.

 

[WizeOne]

Re: Advice for attached carport plans?

Bob, I don't think he could get too much pitch on that roof. His garage looks like it is a 3/12 to 4/12 pitch (which is pretty shallow) and to tie his carport roof into it, higher up the rafters, (or anywhere for that matter, even at the wall where I suggested earlier) he would have to go shallower than that pitch.

Also scoutabout, only going out 10 feet wide, I do not think you need that midspan support (like FM220's, or an additional support at the garage wall)

As I am thinking, for the cleanest look and snow load capability, I would go back to my original suggestion of trimming the rafter tails back to the wall plate. Lag a 2 x 6 header to both the exposed rafter tails and top wall plate. Screw in suitable joist hangers to the new wall plate as well as using screws to hold the carport rafter 2 x 6's to the hangers.

With a wet snow load of 60 pounds per square foot, with an acceptable deflection limit of L/240, you can place those rafters every 16" and have them span 10'. (using douglas fir) I would keep as much pitch as you can handle but these are load calculations for basically a flat roof. Your pitch would also determing your roofing material but as Bob suggested, metal would be best. I'd install it over 1/2" sheating covered with a medium weight felt. What roofing material do you have on the garage roof?

On the outer support, in 20 ft, I would use a 4 x 6 (two 2 x 6's on end) header with 4 x4 posts every 4 feet. Also lag in 4 x 4 diagonal braces as in FM220's picture.

You would not have to bury the posts in the ground. Get some pre-cast concrete pier blocks with brackets embedded in them (assuming here that the ground is undisturbed & firm) and place them all level with each other even if you have to dig some of them in a bit. Be sure to use pressure treated 4 x 4's and run you roof rafters at least 6" or 12" over the header. Also, I would not use a gutter, considering your snow.

 

[slia67]

Re: Advice for attached carport plans?

Don't you think that reinforcing the garages connecting wall with 4x4s would be a wise idea?

Just my non-expert thought.

 

[WizeOne]

Re: Advice for attached carport plans?

Don't you think that reinforcing the garages connecting wall with 4x4s would be a wise idea?

Just my non-expert thought.

Nope, do not think it is necessary. That garage wall should have a double 2 x 4 top plate with studs every 16". With the wall plate lagged into both the rafter tail ends and the double top plate there will be plenty of shear strength and the added load on the carport roof will be evenly spread over that garage wall. I am assuming here that the garage walls are tied together with engineered roof trusses not a flimsy stick built roof. If not trussed then the outer walls may not have enough lateral stability to withstand any additional load. That might make the whole garage tilt over as the load pushes down on the garage roof then out on the garage walls then the carport load tries to pull it away from the garage wall.

Here is a good link for just what you want to build scoutabout. Notice that is 15' wide, not 10' and they are using 2 x 6 rafters on 24" centers, not 16" centers. Also note their stated load experience then compare it to what yours would be with a shorter span, closer rafters and closer outside wall supports.

They also talk about your options for outside support footings.

http://www.extremehowto.com/xh/article.asp?article_id=60455

Have at it. A satisfying spring/summer project for sure. Piece of cake!

 

[scoutabout]

Re: Advice for attached carport plans?

...I am assuming here that the garage walls are tied together with engineered roof trusses not a flimsy stick built roof. ...

Ah...glad this came up. There are no engineered roof trusses. Not sure of the vintage of this building but it appears to be old fashioned 2 x 4s (the kind that actually look to be a full 4 inches wide) framing the roof. There are cross members which are honest to God tree trunks (small ones - 5 or so inches across, stripped of bark running side to side at the top of the walls sitting on the single 2 x 4 top plate. These cross members are staggered between the roof framing and so are not tied to the roof structure that I can see.

Opinions? Perhaps there's some remedial work to be done tying the roof structure together better before anything else. I'll see if I can find a photo.

edit: found one - ignore the junk and weathered boards stored up there -- you can make out the roof framing and crossmembers which are lighter in colour.

 

[WizeOne]

Re: Advice for attached carport plans?

Well we could reach the limit of my knowledge here. Suffice it to say, engineered trusses, by making a virtual triangle, force the roof load straight down. Partway open a pair of scissors, place the tips on a flat surface, then push at the apex of the blades. Unless the tips of the scissor blades are securely locked in place with a horizontal member, that V will try to flatten out, taking anything those tips are attached to with them.

IMHO, the only remedial measures you could take would be to assure that each garage roof rafter is securely tied, across the span of the garage to the opposite rafter on the other side. At the same time, there should be a member that comes down from the top intersection of each rafter set to the lower transverse cross tie. Now, of course, you would have the beginning of a prefab engineered truss. The only diferrence would be that they would have several more members, running at angles between the rafter and the lower cross brace.

It also appears that you roof rafters are at least 2 x 6 while you rafter tails at the eves do not appear to be so??? Your wall top plate is also a single 2 x so it would not carry the load of a double plate.

Maybe you would be well advised to provide, at least nominal additional support of the carport roof on the outside of the garage wall.

 

[fishingman220]

Re: Advice for attached carport plans?

mine are 6x6 posts, in the ground 2ft, i was saying i would put the posts where mine are in the middle comming away from the garage, i would put yours right again the garage, to help with the weight of the snow

 

[fishingman220]

Re: Advice for attached carport plans?

and no it doesnt leak at all, all the paper and shingles are under exsiting shingles

 

[scoutabout]

Re: Advice for attached carport plans?

Thanks guys -- some great ideas and pics. Once the road in is passable in other few weeks, I'm going to spend a beer or two considering how I might improve that roof structure. I'll probably also incorporate vertical posts along the outside wall too.

Fishingman, I really like the way you handled that roof. I'll probably try a combination of the overlap like you which will get me some more headroom as well as tying to adjacent vertical posts.

 

[lowkee]

Re: Advice for attached carport plans?

Given the amount of snow, I think I would recommend making a stand-alone structure. The pitch you are going to end up using will likely be in the 1.5/12 range without going all the way up the roof, which is just plain crazy given the snow pack. 4/12 is the minimum for any unheated building with snow, IMO.

My recommendation:

Buy some 4x4 posts
Space them at 4 feet
Mount them above some 5gallon buckets filled with concrete, surrounded by crushed rock
Screw (or bolt) 2x8 boards horizontally along the top edge of the 4x4 posts for lateral support
Make basic roof trusses (5/12 pitch) using 2x6 planks, spaced every 2 feet (18 inches would be better)
Nail some 1/2" plywood to the trusses in a staggered pattern (staggering is very important)
Tar paper and shingle the roof *OR* nail metal barn roofing on top of the trusses (space the trusses close for metal roofing)

Meets code? Likely not. Will last? Probably longer than any boat under it.