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My house is 100 years old in Ontario, Canada. My attic is unfinished, and the joists are true to size (2” x 4”) rough sawn 2x4s (two 10’ pieces make up the 20’ span), 16” OC, cathedral roof. Supported below through the Center of the entire house. Not sure of the wood species but the colouring is very dark brown.

If I want to build a room on them what’s the most cost effective way to beef up the joists so they are very level and strong? Because of the roof slope the walkable area would be 11 feet wide.

Do I sister additional 2x4s for just the usable area? Sister a bigger board? Add sleepers across the opposite directions of some kind?

I will be putting 3/4 ply on top and vinyl plank on that.

Any help would be appreciated.

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    Can you post a picture of the existing joists and where they connect?
    – gnicko
    Oct 20 '21 at 19:46
  • Sistering is a bad idea for attic conversions. You might get away with it for occasional attic use, storage etc. But for a full conversion, sistering is likely to damage the ceiling below.
    – handyman
    Oct 20 '21 at 22:44
  • @TylerH OP said the span is 20 feet, only about 10 or 11 feet walkable. Need to know if there is a supporting wall below.
    – crip659
    Oct 20 '21 at 23:44
  • The joists are pretty level right now. I’m not sure the species of wood, but it is true to size 2”x4” rough sawn. It is very dark brown. Looks in good shape. The joists butt together in the middle and are supported through the Center of the home on the main floor (wood frame) and in the basement (wood and brick columns), for the full run of the house. Oct 21 '21 at 11:47
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    @JohnnyBender For what it's worth, a thicker joist is about double the strength as a joist one size thinner (e.g. 2x6 compared to 2x4) , in terms of load it can carry when in place. Can you take a photo of the space (showing the joists) and add it to your post? Images always help clarify things.
    – TylerH
    Oct 21 '21 at 13:13
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Usually you'd add new joists of the correct size, in between (but not fastened too) the old joists. Pack them up slightly off the wallplate (10mm or so) so any flex in the new joists won't touch the existing ceiling.

This method separates the old existing ceiling from the new floor and ensures that the new loads from above are transferred to the wallplate (and not the old ceiling).

p.s. measure the distance between the wall plates (or other supporting walls) and then use a joist span table to determine the new joist height

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  • What about sistering a 12 foot 2x6 to the centres? That would strengthen the butting 2x4s at least? Oct 21 '21 at 13:03
  • Or on top solutions so I don’t have to deal with wires or run the unused span? Running 2x4 on top in the opposite direction (on its side)? Or 2 layers of glued down ply in opposing directions? Oct 21 '21 at 13:06
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    @JohnnyBender both of your comment questions go contrary to this answer's advice to not attach the new floor to the old ceiling. Attaching the new floor to the old ceiling introduces a lot of stress into the old ceiling that can lead to damaging the old ceiling. It's your house, so you can choose to do so, but this answer is recommending against it.
    – FreeMan
    Oct 21 '21 at 15:27
  • @FreeMan I’m just asking a lot of questions so I can learn. Oct 22 '21 at 14:34
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    No, I’m only concerned with structural soundness. I may just have to hire an engineer to be safe. Oct 22 '21 at 19:48
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I doubt 2x4 joists on 16" centers are going to support a floor, you may have to run 2x6's over the span all the way to the top plates of the walls. This is probably something that a structural engineer should look at.

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  • I can’t seem to find a load table that shows TRUE SIZE wood. My 2x4s are not (38 x 89 mm) they are true size (50.8 x 101.6 mm). So 12mm thicker than a 2x6 but 38mm shy in height. So somewhere in between the load of a 2x4 and 2x6. Oct 22 '21 at 14:41

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