How to replace bottom of deck post?

[ScubaAl]

I have an existing deck supported on 90mmx90mm posts that go into stirrups in concrete. Bottom 100m of post has rotted and stirrup rusted out due to build up of mud/water. I will support deck(!) and then remove concrete pad and stirrup to replace with new. Question - can I joint a short piece (say 400mm) of new 90x90 into the bottom of existing post (which is in great condition apart from the bottom)? If so, what joint should I use? I was thinking of a lap joint + Gal plates + coach bolts?

It will be much easier than dismantling the entire deck

[MitchellMc]

Welcome to the Bunnings Workshop community @ScubaAl. It's sensational to have you join us, and many thanks for your question about deck posts.

Could you just cut the post off and use a longer stirrup?

Any reason why you can't replace the whole post? How's it connected to the joist/bearer? You're not going to be able to buy just a 400mm length of 90 x90mm post and will probably have to get at least a 1.8m piece. If you can replace the lot, then I'd recommend doing so.

If you want to join the sections, then using a lap joint and gal plates sounds like a good idea. I'd prefer you to use Gal bolts right through the lap-jointed area instead of coach screws.

Please let me know if you have any questions.

Mitchell

[ScubaAl]

Thanks for the very quick reply

The main issue is that this deck post is 3.5m high - going up through the deck, lap jointed into into the bearer, supporting the handrail and out to a neat finial. So to replace the entire post I'd basically have to dismantle the entire deck, lifting all the boards, taking off the handrails so I could then unbolt the bearer, knock them out of the lap joints into the upright and put it all back together. All because 100m of post has rotted at the bottom. I know I'll have to buy more post than I need but thats trivial compared to dismantling an entire deck and rebuilding it. The underlying question is whether there is anything against using a lap joint to join the bottom of the support post - given that it already has lap joints where the bearer is cut into it anyway?

[MitchellMc]

Gotcha, @ScubaAl. I can see why you're trying to join it now.

So, is it just 100mm to good solid timber? Can you remove the current stirrup and exchange it for one 100mm longer?

Mitchell

[ScubaAl]

Hmm. I agree that would be a great solution. The problem is that bottoms of the post is already off the ground on a stirrup - so the good timber is 400mm up from the pad/ground. And I think that the max gap between the bottom of a stirrup and the concrete is 300mm? So although you can get 600mm stirrups they are still supposed to be used with a max 300mm gap - at least thats what I read on the Pryda website. Makes me wonder whats the point of a 600mm stirrup - you have to go 300mm DOWN into the concrete? I'd love to be told I've got that wrong and can just get away with a new stirrup

[EricL]

Hi @ScubaAl

I've seen some lap joint repairs on deck posts and I reckon it can be applied to your situation. However, it always goes towards council compliance. The repair job you're about to undertake could possibly fall into the territory of work signed off by a builder. I suggest asking your local council about the rules of structural deck repair. In this manner, you can be secure in the knowledge that you checked with the council before you carried out the repairs.

The advantages to using a lap joint are that you get a very strong vertical and lateral joint, it promotes long grain to long grain connection and will resist twisting. This also serves as a good opportunity to re-assess the drainage situation in that area. Perhaps you can dig out a channel to re-direct the water or perhaps place a drain coil to carry the water away. 

Please keep us updated with your progress, we look forward to seeing your deck post repaired.

If you need further assistance, please let us know.

Eric

[ScubaAl]

Update - for anyone else with the same issue. In the end I cut the rotten 100mm off the bottom of the post. Fortunately the rest of the post was solid. I then installed a longer stirrup down into a concrete pad. New pad was bonded to old concrete beneath using 12mm reo dowels chemset into the old concrete and using boncrete on the old surface and as admix. I also took the time to reprofile a drainage channel into the top of the new pad away from the stirrup - so water doesnt sit there and cause corrosion in the future.

The new footing is now bomb-proof and by far the strongest part of the house! Best of all I have introduced no additional timber joints, with potential weakness, and so the deck design remains the same. Highly recommend the Ramset 502 Chemcrete stuff. Really easy to use if you spring 70 bucks for the proper injection gun from Bunnings. Just make sure you clean the dust out of the holes thoroughly.

[EricL]

Hi @ScubaAl

That is so good to hear. I'm glad you've found a solution to your decking post. Would it be possible for you to post a photo of the repair? I'm sure our members who are in a similar situation would appreciate the information. I hope you find a way to divert the water so that you won't get any more flooding in that area. We look forward to seeing the photos. 

Eric