Expert: Daniel Humphrey - 3/24/2008

Question
We have an existing back porch that is 10'x30' and we want to expand the 10' depth to 16'.  Can we do this and just have the column supports at 16', by sistering the roof joists?  We plan to tie an additional new porch (also covered) approximately, 16'x16' to the existing porch.

Thanks,

Debbie

Answer
Debbie,

That will be a big space!

Of course you can do this if you get the structure right--size the new rafters correctly and attach them to the exiting ones and to the wall plate. Since the new rafters will likely be proportionally deeper for the 16-foot span than the ones for the 10-foot span, you may not have room on top of the house wall plate for slipping the new ones in beside the old ones. You may have to use joist hangers. Make sure the old rafters are tied in suffiiently also, since you will be stressing this connection, especially if you have only a post-and-plate construction for the outer wall.

For that outer wall, use substantial posts properly attached to the foundation, and use a continuous top plate for both the planned expansion and the new porch. The foundation itself needs to link all the posts and preferably tie into the house foundation. All this structure needs to be secure in itself, since the porch roof cannot supply much in that respect.

When you get further along in your plans, feel free to ask more questions.

I'll be gone for work, starting April 1, and will not be available for a while.

Daniel