Undercoating a deck

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Forum topic by baby86bear posted 05-25-2012 01:59 PM 2068 views 0 times favorited 6 replies Add to Favorites Watch
View baby86bear's profile


15 posts in 2327 days

05-25-2012 01:59 PM

Topic tags/keywords: question

Good Morning fellow Jocks,

I am about to build a deck and i want to know what is the best thing i can spray to the galvanized holders to prevent rust and to spray the under side of the decking joist. I will be using pressure treated lumber but, just want to be sure all is good. Some of the joist will be resting on the ground..

thanks for all and any help…. plus…. working on a house that i am gutting and redoing… will post pictures.

6 replies so far

View ajosephg's profile


1880 posts in 3554 days

#1 posted 05-25-2012 02:03 PM

I don’t think coating the under side brings anything to the party except extra expense. However – you should not allow any wood to contact soil – put concrete footings or blocks beneath.

-- Joe

View RussellAP's profile


3104 posts in 2280 days

#2 posted 05-25-2012 02:09 PM

You shouldn’t have to worry about the joist hangers, and don’t treat the pressure treated wood for at least a year. I wouldn’t put any joist on the ground, rather dig it out and put in some concrete blocks or piers and line them with a product that is a blue plastic that goes between the wood and the concrete. Can’t recall the name of the plastic, but it’s 4” wide and comes in a roll.

-- A positive attitude will take you much further than positive thinking ever will.

View dhazelton's profile


2766 posts in 2290 days

#3 posted 05-25-2012 02:12 PM

I made a small deck/landing that rests directly on the ground as it was only a 9 inch step up to my door. I dug things out and put in a couple inches of gravel, so it’s at least sitting on something with good drainage. If you do that then you shouldn’t need to do anything to joists or hangers, and make sure you use the correct galvinized hanger nails (or screws if you wish).

View Blackbear's profile


137 posts in 2212 days

#4 posted 05-25-2012 02:40 PM

What Russell was suggesting sounds a bit like sill seal? It stops wood from wicking moisture from the concrete.

Baby86bear, I live in a climate with a fair amout of rain and snow. My deck is presure treated and showing no real signs of water damage after 5 years. The biggest thing is some slight mildew in a few shady spots. I am about to use a product to scrub the deck and then seal it with a waterseal.

View RussellAP's profile


3104 posts in 2280 days

#5 posted 05-25-2012 02:51 PM

Another thing I picked up watching Holmes on Homes about joist hangers is to first screw the boards in place so they are exactly where you want them, then nail or screw the joist hanger under them tight. If you put the joist hanger up first the boards will be helter skelter and an uneven surface is guaranteed.

-- A positive attitude will take you much further than positive thinking ever will.

View a1Jim's profile


117085 posts in 3570 days

#6 posted 05-25-2012 03:34 PM

I usually build 10-20 decks a year .Most Pressure treated wood is rated for 40 years, the the newest type of Simson Joist hangers the ones that have a “Z” in there number are made for the new type of PT wood and should hold up for years also. Some PT wood is rated for ground contact and some is not depending on the area your in. In my area it is code to put a 6mil vapor barrier(black plastic) on the ground to help keep moister off the bottom deck. I pre-install my joist hangers using a small piece of the joist that are going in them as a gauge to get the hangers set at the correct height. Be careful what kind of deck material you use, for the deck surface never use a doug fir or non outdoor rated woods even if you stain both sides first because after a couple years your finish will not protect the bottom of the decking material. Another mistake home owners make building decks is when they use composite decking the place the joist to far apart ,most composite manufacturer recommend 16”-12” on center. If you feel better painting the Joist hangers before you install them it won’t hurt a thing and my prolong their life but is not necessarily in my opinion.

-- wood crafting & woodworking classes

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