Screwing wooden decking to aluminum rails
BiggFrank
| Posted in Construction Techniques on
I have been asked to replace a deck on a tiny house. The original deck was made of 1/2 inch oak boards fastened to square tube aluminum rails with steel screws. No prizes for guessing what happened, the screws all corroded and broke off due to electro-galvanic corrosion (to make matters worse it is on the coast and so there is salt as well as moisture.) I am looking for suggestions on screws for the new deck. My first thought was self tapping stainless steel, but read warnings that this wont work if there is salt present, which there is. My second thought was to use aluminum screws, but they are hard to find and presumably so soft they would strip easily. Would ceramic coated deck screws work ? (I have my doubts.)
Replies
I would cover the aluminum with Vycor, bolt a PT 2 x 4" on top and treat the rest like a regular deck.
Thanks, that is a great idea and would probably be the best way to go, unfortunately the deck is actually hinged and folds up against the side of the house for moving (it is on a trailer) and that would make it too thick to fold (that is why the decking is only 1/2" thick.) If I don't get any other suggestions I may try and persuade her to change that part of the design though.
Then bolt the 2X4 or a 2X2 to the side and screw the deck to that. How often does the house actually move? Is there a reason the deck couldn't be removed to move the house? No reflectionon you but it sounds like a dumb plan.
It's vaguely possible that you might be able to use aluminum rivets to attach aluminum straps to the rails, then use more substantial screws through the straps and into the planking.
Thats a good, out of the box, idea that would be worth investigating. Unfortunately the underside of the aluminum framing is covered in sheet metal so not accessible. It is a very expensive and unusual tiny house / RV / trailer hybrid, I should have taken some photos to post, but it is 100 miles away so I can't.
Stainless screws required for the salt environment. Aluminum will corrode to protect the stainless if moisture is present. Need to eliminate the moisture to break the galvanic cell circuit.
Type 316 is best, 305 will work if you keep the salt off by sealing any exposed surfaces.
Srewing boards to the aluminum tubes:
Nitrile sealant on top of aluminum to seal around the screw shank.
Deck boards sealed / waterproofed / finished on all sides prior to installation. Plugged holes for the screws, or hidden fastener system.
Corrosion Inhibiting Compound (CIC) applied inside the aluminum tubes after installation to fully coat the tube and screws.
NOTE: A penetrating water displacing CIC that will creep into the screw holes would be best for corrosion protection. These are hydrocarbon based, and is not compatable with Vycor or similar flashing. Use a Nitrile sealing tape instead Soilid tape if available, closed cell foam is acceptable. Open cell foam ( weather stripping tape) will trap moisture, and should not be used.
Alternates: Use aluminum deck boards, and rivet in place. Apply CIC to inside of tubes to coat rivet tails.
No metal to metal contact.
Use stainless machine bolts all the way through the rails. Put a plastic sleeve, such as a piece of vinyl tubing, through the rail where the screw penetrates. Use a neophrene washer on the nut and between the wood and the rail. There is no metal to metal contact and you've only increased the over all dimension by the thickness of the nut. I would assume that with the nut away from the house when the deck is folded it wouldn't interfere.
Mike
Thanks for all of your great suggestions, the suggestion by Florida inspired me to come up with a solution. I will use aluminum machine screws to fasten wooden strips to the sides of the aluminum rails. I can then use wood screws in the usual way.
I found it too hard to get aluminum screws so went with mikes suggestion of stainless steel screws, neoprene washers and plastic sleeves (made from old irrigation tubing)