Deck stair stringers--how to deal with Pressure Treated Wood shrinkage

[Mark37]

Bought my stringer and riser material a week ago.
I have the material stored with the boards separated by stickers.
The 2x12 material started out at 11 3/16"  and now measures 11 1/8".  If I cut the tread / riser notches now and the wood shrinks further, the stinger cuts will deviate from straight, square, and plumb, and also it would mess with the miter joints where the risers meet the end stringer.
I guess the ideal would be to let the boards season for weeks or months,  but that is not really practical.

I found this on the Fine Homebuilding web site How to avoid sloped stringers on deck stairs  but I don't have the experience to estimate how much more the wood might shrink

Any other suggestions would be greatly appreciated.

 

[Cheese]

Do you have a moisture meter to get a better idea of how wet the treated lumber is?

 

[Mark37]

Thanks for the response.  50 years of woodworking and I've never bought a moisture meter.  But I'm going to go get one Saturday.

 

[Bob D.]

Are there alternatives to pressure-treated wood that would fit your application such as treated LVL?
https://pacificwoodtech.com/products/pwt-treated/

The install guide talks about stringer use.
https://pacificwoodtech.com/wp-content/uploads/2022/06/PWT-Treated-Install-Guide-2022-06-27.pdf

 

[peter halle]

You can also purchase pressure treated lumber that is kiln dried after treatment.  Premium price and not at a big box store.  I am sorry I have only used it once and don't remember brand names although I suspect that would vary in different parts of the country.  It is called KDAT lumber if you decide to investigate.

Good Luck.

Peter

 

[Dane]

If you are inclined to use a different material- the Pylex steel stair stringers are pretty decent and eliminate the issues of the crappy wood they use for PT these days.

 

[Muttley000]

You can also purchase pressure treated lumber that is kiln dried after treatment.  Premium price and not at a big box store.  I am sorry I have only used it once and don't remember brand names although I suspect that would vary in different parts of the country.  It is called KDAT lumber if you decide to investigate.

Good Luck.

Peter

Agree with this, or estimating with the moisture meter.  Construction lumber quality is in a sad state based on my last few trips to the yard.

 

[Mark37]

I appreciate the suggestions.  Unfortunately the Pylex steel stringers aren't going to work for the rise and run I need to use on this stairway.  I'll make some calls Monday to see about the KDAT lumber, but its not listed on the website of the lumber yard I normally go to.

I bought a Lignomat moisture meter.  As a reference, I had dry a PT 2x4 that's been in my shop for a several years and for the last several months the RH in the shop has been controlled to 50%.  The moisture meter read 10%, which according to the Bruce Hoadley book Understanding Wood, is about what you would expect at 50% RH.  I was concerned the wood treatment chemicals might affect the moisture readings, but it seems reasonably accurate.  I'm going to call Lignomat Monday and ask about this. 

I also weighed the boards, and carefully measured width at a few spots.  As might be expected, the widths were narrower, and moisture readings were also a little lower at the ends, where drying would be fastest. 

The good news is the highest moisture readings I got were about 19%.  Taking some readings on PT deck joists in the existing deck,
I think my new stringers will eventually get to about 15%.  So I can use the data in the Hoadley book to estimate the shrinkage going from 19% to 15%

One other caveat is Lignomat says the meter is not accurate for lumber thicker than 5/4

 

[Mike Goetzke]

I appreciate the suggestions.  Unfortunately the Pylex steel stringers aren't going to work for the rise and run I need to use on this stairway.  I'll make some calls Monday to see about the KDAT lumber, but its not listed on the website of the lumber yard I normally go to.

I bought a Lignomat moisture meter.  As a reference, I had dry a PT 2x4 that's been in my shop for a several years and for the last several months the RH in the shop has been controlled to 50%.  The moisture meter read 10%, which according to the Bruce Hoadley book Understanding Wood, is about what you would expect at 50% RH.  I was concerned the wood treatment chemicals might affect the moisture readings, but it seems reasonably accurate.  I'm going to call Lignomat Monday and ask about this. 

I also weighed the boards, and carefully measured width at a few spots.  As might be expected, the widths were narrower, and moisture readings were also a little lower at the ends, where drying would be fastest. 

The good news is the highest moisture readings I got were about 19%.  Taking some readings on PT deck joists in the existing deck,
I think my new stringers will eventually get to about 15%.  So I can use the data in the Hoadley book to estimate the shrinkage going from 19% to 15%

One other caveat is Lignomat says the meter is not accurate for lumber thicker than 5/4

Mark - my experience is that the PT lumber moisture content varies quite a bit at the big box stores. My recent project was a pergola that I used Cedartone PT lumber. I swear the 2x8x10' boards weighed as much as the 2x8x20' boards. My project wasn't as critical dimensionally as yours. With the high moisture content wood my experience has been to not try to dry it first or it will become far from straight.

Mike

 

[Mark37]

Since I first posted, the boards have lost some weight and the moisture meter readings have gone down a couple of %.  I don’t think I really have a handle on how much more they will shrink, but I *think/guess* it will not be too drastic.  So I added a correction of .1” per the Fine Homebuilding link in the first post. The correction I used is less than the 1/8-1/4" suggested on Fine Homebuilding.  Time will tell.

With the correction, the riser and tread cuts are not square to each other, so a framing square can't be used for layout.  I made up a pattern that runs against an 8 foot straight edge clamped to the board. The straight edge was one of the best $20 I've ever spent.
[attachimg=1]. 

 

[Mark37]

my experience has been to not try to dry it first or it will become far from straight.

Mike

Indeed, the stringers became "far from straight."  I'm used to jointing boards flat and planing to thickness, so this drives me nuts

I've added blocking that somewhat corrects the warping and twisting

 

[Mark37]

I rough cut the outside stringer.  This board (non-incised pressure treated) dried from 40.2 lbs down to 36.2, still feels heavier than all the other stringers.

Checking that my previously marked cut lines on the outside face line up with the rest of the carcass.  The risers will be mitered to the outside stringer, so I want to make sure I've got every cut where it needs to be.  I can't remember the name for the jig I'm using.    So far so good

 

[Mark37]

Had a break in the Northwest rainy weather this afternoon and got the treads on. 
Kudos to my wife who pre-finished them.
[attachimg=1][attachimg=2]