Notching joists to lower them in a garage.

[jamesBen80]

I'm boarding out my loft in the garage. Currently there a 2x4's every 4th rafter acting as a tie. I've gone for 2x8 c24 joists at 450 centers to match the rafter spacing. It's a 4.5m span and I was wondering if I can notch the underside of the new 2x8 to sit them a couple of inches lower. As it will gain a bit of head room. I'm going to screw them down into the timber wall plate and also bolt them through the rafters to tie it all in. Then board it out with 18mm t&g chipboard. I'm looking at upgrading the 3x2 rafters in the future too as they've sagged a fair bit over the years and I'd like to replace the felt roof with slate and possibly solar in the future.

 

[King Billy]

1. It's only a garage.
2. Your chosen joists are OK for deadloads of not more than 0.25 kN/m² on your required span. This is for light loading applications.
3. But supposing you're using 47mm X 195mm, which is good for 4.38 M span, and you want to take multiple 25mm cuts out of the underside, which will reduce them to 47mm X 170mm, which is only good for 3.74 M span.
You're already designed for light loading. I would suggest you reconsider your approach. But it is only a garage.

 

[noseall]

I'm boarding out my loft in the garage. Currently there a 2x4's every 4th rafter acting as a tie. I've gone for 2x8 c24 joists at 450 centers to match the rafter spacing. It's a 4.5m span and I was wondering if I can notch the underside of the new 2x8 to sit them a couple of inches lower. As it will gain a bit of head room. I'm going to screw them down into the timber wall plate and also bolt them through the rafters to tie it all in. Then board it out with 18mm t&g chipboard. I'm looking at upgrading the 3x2 rafters in the future too as they've sagged a fair bit over the years and I'd like to replace the felt roof with slate and possibly solar in the future.

Use long leg jiffy hangers fixed to the wall plate.

 

[jamesBen80]

I thought of joist hangers to drop the joists lower but I really wanted to bolt the ends to the rafters to create a triangle truss for strength. I'm probably only going to store light things up there so I don't think I'll be going over 0.25KN / m2. I'm not concerned about bounce or flex cracking the plaster ceiling like in a house, it's only a garage.

 

[tony1851]

As long as you're only going to be storing light stuff up there (kids old toys, suitcases, decorations etc) you could notch the ends up to nearly half the depth of the joists - it won't affect the deflection or the bending strength; all it affects is the shear stress at the bearing, which is not likely to be great in your case.
Alternatively, consider saving money and use 6 x 2 instead of 8 x 2 - you could also notch the 6 x 2 to get the tops even lower.
There are simple equations in the timber code which give you how much you can cut out at each end without exceding the allowable shear stress. But up to 50% should be ok if you dont have a ceiling and don't mind just a little movement when moving about near the middle.
Timber is very forgiving when overloading! Dont be too tied down by the figures in the load/span tables - they are very conservative.
When fixing the chipboard down, consider gluing as well as screwing - that adds a little to the strength of the joists which then begin to act like a series of 'T' beams. Soudal PU glue is the one generally used.

 

[stevie888]

Good stuff there from Tony.

One additional comment: bottom notching is potentially more deleterious than top as the bending will tend to open a split from the notch. A wrqp with builders band would prevent this.