Thread: 2-part floor joist to allow for stair head height

Cut the joist off short of its thickness
Put a trimmer joist (ie a noggin of the same size as the joists) between the adjacent joists. Nail the ends to the adjacent joists and the centre of the trimmer to the shortened joist

Bob,

Thanks for the reply!

Unfortunately I dont think that will work. The joist at one end of the trimmer would be further up the stairs and hence would voilate the height requirement even more. To avoid this, The trimmer would have to go all the way to the end of the stairs, which is a long way.

Another problem is that to save height, the floor joists will be running
parallel to and alternating with the existing ceiling joists. So the existing
ceiling joists would get in the way of a trimmer.

Thanks,
Rossco.

If I read that diagram correctly, you are planning to put one joist on top of another. Forgive me for stating the obvious, but isn't that going to get in the way of your flooring? In any case, I doubt that set up would be acceptable to an building inspector, unless you had some engineering detail for it.

The way Bob has outlined is the way it is done. If the second joist is in the way, it goes too and you carry on your trimmer to the next joist. The size of the trimmer and the trimming joists is dependant on the width of the opening. For 1000mm or less, they can be the same size as the floor joists. For larger openings up to 3000mm, you increase the width of the trimmer and trimming joists by 1/5 for every 300mm over 1000mm.

But I'm not all that clear on what you're trying to do. Are you wanting to create a void in the floor, or are you just stepping the ceiling up at that point? The normal thing to do would be to have a hole in the floor surrounded by a balustrade above the stairs.

It could work, without the details I don't want to comment but the trimmer is a simpler way to do it.

But from your posts it seems like the stairs go up and turn left thus eliminating the trimmer option.

it seems like the stairs go up and turn left

How did you pick that up?

Maybe some photos if you can Rossco, to save us all getting confused.

Thanks for all the replies. Here are some pictures.

The first picture shows the attic from the top. The stairs are marked,
with the arrow going up. The existing ceiling joists are marked in green
(they would be cut where they cross the stairs). Marked in blue are four
loadbearing walls to be constructed to hold roof loads (they go up to
the purlins halfway up the roof).

The next picture shows what I had proposed to do. The new floor joists
are marked in red. There are double joists at the ends to hold the
loadbearing walls parallel to the joists. The other two loadbearing
walls are perpendicular to the joists. I had planned on not flooring the
small section south of the stairs (it has low height anyway).

The last picture shows how to do this with a trimmer. It's better since
it allows me to put flooring joists south of the stairs. My only concern
is that there is a lot of load on the two joists at the ends of the trimmer.

If the trimmer design will be strong enough, I will stick with it. After
thinking about it, the existing ceiling joists won't be an issue for the
trimmer because they can be attached to it as well.

Thanks again for the responses, it is a lot clearer in my mind now.

Rossco.

You should run an extra member to stiffen the ceiling joists that connect to the trimmer.

That's pretty much what I thought it looked like.

With the trimmers, you have to beef up the two joists that the ends of the trimmer are attached to to take the weight. They effectively carry half the load of all the truncated joists in between. The guide I gave you above concerning increasing the width of the trimmer and the trimmer joists (the name for the two full joists at each end of the trimmer) is from the NSW framing manual. You use it in conjunction with the span tables for joists.

Perhaps an alternative would be to have a beam underneath supported by those blue load-bearing walls? Your truncated joists can sit on that. You don't gain too much except you wouldn't need the heavier trimmer joists, which might cause you a headache with the ceiling below.

Or enclose the stairwell on both sides and the new wall on the currently open side of the stairs becomes a load-bearing wall for the joists above. Being load-bearing, it would have to extend to the floor, but you could build in a cupboard underneath that is entered from the high end of the staircase to the right of the diagram.

Thanks to everyone who responded. I found the section in the framing standard that you mentioned.

I'll nail-laminate two floor joists (each 190x45) together to make the trimmer and trimming joists. I'm hoping I can find some pretty big joist hangers (190x90) to attach the trimmer. If not, I suppose I'll have to improvise with heavy-duty brackets or similar.

You may look into a truss boot due to the 90mm.