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I've got a lot of 7x2 joists left over from a previous project. Now I have a new ceiling to put up but they require only 5x2 joists, so I'm thinking of trimming the 7x2 so as not to have them go to waste.

Would trimming a 7x2 to a 5x2 (or reducing the height by whatever amount) give the same structural performance as off-the-shelf 5x2s?

  • Where are you that lumber is described in those sizes? Is that inches? – isherwood Oct 15 at 20:31
  • Yes, those are inches. I'm in the UK but used inches just for familiarity (I'm assuming most visitors on here are from America). The actual sizes of the joists are 47mm x 122mm and 47mm x 170mm C24 grade. – JamesDean Oct 15 at 20:43
  • They'd be rough equivalents of two-by-sixes and two-by-eights. – isherwood Oct 15 at 20:46
  • Please take the tour so you're familiar with our routine. – isherwood Oct 15 at 20:47
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Yes, lumber cut to a smaller dimension is essentially the same as lumber cut to that size at the mill.

  • Common sense told me it would be the case but given they are 'c24' graded, I didn't know if I'd be changing its properties if I shaved off 2 inches. – JamesDean Oct 15 at 20:40
  • Grade is independent of size, in my experience. – isherwood Oct 15 at 20:48
  • @isherwood Yes, lumber grades vary by size of lumber and location of use. A 2x6 for “Light Framing” is graded differently than a 4x10 for a “Beam”. For the same stress loads, a 2x6 might be “standard and better” or a “#2 and better”, while a 4x10 beam may need to be “Select” or a “Dense” for the same stress loads. – Lee Sam Oct 15 at 21:44
  • Yes, but that type of beam isn't in common use on a residential jobsite. We're talking about two-by in this case. – isherwood Oct 16 at 12:48
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I would not trim the lumber down. Your ceiling joists that call for 5x2 are just that, ceiling joists. If there was a floor above they would be floor joists from above and then they would have a greater dimension.

Given that we've established that this ceiling will not have a floor above it I see no reason for cutting - just leave them as 7x2 and set them so that the bottom edge is where you want ceiling to be.

If the extra height of the ceiling joists interferes with the top of say the rafters (i.e. underside of the roof sheathing) then you simply need to trim off the top corners accordingly.

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  • Is your suggestion not to trim the lumber a practical one because they are just ceiling joists? If I did decide to cut them to 5x2, would they still be structurally identical to regular 5x2s? – JamesDean Oct 15 at 21:14
  • My comment is based upon not wasting extra rigidity for your ceiling that you get basically for free. There could be some sacrifice in strength depending upon how ideal the saw mill ripped the 7x2's out of the log. Your ideal dimensional lumber would have the annual ring pattern as parallel to the wide dimension of the timber as possible. If your 7x2s are ripped where this parallel pattern is mostly across the timber then cutting them down has no sacrifice. On the other hand as you get closer and closer to the middle of the log the ring pattern will be in crescent patterns when (continued) – Michael Karas Oct 15 at 21:53
  • (continued from above) viewed from the end of the timber. In the extreme case of this you would ideally need to cut down your timbers by removing an equal amount from each edge. – Michael Karas Oct 15 at 21:55
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I agree with Isherwood that lumber custom trimmed to a smaller size will have the same strength as lumber milled at a mill site...generally.

There is a limit to this method in size and location of use, but probably doesn’t affect you in this location or size of material. (I’d be careful not to leave a large knot on the edge of the member after trimming it down.)

However, I also agree with @Michael Karas that you could use the slightly larger lumber (if space allows) and just trim the members where necessary to fit, (as long as they are not trimmed smaller than the matching members).

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