Removing part of a load bearing wall - what size header
I'm removing part of a load bearing wall that is in the center of the span directly under the ridgeline. How do I know what size and type of header to use. All of the web site have complicated formulas that are hard to calculate. Is there a easy matrix to follow? The room is 24 x 24 and I want to open up 13' of the wall in the middle?
Have you considered that the reason the sites have complicated formulas is because this is a complicated problem? You are looking to create a 13 foot header, this is relatively large, and is by no means simple to compute. At that size, there are no simple matrix tables available.
The reason this is a complex problem is that the header size depends on the loads, which depends on the specific geometry of your house, and the end support conditions. A large header like the one you want to install requires proper detailing of the connections to the supports, and the loads need to be carried down to footers.
I recommend you hire an engineer or an architect to design the header and detail the connections. Even if you found a website where someone was foolish enough to give you a size sight unseen, would you really trust such a response?
Alternatively, you may be able to find a local lumberyard that will size the header for you if you purchase it from them. Whether this saves money in the end is not obvious, since they clearly have to build the cost of sizing the beam into the purchase price, and you still have to install it yourself, with no connection detail drawings. And you likely need a building permit, which will require stamped drawings in many jurisdictions, and the lumberyard paper may not be acceptable.