1/2″ Plywood Roof on 24″ centre – Sagging Fix?
MM1973
| Posted in Construction Techniques on
Just took over GC’ing my reno, as we parted ways with our GC, I am trying to improve several things before Insulation. We put a second floor on. The second floor roof is 24″ OC with 1/2″ PLY with H clips. IS this sufficient? The lower Roof is 16″ OC with no H clips, 1/2″ PLY. I have looked at the stamps to see what the rating is but it was no number except for the manufacturer and 12.5mm or 1/2″.
Now that everything is still open I am debating blocking between the trusses and rafters to prevent future sagging.
1. How much of a concern is sagging if using H clips?
2. Should I block?
3. How much Should I block? Every 2′ on every run? Should I block every run? Should I block at all?
thanks
Replies
If the proper roof sheathing is installed, what you are describing is code approved in most regions and is the standard practice. Double check with your local building code for rafter spacing and confirm that you have proper sheathing (which you probably do).
That’s the thing I cannot find the rating numbers on the stamp. I am a little concerned. I just think I could put in support blocks now while we are still open but don’t know how often on a run.
I wouldn’t bother with the blocking if you have h clips as it standard practice. You are allowed to eliminate the h clips on 16 oc b/c the sheets will not deflect as much. If your plywood is not rated for 24 oc than you’ll have to put in more rafters or install new plywood as the span rating is based of perpendicular orientation to the rafters. I highly doubt you have the wrong plywood as you kind of have to go out of your way to track down plywood sheathing that isn’t rated for roof and wall. It could be that the plywood labels are on the top side of the roof and felt or roofing is covering the labels. If you can pull back a little felt you may find a label. Also, the plywood may have a certain number of spray painted lines on the edges that will indicate from the manufacturer the type of sheathing it is. If you contact the supplier they should be able to tell you if the proper sheathing was purchased. Make a call to your supplier before you put in any more time or $. I think you’ll find everything is alright.
I will look into it and let you know thanks
Not sure what these mean, see photos. There are three black lines along the side edge. This is Canadian by the way.
I’m not familiar with Canadian stamp, sorry. All my work has been in the US and Scotland, so cannot speak on the Canadian grading. If the material was purchased through a building supply house I’m sure they sent the proper sheathing. Give them a call and they will probably know.
It may be standard practice but the plywood still sags between the trusses. Early in the morning when dew is on them they look terrible.
You are right, there is already sag, however slight and the roof is only
6 months old. Even with the H clips, in fact you can see it most at the H clips We have a snowy climate 5 months out of the year so there is weight on the roof. I have started blocking. Anyone have an idea in a single rafter run how often you would block? Also, should I do every run? Thanks for the help.
I'm sorry, I missed that you were asking about blocking. When we block sags up we do each edge and one in the middle. We lay the 2 x 4' flat and nail them to the trusses or rafters on each end. It's a PITA job but if you go at it you can get a lot done in a short time. Precut your pieces, use a nail gun and wear good hearing and eye protection.
Awesome thanks