Timber for decking - issues with bowing.

[Craig Farrall]

Hi All,

I must caveat this by saying that I am a novice when it comes to DIY and this is quite a big project that I am figuring as I go along.

I am building a pergola / decking area currently and I have done the majority of this from the posts, the decking and most of the cross beams. I have noticed though, that the board is currently bowing considerably, which is throwing off the straightness, which is going to cause issues when I come to lay the actual deck boards.

I have added a couple of pictures to illustrate this - you will see that the middle of the large board (4.8m) bows out. On one of the pictures you can see what this looks like with another beam just placed on top, to see this in full effect.

I am not entirely sure what is causing this, whether it is the timber itself, or the cross beams that are attached to it (even though they aren't too tight which makes me think it's not that. All of the smaller bits of timber that are sandwiching the two boards together in the picture are identical in size, so definitely won't be caused by that.

The cross beams are currently nailed in, so ideally don't want to be pulling all of those out - however if this is what is needed to fix, I can do this.

I obviously have an issue, that may well be caused by my lack of knowledge on this, but I am wondering - what can I do to fix this?

Just another small caveat, as seen in the pictures I've not done all of the cross beams and the decking will be raised from the ground at a later point.

Appreciate any help / advice you can provide here.

 

[foxhole]

Nothing I can see prevents the timber bowing, should have been constructed by fixing to the posts first, they can’t move so neither can anything fixed to them.You will struggle to lift a large structure and fix it and keep it square and straight.
Difficult to suggest anything without knowing your design plan? Certainly wouldn’t use any nails , screws allow removal and adjustments .Did you check timber for straight before use?

 

[Craig Farrall]

Nothing I can see prevents the timber bowing, should have been constructed by fixing to the posts first, they can’t move so neither can anything fixed to them.You will struggle to lift a large structure and fix it and keep it square and straight.
Difficult to suggest anything without knowing your design plan? Certainly wouldn’t use any nails , screws allow removal and adjustments .Did you check timber for straight before use?

Thanks for the response! So it looks like the approach I have taken isn't the best method. I assumed that with the help of someone that once this was semi-constructed it could be lifted / add blocks underneath to raise the amount needed and then fix to the posts etc.

The nails were used to attached the joist hangers to the cross beams - which from research was the best way to go. I can't remember exactly if that piece was checked before hand, but I think I have used any bow pieces to bow inwards at the two ends and not the middle, if that makes sense - maybe I should have flipped it the other way, so the attachment of other beams could help with that, I don't know - as I said, I am definitely a novice at this.

I have a similar issue with the opposite end too, but not as defined as this one.

Any suggestions on what I could do next?

 

[JohnD]

what size/type nails did you use?

 

[^woody^]

If all the joists are the same size and fixed as they are with hangers, then the end timber can't bow.

Run a string line across those joist ends and check them

 

[foxhole]

If all the joists are the same size and fixed as they are with hangers, then the end timber can't bow.

Run a string line across those joist ends and check them

It can if the joints are not all tight and fixed securely there is clear daylight between some of the timbers and the end bowed timber.The other end may be bowed , it's a floating frame at the moment, nothing to keep it square.

 

[^woody^]

It can if the joints are not all tight and fixed securely there is clear daylight between some of the timbers and the end bowed timber.The other end may be bowed , it's a floating frame at the moment, nothing to keep it square.

The timber will remain in the same position as fixed with those hangers. It won't move afterwards.

 

[foxhole]

The timber will remain in the same position as fixed with those hangers. It won't move afterwards.

It clearly has.

 

[^woody^]

It clearly has.

What, stayed in the same position as fixed?

 

[foxhole]

What, stayed in the same position as fixed?

Moved

 

[^woody^]

Impossible.

Has the metal hanger stretched somehow?

 

[foxhole]

Impossible.

Has the metal hanger stretched somehow?

Doesn’t need to stretch, a timber frame can move unless it’s fixed and braced to something immovable .

 

[^woody^]

Doesn’t need to stretch, a timber frame can move unless it’s fixed and braced to something immovable .

A frame as a whole may move if not secure, but individual timbers making up the frame can't once fixed

You can see a gap between the joist and the bent timber. That has not bent after fixing because it physically can't have if the hanger was securing it.

 

[foxhole]

Joists still warp and twist without noggins even though they are firmly fixed both ends.

 

[^woody^]

Joists still warp and twist without noggins even though they are firmly fixed both ends.

I don't know if you are seeing the same photo as me.

Its not the joist that as twisted but the timber fixed to the end of the joists. Nothing to do with joists or lack of noggins

There is no way that that end timber can twist, bend, deflect or move once it is fixed to those joists with those hangers. No. Way