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I am looking to add plywood flooring in my attic for storage page. There is a lot of space, very large areas. From what I've been reading, it looks to be ill-advised to add flooring if the roof was constructed with trusses instead of rafters (one such site). I have no idea what these are. Would someone be able to explain what these are and if my home has it.

Here's an overview: Overview image of attic

Other pictures are here.

Also I am aware that I would need to extend the floor joists keep the insulation intact.

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    I believe someone needs to examine the construction and condition of your house to answer this. – keshlam Feb 10 '16 at 17:20
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    Those aren't trusses...though I have no idea what those random angled posts are. In fact...I'm kind of worried about those. Those seem to be some sort of ad-hoc attempt at supporting ridges that weren't engineered properly. That said, given that they aren't trusses, you should be OK assuming your ceiling joists are large enough. Realize, however, that that will greatly reduce the amount of blown-in-insulation that can rest there. So it may not be a benefit in the long run. – DA01 Feb 10 '16 at 17:40
  • The type of roof would really not matter compared to the support of the ceiling (new floor). My shop is trussed 36' clear and one of my bays is 2 stories. The ceiling first floor, floor of the 2nd floor is 14” LVL I beams on 16” centers. This can hold several hundred pounds per square foot. If you have 2X4 or even 2X6 With the span of a room you may not be able to put much more than the sheeting up there without doing damage. This is what you really need to be looking at. – Ed Beal Feb 10 '16 at 18:11
  • This is totally dependent on the size of the joists which are covered by insulation. While the pictures of the oddly place supports and such are great (because they do make me feel better about my house construction) they don't really matter. Although I would like to kick one and see what happens. – DMoore Feb 10 '16 at 18:48
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This is "Ridge board and rafters":

enter image description here

These are roof Trusses:

enter image description here

From your pictures, you clearly have ridge boards with rafters coming off of them.

The reason trusses shouldn't hold the weight of flooring or heavy boxes is because they are typically made from relatively small boards (2x4's in some cases) and the bottom board is already carrying a significant roof load because of the design of the angled elements.

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    In the UK all modern homes are made using trusses, and we are fine to lay boarding across them - they dont get rated for the same load bearing as a normal floor, and arent supposed to be used as a normal room, but my building surveyor friend had no issues with us boarding our attic and using it for storage. – Moo Feb 11 '16 at 20:39
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The roof itself doesn't matter. It is the boards on the bottom (truss chords) or ceiling joists that will carry the weight of whatever you are storing.

If it is a few boxes of christmas ornaments that is one thing, but if you try to store your bowling ball collection up there and build a weight room then you will be in trouble.

Any normal floor is built on joists that span a distance between vertical supports and have to be sized for that span. Check the span tables here.

You will notice the absence of 2x4's on the span tables. Your ceiling joists, or the boards that will become the floor joists of this bonus room, are most likely 2x4's which means they would have to be reinforced to be used as a floor.

Good luck with your project!

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