How can I reduce the effects of dilatation of wood on a steel deck?

Question

I am planning to build an outdoor deck at my house. The dimensions will be 3x9 m. The base of the deck will be short concrete columns and on them I'd like to place a frame made of steel bars. The top of the deck will be some kind of wood or artificial wood (WPC).

I live in the Czech Republic where we have relatively cold winters (minimal temperature usually -5°C to -10°C) and warm summers (over 30°C). I am worried that the temperature differences might cause dilatation [expansion/contraction] of the steel which would damage the boards or even the screws (shearing forces). Is there a secure way how to attach the wood to the steel, so that it is not affected by the "movement" of steel?

I found a forum post which recommended attaching thinner wood beams to the steel and then screwing the boards into the wood beams. However, I don't see how it would prevent the dilatation damage - there would still be wood attached to the steel.

On another forum, I found a post recommending to create segments of 5-6 boards screwed into wooden beams which are only layed down on the steel and attached with a thin metal strip just to prevent larger movement (see the image below). This method of attachment seems a bit too weak to me - I fear that the board segments might move under my feet.

Do you know any better way how to attach wooden boards to steel frame?

Answer 1

The solution for attaching wood to steel is the same as for attaching wood to wood - you have to adjust the holes to allow for the expansion (a more common term to native English speakers than "dilation", though I understood what you meant).

Wood will really only expand across the grain of the board (so in your drawing, it will expand the decking across the width of each deck board, but not in any noticeable amount in the length of the board) and this is due to moisture changes, not so much temperature changes. For a cross-grain wood to wood joint, your expansion will happen in two different directions, so one of the holes needs to be elongated to allow the screws to move in one direction. Usually, the deck board hole would be elongated slightly across its face to allow for this expansion.

I presume, since you're attaching wood to metal, that you're using nuts and bolts to go through pre-drilled holes in both the wood and the steel. Since the steel will expand in all directions more-or-less equally, you simply need a larger hole pre-drilled in the deck board to provide clearance, then use a bbolt with a big enough washer to prevent it from pulling through the hole. This hole should be large enough that the screw will drop straight through the clearance hole without the threads catching on the wood board. It will hold the board in place by acting as a clamp, not because of the threads - this is the way screws are designed to work. When you tighten, get everything just snug, but don't tighten it until you're leaving marks in the decking.

To be honest, though, I'm not sure if the steel will expand enough for you to need to worry about this. I'd recommend finding some tables online to see what the expansion rate is for the type of steel and size/shape of beams you're using. You may find that for your expected temperature range, the expected expansion isn't going to be a problem for you.

If you're going to use an artificial/composite wood product, then you'll also need to look at the installation instructions for it to see what the expected expansion rates are for it and determine how big an issue that's going to be.

Here in the US, though, most people really don't worry much about the expansion of decking material when building outdoors. I built a deck a few years ago using wooden joists (not steel) and wooden decking. Our temps range from around -20°C to 35°C (0-95°F sometimes we get below 0°F temps) and I didn't do anything but use decking screws to attach the decking to the joists. I made no special accommodations to allow for expansion. I've had a few deck boards split, but that is, I believe due primarily to screws too close to the ends of some of the boards and preexisting cracks in the boards - i.e. I knew they'd be splitting when I installed them, but put them in anyway.

---

I'm not sure, actually, how you're going to attach this. My presumed nut-and-bolt method will either leaving the bolt heads above the surface of the deck as trip hazards, or will require that every single one will also need to be counter-bored to recess the bolt head and washer into the decking which will be incredibly time consuming as well as creating little wells to hold rain water in the wood (instead of allowing it to drain off the surface) speeding the rotting of the wood. Maybe some clarification on how you plan on attaching the decking would be in order. You can edit that into your original question and I'll edit to match.

Answer 2

One approach that helps to allow for movement (of both wood and steel) is to use the "deck clip" or "invisible fastening" arrangement where the edges of the boards are slotted, and the fasteners hold a clip that goes into the slots, rather than through-bolting at all. The clips can slide to allow movement.

This is one example, no endorsement is implied, there are MANY competing manufacturers or you could easily make your own stainless steel plates with a hole for a machine screw (since you are going into steel. Drill and tap the holes in the steel, use stainless steel plates and screws.) See what's easily available in your local market.