Reply To: Revisions to deck post-tender

Holman’s Hanymen

Hello Ivan

I’m a bit of a novice with decks but a nice design.

I am just in the process of researching details for decks to cost out some ideas for a customer and the issues are very relevant.  Unfortunately I don’t have the guides and codes available so am trying to put together details from scratch so that I can also apply to other jobs if they arise.  Whilst I am a jack of all trades (and consider myself to be very practical) I also have an engineering background.

I generally support the points Karl has made and if the contractor wishes to change the design he should provide justification (structural and durability) and any cost saving of his alternative design.  Any changes can then be considered against the cost savings with your client.  

    •  I spec’d the deck with 100x100mm posts at maximum 1.2m centres, with notched tops to posts. Contractor wants to do without notched tops and just use 2 No. bolts to screw bearers to side of upright posts – any thoughts?

    Notches provide an ideal way for the weight of the joists to bear directly on the support without the load being transferred through the fixings.  Notches are a fag to cut but the joists could be supported on a flat topped post using timber plate connectors.  If bolts are going to be used I would use toothed timber connectors – possible belt & braces but ensures mechanical resistance.  Also, if bolts are used (and not recessed) they will obstruct any vertical boards applied as skirtings.

    •  I specified galvanised decking post shoes at the base of each post rather than set the posts into concrete by upto 600mm. This was partly with the view that if the deck ever needs replacing, the same foundations could be used, rather than having to re-dig. Contractor wants to set posts into foundations and avoid using galvanised shoes.

    Fixing posts into the ground provides lateral stability but maintenance will be more difficult in the future.  If wood is in contact with the ground ensure it is treated to Class 4 to provide a 15 year life.  Use of shoes is a good sustainable feature that is likely to work in practise but for good measure you could add a couple of concrete posts to provide the lateral stability of the deck if it is not tied back to a building

    • I spec’d deck with both joists at 400mm centres and bearers beneath the joists at 1.2m centres, the contractor says he never uses bearers beneath joists, I want to leave them in. All are specified as 150x50mm and the maximum spans are 3.5m for joists and 4.8m for bearers

           Should provide a very solid deck.  If bearers are used as you have shown I reckon the joists could be reduced to 4 x 2 – say the bearers notched  / supported on the posts and the 4 x 2 laid on the bearers.  A potential saving for the client !.  (I reckon a 1.5m grid with 6 x 2 bearers and 4 x 2 joists at up to 600 centres will sustain the residential loading of 1.5Kn/m2 with C16 stress graded timber – not sure whether Karl would agree.  However, design is choice and one doesn’t have to adopt the minimum in design!.

Lastly, it is good practice for the cuts on preserved timber to be treated with preservative – there may be a note on your drgs (or on a separate spec?) but I didn’t notice it.

Hope the points are helpful.


PS  I was intending to post the above on Friday but was thwarted by technical difficulties.  Having undertaken some more research the notching of posts seems to be well covered by TRADA and TDA but I am sure a robust detail could be developed using timber plate connectors to avoid notching.  I also note that the TDA recommends a design load for decks of 3KN/m2 which is different to the loadings currently used for buildings in the UK (residential 1.5, office 2.5 and commercial 4.0).  I doubt whether the smaller joists I outlined above would meet the 3KN/m2 loading but does anybody know the background to the 3KN/mm2 loading adopted by TDA.