Deck concerns

My brother in law has had a deck designed (8m by 4m) by a "mate" of his. The design is actually pretty robust, and you could prpbably land a plane on it, it's spec'd up that much. The only concern I have is the joists (I think they're called).

They have metal posts supporting the deck, with U-channel running around the perimeter to hold the deck. The channel is plenty wide enough,150 mm inside I think, but the plan calls for the 50mm x 150mm joists to sit in the channel either side, and get knocked into place in the channel, almost a press fit if you get the drift. I've put a sketch which tries to explain it. There is no fastening of the joists to the U-channel. The decking fastens to the top of the joists and I guess the whole thing "floats" in the channel. I'm concerned about the joists warping. They're 50 x150 spanning 4 metres. The plans have all been run past the council but I think they're more concerned with the engineering aspects, and assume the fasstening is all taken care of.

Any opinions gratefully accepted because my bro in law has not a lot of idea about these things and is relying on his mate. We're both building it with him, but the mate can shoot through anytime, whereas I'm married to his sister and he can hunt me down like a dawg.

John,
bit of an unusual design which has a few minor problems, but it's not unworkable. The sheer mass of all the decking and joists will stop it shifting once it's all nailed together, but it may move a bit while you're fixing it. You could fix the joist by drilling a hole in the bottom flange of the channel and driving a roofing screw up into the joist. (14gauge by 50mm)

Ideally the joists should have a rebate on the top edge so that their top surface is flush with that of the steel. There will be no (easy) way to fix the perimeter boards to the steel channel.

Corrosion may be a problem unless the steel is galvanised as the timber will tend to trap moisture where it sits on the steel.

You'll need to fix temporary spacers (ie lengths of decking or similar) along the top of the joists to hold them in place, and possibly a few diagonals under as well.

Unless you're hardpressed to get your 600 clearance from ground to joist it would be easier in the long run to weld or bolt some cleats to the steel and sit your joists on top, bolted to the cleats to hold them in place. You may also find that suitable gal RHS will be lighter and may work out to be cheaper.

Mick

PS, if the above proves unsuitable, hopefully your B.I.L. will find it easier to hunt down you than me. :D

Thanks Mick. When I saw the plans I thought it was a pretty complicated way to do things. There's plenty of timbers that will span 4 metres and I would have tended to leave the U-channel out of the equation altogether and take a more traditional bearer/joist/decking approach with timber.

However, he's got his plans and his council approval already, and unless I'm real brave I won't redesign it for him and put him back a few months.

I'll take a note of the temporary "keep it all lined up while we're doing it" measures you mentioned, but do you think the joists risk twisting in the U-channel without being fastened? Over time I mean.

I guess it depends on the timber you're using, its species and how well it's been seasoned. If you're worried about them carrying on then buy additional joist material and nail bridging pieces between each joist, one at each end and then maybe two equally spaced runs. By the time you do all that, it would definitely be easier, quicker and cheaper to take a more traditional approach.

Mick

Thanks for all the help Mick. I'm getting the impression that B.I.L. is getting a bit frustrated with his mate actually. He doesn't know much about this stuff, and has been relying on his mate, and his mate's mates for advice. I only know what I know from 2 pergolas and a deck over the last 15 years, but they're still standing, and I did get his sister out of the house so I suppose he has a soft spot for me. I think I'll push and see of he wants to reconsider all timber. I'd still keep the metal posts in but maybe we can rethink the rest. I hadn't thought about the moisture causing corrosion and a couple of other things you brought up so I'll throw 'em into the pot and see what boils.

You can use soild blocking between the joists, this will stop the joists from wanting to roll over and if used mid span can reduce the spring in the finshed deck.


Good point mick about the rot, where the joists butt into the c section.

If i was building this i would cut my joists tight then paint the ends to re-seal the cut end, then smash them into position in the section.

If your frame is not square you will have to measure every joists to fit, to check if square use the 3, 4 ,5 rule.

A question whats the plan to cover the outside of the steel around the edge. are you going to screw so battens on the side with metal tek screws then fixing decking on for a fascia

There is no plan Gaza, as far as I can see, to finish off the channel. They'll probably paint it. I feel a bit awkward here because I've come into this after all the plans are done and now he's asking my advice because I don't think he has a lot of faith in his mate. It's going to be a case of making what they've got work, or pee them right off by going back to basics.

I'll probably tell them to use blocks between the joists, as that's my biggest concern, the twisting. If I cut them tight and belt them in I just know that when I belt one in, the previous one will loosen up. And it's a pain in the butt measuring and cutting that precisely too.

We'll see what happens tomorrow night when we get together and discuss, but thanks guys for the tips so far. As Arnie says, I'll be baaack

Hey Rookie

Suggestion to protect the steel is to run some PVC tape (duct tape or equiv.) along the surface that you will befitting the timber to.

I used it on my deck built 6 year ago under the decking boards as they are fixed to 100X50 Galv RHS joists. This stops the moisture in the wood contacting the galv and accelerating the corrosion process.

I had to lift some boards a few months ago and peel back the tape, good as new both board and steel looked A1.

Your only problems is that the tape wont take too much abuse, so I would suggest that the ends of the joists be sized so they are a clearance fit inside the flanges of the channel. Mick's idea of a securing screw underneathe should be considered.

Hope this helps

Just an idea on finishing the C channel. You can nail 2 rows of 90mm boards as a facia using stainless twist shank nails with a nail gun. I was introduced to this method by a fencing friend of mine after I winged about how long it took me to predrill and screw decking to a steel gate frame. Next time I needed a gate I went with palings and he nailed them all on in about 5mins, and it looked better too.
Cheers
Michael

Thanks John, Michael

I'll definitely give the duct tape idea a go and I'll mention the finish, although the bro-in-law doesn't seem to mind all the channel around the edge. Beats me. I wouldn't have used it in the first place. :confused: