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Lean-To Shed Roof attached to back of my garage


I have a detatched garage with an old pool deck butting up to the back of it.

I'd like to build someithing like the photo below and have no idea where to start.

I'll park my snowblower, riding mower and trash/recycle bins there.


I live in MA and we get a lot of snow, but I'd like to try and use a clear corrugated plastic roof.


Is there somewhere I can go to learn how to:

Attach a ledger board to the back of the existing garage

Attach posts to an existing cement pad

Size the material appropriately considering the heavy snow


Basically, is there a source for actual plans that I can work with? They don't have to be free....




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Posted 2011-11-09T22:49:19+0000  by SteveInMA SteveInMA

Hi, hello and thank you for joining How-To-Community.


Steve I couldn’t find any plans that are accessible to me that I could refer you to so I decided to try to help you with this project by posting some step by step instructions. Please feel free to ask any questions you may have and I also encourage our members to chime in with their comments as well.


So let’s get you started.


Start off you project by inspecting your existing garage and correcting any issues it may have. These are some of the things you should look for;


Rot damage to the bottom plates and exterior sheathing.

Presence of anchor bolts and diagonal bracing, if your garage doesn’t have anchor bolts every at least 36” inches you should add some more.

Also if your garage is sheathed with particle board material and plywood was not used for the corners you should consider adding some diagonal bracing to the corners to minimize racking considering this is a free standing structure and you’re about to lean another structure to it.


For the diagonal bracing you can use dimensional 1x4 lumbers, steel strapping or plywood sheathing, I prefer steel strapping. Note that diagonal bracing can be added from the inside of the garage.

Steve this is how installed bracing should look like;

Diagonal bracing.JPG



Once all of this is corrected (if it needs to be) you can proceed and start planning on the lean on shed addition. Ideally ledger board should be attached to the band (ledger) board that’s on the building, in your case that is not an option and depending on the height of your existing garage and location of the shed you next best option would be to attach or rest new ledger board on to the top plates.


Ledger should be fastened directly (siding removed) to the wall studs (ledger) using min of ½”D lag bolt in combination with fender washers. This is what I’m use to seeing; you should confirm this with your local building department before construction.


Once the ledger is attached you can start working on the post footing. Having said you reside in MA your concrete footings should be placed bellow the frost line and the post supporting the structure should be installed on top of the ABA standoff type of post base. Again confirm this one as well with your building department; they may have some additional requirements that are specific to your area.


This is how post installed on the ABA standoff base should look like;




Once the posts are installed you can start working on the top beam.

There are two different ways of installing a top beam; on top of the post using hardware or on the side of the post like pictured in your image.

I would recommend duplicating what’s pictured in the image you have attached in your post.

Sizing the materials;

This is something that greatly depends on size of the structure you are planning on constructing (span) and specie of the lumber used.


I would suggest using this AWC CALCULATOR in order to determine size of the rafters needed.

Rafters should be attached to the ledger boards with sloped joist hangers and to the beam using type H ties. I would recommend spacing roof rafters every 16” inches on center.

See attached images;

H.JPGNail detail.JPGHanger.JPG


Once the rafters are installed you next step should be installing cross bracing (furring strips) as a support for the polycarbonate panels .I would recommend  spacing furring strips same as rafters or 16” inches on center.


These are detailed instructions for polycarbonate roofing and also roof to wall flashing detail;



Hope this helps Steve and good luck with your project.



Best Answer

Posted 2011-11-14T21:38:57+0000  by George_HD_CHI

Wonderful response! Thank you very much.

Posted 2011-11-15T17:16:26+0000  by SteveInMA
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