Deck joists do not need to be perfect, but they need to be consistent. How consistent depends on the decking material used. More rigid decking material will hide the joist inconsistency better.
Deck joists supporting 1 ½” (38mm) or 5/4” (32mm) dimensional wood decking do not need to be planed. Composite or PVC synthetic decking will highlight waves between joist on the deck, requiring greater care in joist installation and planing of inconsistent joist.
The type of decking and joisting practises will determine the level of planing required. Decking material reflecting the joists’ change in height differently. Synthetic decking being the worse. Both in the fact that most composite decking is 1” (25mm) or less thick but also the flexibility of the material. The deflection increasing as the decking warms. On a hot day, PVC decking can have a slight wave as the material droops between joists.
Wood decking being more rigid, will not need any joist plaining. The rigidity allows the decking to minimize the differences between joist heights. The decking gradually rising and falling with the changing joist’s height, absorbing the change. Making joist differences indistinguishable when scanning over the decking from the ground.
This is the biggest reason to plane deck joists, is distinguishable height differences. If not, the joists do not need to be planed. If changes in the joists are noticeable, it needs to be corrected. There are a number of good construction practises that will minimize differences between joists. The following five construction practises will minimize required joist plaining.
Joisting practises reducing deck joist planing
The goal here is consistency. If the joists are consistent, one to another, they will not need to be planed. Taking a few extra minutes in the frontend and save hours planing your joist later.
1. Cull defective joist to lessen planing
Before installing any joist, take a second to look down it. If it has a huge crown, bow or twist, don’t use it. Installing the joist will not change the fact that its garbage. It will be better to cut it up and use it as deck blocking. Every quality deck needs blocking to reduce deflection, twisting and support critical points of the deck. Plus, this is a great way to use below standard joists.
I have written an article including 27 tips about joist blocking. Click here to learn more about why and how to use deck blocking.
But the best way to reduce planing of joists is don’t installed joists that are twisted or have massive crowns.
2. Group joist by size to reduce planing
Surprise, joists are not all the same size. Either when they are planed at the mill or when dried. But a 2×8 joist deck can vary from 7” (178) to as big as 7 ½” (190mm). 2×8 deck joists are commonly 7 ¼” (184mm) but are not consistent enough to not check.
While checking the joist for defections, sort them into piles according to size. Then install the joist as groups. Starting on the one side of the deck with the larger joist, then placing the next group size beside them. Creating mini-steps between joists. Gradually reducing the joist height by unnoticeable marginal amounts, not up and down with each joist.
It’s the up and down in joist heights that must be reduced, not a consistent, gradual drop in height. A deck sloped a 1/8” (3mm) for drainage is unnoticeable by the naked eye. The same is true for a consistent drop in elevation between the joist. If the joists are consistent, the decking will be also.
3. Install hangers after flushing joists to the ledger and flush beams reduces planing
To install joist inconsistently in height, its a good practise to toenail or angle screw the joist flush to the ledger before installing the hangers. This will ensure that the top of the joists are consistent with the ledger and each other.
It may seem counterproductive compared to preinstalling the hangers before installing the ledger, but it will be more efficient than adjusting hangers or joists later. Consistent joists and hangers will save you time in the long run. A make a better-looking deck.
4. Consisting crowning of deck joist reduces planing
After you have removed joist with excessive crowns, you still need to account for their crown. All joists have a slight rise, a crown that needs to be accommodated for during construction.
By consistently installing joists with the crown up not only will make for a stronger deck but also minimize waves in the decking. It is not true that Saskatchewan is flat, its just that it doesn’t put its valleys beside its hills. The same is with deck joist. If all your deck joists are crowned uniformly, the difference will be minimal. The joist won’t require plaining. But if one joist has a slight crown, 1/8” up and the one beside it the same down. You now have a difference of a ¼” between joists. But if they are both crowned up, they are consistently the same height. Being level from one joist to the other.
5. Using Roberts Ridge reduces required joist planing
This is a neat trick invented by Eric Roberts for levelling mid-span joist heights. Lay a straight 2×4 on edge midspan of the joist before installing blocking. Screw 6-inch TimberLok or HeadLok screws through the 2×4 “ridge” into each joist below, pulling the joists flush to the bottom of the ridge. They need to be stronger than #8 deck screws to pull the joist up. Then install the midspan blocking while the joists are being held in place by the ridge. The blocking holds the joist into place not only horizontally but also vertically. After removing the 2×4 and install the joist’s flashing and decking.
The joist pulled up to the same height before installing the blocking. They remain more consistent in height with the blocking.
To read more about Roberts Ridge, click here to go to Deck Magazine
6. Bonus tip, use Elevation metal joist from Trex
Well, if its true, straight, and consistent joist you want, there is no need for any tricks. Elevation joists are more costly than wood joists, but so is their quality. You may have noticed that all the tricks are to make inconsistencies less noticeable, for you don’t have to plane the joist. But Trex’s elevation joists’ are consistent in height every time, perfectly level and resistant to rot and decay.
Incorporating the five tips above will minimize the need to plane deck joist by reducing extremes between joists. Creating a consistent decking surface. Translating into a level looking deck to be enjoyed.
But before patting yourself on the back and installing the decking, take a moment to check while the joist can be easily planed.
Checking and marking joists needing to be planed
There are three ways to check if joists need to be planed, string line, 6’ level or a straight board. Remember, you are looking for consistency between joists of less than 1/8” (3mm). A discrepancy of less than 1/8” (3mm) will not be noticeable on the deck and is hard to see from the edge of the deck.
The easiest is a straight board or 6’ level, but a string line is more accurate. Pick the straightest board you can find and drag it along the top of the joist. Watching for where the board lifts off the other joist. The first joist that lifts the board, the one it rocks on needs to be planed down. Mark the top of the joist where the crown begins and then drag the board from the other side. Marking where the joist is higher than its neighbours. This is the section that will need to be planed down.
The same can be done with a string line. Screw a screw halfway in, mid-span on the edge joist. Loop the string line on the screw and drag it across the joist from the other side of the deck. Marking any joist that pushes up on the string line. The same as with the board marking it from both sides for you know what part of the joist needs to be planed down.
Take a level and hold it up against the joist connecting the two marks and draw a line between them. Marking how much material needs to be planed off the joist. Repeat this again on the other side, giving yourself two consistent guiding lines to work off.
3 Ways to plane down deck joists
Once locating the joists needing to be planed, you have three options to level out the joist: a wood planer, belt sander or circular saw. The idea is simple, remove the material above the line. Reseal with end treatment, and voilà, you have a consistent decking surface.
Wood planer, either power or hand, is the most efficient and effective way to level a high point on a joist. Simply scape back and forth till the joist is level with your level line.
Belt sander can also be used to plane down high material on the joist. Similar to a wood planer except a little slower as you are grinding of the wood not cutting it off. Use a low grit sandpaper like 60 grit, often used to remove old varnish on hardwood flooring. Something with real bite, to eat away the material. Take your time, apply slight pressure and move the sander back and forth, sanding the high material off. It will take a little longer than a planer but still reasonably effective with the added advantage of removing material in smaller layers, giving you a greater level of control.
Circular saw cutting of the high joist material. This is dangerous and tricky as a circular saw ideally has more than a ¼” (6mm) of material to hold the blade while cutting. If you have a crown difference more than ¼” or refer back to tip #1 in deck joist installations. But carefully holding back the guard starting with the blade resting on top of the joist and carefully cut just above the line to plane the joist even with the other joist. Go slowly as I said the blade ideally has some material above it to guide it and will wander easily.
In a much more crude and dangerous way, I learned as a framer. Set the saw blade at a 1/8” (3mm) and pull the saw over the joist perpendicular to the joist. Chipping off the high material as the blade runs spins over the joist. It will not be a perfect cut and is challenging to remove material above line consistently, but it can be done. Not recommended, but in a bine, it may be a solution.
Conclusion about planing deck joist
Using the five tips in selecting, sorting and installing the joists there should be little need to plane your deck joist. As they will be consistent with only minor and gradual differences in joist’s height. Unnoticeable under your decking. But still, check your joist for humps before installing decking, ensuring you will have no problems in the future.
If a super straight deck is on the wish list, check out Trex’s Elevation joist, the straightest deck joist out there. Click here to read more about Elevation Joist.
Take care in installing your joist, and you will be able to enjoy a level deck for years to come.