I have a project coming up and we're installing white oak treads on a system that has steel skirt boards. Will there be issues with warping if these are scribed tight to the skirt? Would it be better to leave a small expansion gap. I've included the drawing detail. I would love to hear people's experience with this situation or something similar.

Click the link below to download the file included with this post.

Stair_detail1.pdf


Click the link below to download the file included with this post.

Stair_detail2.pdf

You can have both bowing and splitting if you fasten it hard to the steel frame. Spec rift or quatersawn and that chance is greatly reduced.

I would worry about black staining of the oak from tannin reacting with the iron.

I cant see where there would be a problem. Wood expands on the width not the length. I would scribe them tight to the skirt and drill the holes in the steel, either drill your center hole for the diameter of the screw and slot the front and rear hole to allow for some expansion both ways. Or drill the rear hole and slot out the middle and front hole so that all of the expansion moves to the front of the tread. If you wanted to leave a 32nd on either side of the tread to be safe you could.

nicko

To surround a piece of solid wood in a welded frame of steel angle invites problems. It's fundamental. Wood expands and contracts across the grain with moisture fluctuations. Make these treads with wood at 8% MC and have them dry to 5-6 % during the winter and you could have splits where the screws attempt to constrain the movement. Make the same treads in the winter with 6% MC wood only to have them swell in the humidity of the summer - you may have cupping or at least severe wood compression within the steel frame.
This design ignores basic wood construction practices. If your area experiences humidity swings seasonally I would recommend an alternative construction or risk call backs for noisy or broken treads.

If you look at the drawing you can see that the treads are not surrounded by steel but sit on top of the steel

I agree with Nicko on fastening. If your worried about tannin bleed into the wood, fit, then remove and finish all sides before a final installation.

It is OK to sit on a metal frame.
It is OK to scribe tight against the metal.
It is OK to accommodate the treads' slight movement across it's width.
It Is OK to not worry about iron staining black, as long as it all is kept dry.

The tread will only require slightly slotted screw holes in the metal frame. Look at the Shrinkulator to determine how much, and have the metal shop modify their work to include that slot.

This may be an open riser design, but if the tread is to be tight against a (wood) riser, you will find it wise to rabbet the riser over the tread. Adjust the slot sizes in metal accordingly - no slot, hole only back by the riser -and max slot length at the most forward - away from riser - part of the metal support frame.

I looked at the drawings again and there is a 1 1/2" angle iron welded pan that the tread drops in. That ain't good. Or maybe it's my eyes ain't good.

The second drawing shows a hidden line which represents the angle under the tread. The last drawing shows the step tread from the bottom.

Nicko

Upon further contemplation(and a glass of wine), the possibility of someone spilling a drink on the stairs would have me say you should absolutely seal the treads all around to prevent tanin stain. I’d like to imagine my stairway will never see excess moisture, until one of my kids overflows the upstairs bath tub!

I don't see anywhere if this is residential or commercial. If it's commercial, people coming in the building will have snow on their shoes and water will certainly be a concern.

What I see is:
There is a 4 pc, 1-1/2" angle iron frame between the steel stringers to place the tread into
The drawings call for 6/4 (1-1/2") treads, negating scribing the treads to the stringers ("skirtboards"); rather, a good fit to the angle iron frame would be required. I'd leave an 1/8"-3/16" gap at the back of the tread to the frame, tight to the fron, snug for length.
The designer/architect is paying attention- chamfer the underside egges to fit into the inside of the angle iron; drill oversize holes and fasten with Truss head screws- this alows for some movement
Rift stock would be best, but flatsawn will work. I'd probably glue up flatsawn to 3 pcs per tread
Simple job

After a second cup of coffee, I reaized that if the treads are 1-1/2", set inside 1-;1/2" angle, then the treads would stick above the angle the thickness of the angle- which I'd guess would be 1/4" + -
So, if you rabbet the treads all the way around, you'd cover the angle iron, and be able to scribe to the stringer. But that 1/4" over at the nosing is a bad idea as it will eventually crack and break off.
Best to clarify with the architect/client

I'm not the original Adam poster

I agree with David. As long as you make allowances for the wood movement in tread width there should be no issues.

The good thing about the steel is it will not move as opposed to regular 2x house framing that shrinks & settles for a the year following construction.

I'm impressed by the Architect/Designers drawings including elongated holes and biscuit/domino's.

The steel rusting and tannins damaging the wood should be completely avoid by painting the steel and the finish on the stair treads.

It all looks well thought out to me, except the micro bevel in the bottom of the treads. You'll have to keep the glue out of them or run the finished treads on the router table with a regular v-bit, which wouldn't be hard. I guess you will be micro beveling everything anyways with a laminate trimmer.

I'm not the original Adam poster

I agree with David. As long as you make allowances for the wood movement in tread width there should be no issues.

The good thing about the steel is it will not move as opposed to regular 2x house framing that shrinks & settles for a the year following construction.

I'm impressed by the Architect/Designers drawings including elongated holes and biscuit/domino's(not that you need either)

Painting the steel & finishing the treads should prevent any staining.

Wooden stair treads installed tightly against a metal skirt board provide a clean and seamless look to the staircase. The metal skirt board acts as a decorative cover for the structural portion of the stairs, and by placing the treads tightly against it, it creates a visually appealing and polished finish.