Thread: digging post/stirrup holes for deck

gday - it depends on a number of factors... so bear with me!

without knowing specifics of your design, some things for you to consider...
- stirrups (a u-shaped piece of flat steel - i think they're called cyclone stirrups) as opposed to the posts (with a steel tube 'welded' to the post's sole-plate) will be stronger
- if you cast your stirrups into the ground, aim to get a minimum of 50mm covering concrete around the steel; likewise if you dynabolt or ramset your stirrups to the top of your footing
- the more posts you have the smaller your footings can be because the more the downward forces are spread over footings, however
- the more lateral forces you have acting on the tops of your posts, the deeper your footings will need to be (or you need to brace your posts)
- if you decide to go deep then you don't necessarily need to have your stirrups going to the base of your footings; you could use reinforcement bent into the u-shape like the stirrup then wire the ends of the re-bar to the sides of the stirrup, once this is cast into the concrete it will be almost as strong because the concrete will lock it in eddy.

our deck (currently in the timber procurement phase) is 30sq.m - 3m x 10m long - the 3m joists come out from the house & are picked up by a bearer resting on 4 posts, roughly 500 above the ground. the base of the footings are down 600, with a 200 deep bed of concrete placed in the bottom. each 100x100 hardwood post is then placed onto the cured concrete pad & backfilled. there is no roof but the posts will support a beer-rail.


if our posts were 1600 above the footings then they must be dug in 800 deep, but the concrete would not change.

the main goal of the stirrup is to tie the post to the footing (as well as provide clearance between the bottom of the post & the ground) - think of the posts as cantilevers and the depth of the footing as the back-span (hold a pencil in your fist to help you envisage).

so on a deck the lateral forces applied to the tops of the posts are generally less than the downward ones, but do exist; they increase the higher your posts are, or if the deck has an open area underneath that has at least one wall exposed to the wind, or if your deck has a roof. the longer your posts are the more these forces are amplified (short pencil vs long pencil).

Hi brynk,

I'm planing a deck of 6.6mx3.2m, it has a very low clearance of 150mm to 250mm, so there is not enough space for bearers. I'm thinking to go straight to mount joists to stirrups and then put decking boards straight onto these joists. Is it all right to do so?

I've been searching this great forum for advice and ideas but haven't found much about setting up the footings in this way.

If I setup the joists from my house out, how do I make sure they're level? What I learnt from this forum is I can cut the tops off if they're wooden posts, so I don't have to worry too much when I plant the posts. Shall I better go this way, use wooden posts instead of stirrups? Then use joist hangers to mount joists to posts?

Even with the later idea, what sort of saw I'd better use to make sure I cut the post tops off at water level when they're already in the ground?

Many thanks,

Ray

gday. You could put stumps in ground and use 140 x 45mm as bearers bolted tot he sides of stumps and use the same size timber as joists mounted 'internally' of the bearers usinh joist hangers.This way all bearers and joists are the same hight and with decking will have total hight of 160mm. Not ideal clearance but will work. i prosume it will be 3.2 out from the house. you will need three rows of posts if using 140 x 45 or a ledger fixed to house and two more rows of posts. To get them level use a straight edge (something straight and long) and sit one end at house where you want the deck to finish and have the other end over the last (or middle) stump hole. use spirit level on straight edge to make sure it is level and measure fron straight edge to bottom of hole (or top of sole plate). Remember to subtract the hight of the decking ie.20mm and cut stumps to this lengh. Put the whole frame up and check level then rapid set concrete the stumps in last. Then all you need is to put the decking on.

Dan, 600 mm is a farily common depth. Depends on the overall size of the deck. The diameter of the hole depends on the area each post is supporting. THe depth is mainly to put the pressure down on compact soil. There would be no need to go any deeper. If you are using stirrups the depth of the hole provides little resistance against lateral forces, as the joint between the stirrup and post acts like a hinge.

My last deck had 3 500w x 500deep post holes to support 3 steel posts. (engineer specs) These were 2.5 metres high under the deck and held a span of 6 metres. 40sqm deck off the side of the house.

Ray for your question just using joists is not a good idea. some of eth posts may settle and the corresponding joist will sink, while the others stay at the same level. If a post settels under a bearer than that movement is spread over the bearer and all the joists connected, making it less noticeable.

Joists don't have to sit on bearers, but can be attached to the side with joist hangers. That way the tops of the bearer and joists are the same

Yes, it is 3.2m out from the house, now I know the secret to setup the stumps!! I’ll use 3 rows, 5 stumps in each row, so the total of 15 stumps, each of them are 1500mm apart.

But mako, I might don’t understand what you’re saying of ‘mount joists internally’ – do you mean I need to cut the joists to 1500mm long each to fit them inside between every two rows of bearers?

<OI will use 90x19 Merbau as decking board, is it ok to set joists every 500mm? In this way, I’ll need probably 13 lines of joists, say 26 pieces of joists all together?
<O</O
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By the way, what sort of tools would be ideal to dig these 15 stump holes? I found a manual ‘cyclone hole digger’ in Bunnings, it costs about $55, would it be ok? Do I have to dig 600mm deep for the 5 holes close to the wall, where the stumps will be only 150mm above the ground?

Cheers,
Ray

I found the answer from another thread, yes, I have to cut the joists

Please still answer the rest of my questions about the hole digging tool.

Thanks,
Ray

if using the 140 x 45 you just nail joist hangers along the bearer and fix in the 140 x 45 joists into the so basically the top of your joists are level with your bearers. So from your house you will have a row of posts spaced at 1500mm with a bearer of 140 x 45 bolted to it. Then you will have another bearer out 1600mm from the 1st one. Then your last (outer) bearer another 1600mm out. this will give you the 3.2m from house. then mark long each bearer 450mm and nail a joist hanger at each mark. Then cut 140 x 45 joist to fit between each bearer, taking into acount the thickness of the bearers to get you 3.2m off house. and also make sure you can get the 1st bearer right up against the house or you will have a gap between your deck and house. If you cant you will have to mount the 1st beaeer to the house directly. The reason i said you can span your joist 1600mm is if you are using the 140 x 45mm method. This size timber can span 2.6m when used for joists. the 1500mm max span quoted earlier was for using 90 x 90 (or 2 90x45's) bearers and 90 x 45 joists. This meathod would siut also if you cant get your 1st row of stumps right up to the house because you can overhang the joist (because they sit on top of bearers) up to 300mm. As for digging the holes it depends on the ground. if it is hard clay you can either put the hard yards in by hand or hire a post hole digger. If you do get the canterlevered on so you dont go for a ride if it grabs. They are about $80 per day i think from kennards or less for 4 hours. If it is rocky a crowbar and shovel is best. As for the depth with a deck this hight (not very high) you would only need to go down about 300-400mm into CLAY. If you have 200mm of fill on top of natural clay you will need to dig 500-600 from surface. use a sole plate and concrete. Also you could span 500mm with 90mm merbau but i would keep it at 450 to reduce bounce. It will make a differance.

Originally Posted by remonyan

I found the answer from another thread, yes, I have to cut the joists

Please still answer the rest of my questions about the hole digging tool.

Thanks,
Ray

If by cyclone post hole digger you mean one like this:



TRUST me, your MUCH better of with one of these 'scissor shovels':



Also a pinch bar would help should you encounter rock or hard clay.

Or you can take the easiest approach and do as mako suggested and hire a petrol powered post hole digger.

Hire shops only charge around $70 for half day use of a petrol powered post hole digger. Should be able to dig a hole within 5 to 10 mintues or so, not a bad option.

I used a petrol powered 2 person hole driller for my last deck and i can tell you its not for the faint hearted. After 25 holes our bodies were aching. You need to be very carefull with them, dont get the missus to help as she wont be able to handle it, especially if its clay and/or rocky.
Im using the conventional shovel, crowbar, pick and wheelbarrow this time.
But as williamstown said its only 5-10 minutes a hole.

if doing it yourself hire the canterlevered petrol powered post hole digger. It has a set of wheels and the augerat one end with handles and the motor at the other end. if it jams it will just stop the auger and not spin the operator. Ive used them hundreds of times without any problems. The only problem is they can be a little gutless in hard ground. If it is a large deck with lots of holes and you need to go deep hire a dimgo for half a day. Great fun to use too.

I might have to leave a 20mm gap between the brick wall and the first bearer, the reason being if I bolt the first bearer to the wall, due to the limit height clearance I have on this end – only 150mm, it will block several air flow windows the builder built in the wall, these small windows are below the floor level, I guess this is designated to let air flow through underneath the floor? In this situation, is 20mm gap big enough? I hope so.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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The post hole digger I looked at Bunnings was the red color ‘scissor shovels’ that Rough Sawn attached here. I also saw the green color real ‘cyclone’ but I didn’t like it. Thanks for clarify it, Rough Sawn. Mako’s ‘canterlevered petrol powered post hole digger

’ sounds interesting, I’ll go to hire shop this Saturday to have a look at it, my concern is like what happened to Dan, it’s probably a bit hard to handle by a newbie like me

Ok Guys, I have dug 15 of my 40 or so holes and have hit water at about 900mm on two of them!! I am on a canal so ground is about 2-3 metres above the tide mark, We had a bit of rain a few weeks ago so Im thinking the water has built up from this. the holes are close to my retaining wall (pics in this link http://www.woodworkforums.ubeaut.com...ad.php?t=56662 )
is this going to cause me any issues? (its all clay if that matters)

remonyan, 20mm clearance for air flow should be fine. But you dont want a 20mm gap between your house and 1st deck board. if you are puttin joists on top of bearers run the joists right to the house. If joists not on top you may have to overhang your 1st deck board over bearer but no more than about 10-15mm. Thsi would leave a gap of around 5mm between deck and house... not ideal visually but should look ok. Also dont worry about using the post hole digger i suggested. very easy and safe. They look a bit intimidating but trust me they are a peice of p***.