I have designed a large raised larch deck - which i've already shown Karl a while ago. We've gone to site and the contractor is suggesting a couple of changes. Whilst i'm happy to help if it either saves money for the client or makes the build easier for the contractor (or both) i'm not prepared to compromise the strength or longevity of the deck. With that in mind, would members please comment on the following:

  • 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?
  • 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.
  • The deck frame is treated softwood but the surface and facia are larch from a local sawmill. I left this specification clause in: 

    "TIMBER DECK BOARDS: to be treated to BS EN 460 for Hazard class 3 with a desired
    service life of 15 – 20 years with Ronseal Decking Protector water repellent preservative
    treatment." However, most people say larch requires no treatment. Thoughts please? 

  • 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

Any comments gratefully received. The facia is drawn incorrectly and has been corrected by verbal agreement. I would share the Sketchup drawing but it's too big for the file size limit here.

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Ivan, My comments below.

This is typical of too many "contractors", this attitude makes us a laughing stock when compared to the rest of Europe and North America where a correctly design deck is specified and the contractor simply does exactly what it says on the tin. All except in the UK where cheap cheap cheap rules the contracting world and all they care about is profit by cutting corners.

Your spec should have been adequately priced for and if the contractor thinks he can then do what he likes after he has quoted, well he simply shouldn't get paid unless his changes are presented in writing, agreed and signed for accordingly. Anything less shouldn't be acceptable.

  • 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?

If the contractor wishes to deviate from spec he must provide adequate calculations to indemnify him against any short comings. Rebated posts are a recommendation from TRADA, he can argue his point with them..... The joist spans and beam spans are laid out in the EC5 span tables, perhaps I should expect the contractor to even know what these are...

  • 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.

Tell him to stick to the bloody spec... unless he accepts a clause the he will replace them when they rot for free?

  • The deck frame is treated softwood but the surface and facia are larch from a local sawmill. I left this specification clause in: 

    "TIMBER DECK BOARDS: to be treated to BS EN 460 for Hazard class 3 with a desired
    service life of 15 – 20 years with Ronseal Decking Protector water repellent preservative
    treatment." However, most people say larch requires no treatment. Thoughts please?

SIBERIAN larch doesn't require any preservative as long as it not in the ground, if you want to keep a reasonable colour then I would recommend OWATROL decking oil products. Other Larch, English or other certainly require treating to Class 3

  • 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

Your spec is water tight so to speak, I may mention that the 3.5m and 4.8m lengths are indeed the lengths of timber you have specified rather than the spans. The single beam span for a 150 x 47 joist at C24 rating is 1.93m (this is between posts) and for this dimension of timber the max beam centre is 1.2m (this is between beams). if the contractor uses C16 then the spans are reduced to 1.78m at the 1.2m. He very much needs to use the beams or he will have 3 or 4 times as many posts to support the joists - which is madness in itself.

So unless the contractor can demonstrate an improved or same longevity, the same structural ability In accordance with EC5 to 1.5kN and still keep the costs to what has has quoted then fair enough - however, I bet he can't.

Hmmm. Well I did think that the Heather Specification which i buy from the SGD had probably been properly researched and would give a watertight and best practice specification so I based the spec on that combined with modifying the sub-frame spec I had for a Millboard installation. I did look at some span tables too. since the posts are at 1.2m centres in both directions, the maximum span is never reached, so i'm happy with that.

Thanks very much for your input Karl and for getting back to me so promptly.

Karl, just one quick point - do you know why this table seems to allow greater spans: http://www.dfpni.gov.uk/d.pdf

(page 12) the span allowed for a 47x145 Joist at between 0.25 and 0.5 Kn dead load seems to be 3.06m? scratching my head a bit here!

This has to be with a combination of Dead Load (load of the structure) and the Imposed Load (Usage Load) and then the addition of the weather outside has a contributing factor. 1.5 kN is for domestic use and 4 kN is for commercial or balconies. I am not aware that internal floors are lower - if anything, the load, I would assume, would be more for internal use as there is much more furniture used and thus the imposed dead load would be far greater...

Ivan Tucker said:

Karl, just one quick point - do you know why decks joists and bearers are specified to 1.5Kn, whilst as far as I can see, joists in (e.g.) house floors seem to be specified to much lower loads?

Thanks Karl

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 f** 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.

Clive

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.

Great comments Clive, you answered my query with respect to the 100mm joists, any dimension can be used in this instance although I always refer to the Span tables. I usually stick with the bigger the better and for the cost savings from 150 to 100 dimension joists wouldn't really make a saving worth worrying about due to the increased amount of the beams that would then be required and also posts...

I can't comment upon the TDCA's want for a 3kN Loading, this doesn't match with TRADA, I would rather use the qualified direction via the span tables promulgated by the eminent Dr's of wood science in house at TRADA after all they wrote the regulations to include EC5.

I shall ask Janet from the TDCA for her comments.

Karl

Many thanks Clive. Yes the cut ends are addressed in the spec which is a companion to the drawings - I am using the SGD Heather Specification and it addresses a lot of treatment issues. I may have to have a look at putting all the spec detail on either the drawing or the spec but not both as people don't seem to refer to the spec on site much, which again can be frustrating when a client has paid you to write it.

In this instance the deck adjoins a brick retaining wall so i think a few decent M12 rawbolts into that will help with lateral resistance, although i am considering allowing a couple of the posts to be set into concrete, and possibly some additional X, Y or Z bracing to tighten things up. I think on a raised deck you particularly want to be certain that you've made sure it's as stable and long-lasting as it can be.

There seems to be a fairly common conception amongst a lot of landscapers that architects/engineers over-specify structural elements due to fears of litigation in case of failure, but personally, if there are span tables and regulations that guide us in designing, that have been carefully worked out by engineers, then i think you have a duty to honour that - or as Karl says, say 'you do the math' as the yanks say, to the contractor who wants to alter designs.

Thank you for the responses and comments.  Very useful to focus the mind and find my way around. 

Unfortunately I could not find the TRADA tables online but tables quoted in manufacturers leaflets (Q-Deck) confirm that 4 x 2’s at 600 cts will span 1.75m using C16 timber and 6 x 2’s will span up to 3.2 metres if used at 400 cts.  I was not able to find tables for single beams (bearers) and hence a “back of the envelope” calculation to confirm that 6 x 2’s at 1.5m centres would be sufficient.  Q-deck only recommend the use of twin beams and TDA only provide details for a single (187 x 69) beam.

The conclusion of the above was that 4 x 2 joists with 6 x 2 beams should be capable of carrying the loads without any additional beams or posts.  I agree with Karl that the cost of downsizing the joists will not be substantial  (around £100 ?).  Alternatively, it is possible that the number of supports could be reduced.  Based on the TDA guidance (using the higher loading and double beams) I reckon 10 supports would be able to support the main deck area (excluding the stairs and any intermediate supports that may be required due to the curved front).

Firstly, related to Ivan’s query, is that an experienced contactor worth his salt would be aware of this and would have suggested a cheaper alternative when he tendered – as opposed to querying notching and eliminating the shoes after submitting a tender !.

Secondly, and less related to Ivan’s query, is the use of tables.  Tables (by their nature) take a simplified approach and consider the worst case loading / design scenario which leads to conservative designs. 

Designers have a responsibility to understand what they are doing and prepare a safe design.  Use of “approved” tables simplifies the design and minimises design costs but consequently member sections tend to be overestimated because they have to consider all design scenarios.  There is a balance of using less economic sections against the additional cost of undertaking structural calculations / employing a structural engineer to validate the design.

Whilst I cant comment on the TRADA tables, those that I was able to view only provided limited information and were not particularly appropriate for use as a design tool.  They tend to concentrate on design solutions with maximum spans as opposed to providing a range of section sizes for different spans / loading scenarios / site constraints etc.  I assume the use of a 1200 post grid in Ivan’s design is to coordinate with the 400 joist spacing for the decking boards.  If this is a common design scenario then tables that provide section sizes for 1200, 1500, and 1800 module designs, in addition to the maximum spans, may provide more flexibility for designers to select more efficient designs.

Ideally, tables should be prepared to meet the needs of the users as opposed to those preparing the Codes whose primary aim is to ensure safety is not compromised.  Unfortunately, the codes are more likely to be influenced by the interests of the larger organisations in the industry (who tend to be represented on the committees responsible for drafting the codes) as opposed to the smaller practises that (I suspect) undertake the majority of the work in the industry.

Perhaps the above is food for thought and apologies if I have wandered away from Ivan’s query and will be pleased to hear TRADA’s feedback  regarding the higher loading now being put forward by TDA.



Karl Harrison said:

Great comments Clive, you answered my query with respect to the 100mm joists, any dimension can be used in this instance although I always refer to the Span tables. I usually stick with the bigger the better and for the cost savings from 150 to 100 dimension joists wouldn't really make a saving worth worrying about due to the increased amount of the beams that would then be required and also posts...

I can't comment upon the TDCA's want for a 3kN Loading, this doesn't match with TRADA, I would rather use the qualified direction via the span tables promulgated by the eminent Dr's of wood science in house at TRADA after all they wrote the regulations to include EC5.

I shall ask Janet from the TDCA for her comments.

Karl

unfortunately I could not find the TRADA tables online but tables quoted in manufacturers leaflets (Q-Deck) confirm that 4 x 2’s at 600 cts will span 1.75m using C16 timber and 6 x 2’s will span up to 3.2 metres if used at 400 cts.  I was not able to find tables for single beams (bearers) and hence a “back of the envelope” calculation to confirm that 6 x 2’s at 1.5m centres would be sufficient.  Q-deck only recommend the use of twin beams and TDA only provide details for a single (187 x 69) beam.

Trada tables are available direct from them, http://www.trada.co.uk/

Firstly, related to Ivan’s query, is that an experienced contactor worth his salt would be aware of this and would have suggested a cheaper alternative when he tendered – as opposed to querying notching and eliminating the shoes after submitting a tender !.

I would be careful with “experienced Contractor” statements; if you are referring to experienced decking companies then I couldn’t agree more. If, however, you refer to Carpenters, Landscapers, Builders and Contractors then I would suggest this is not the case. I have seen hundreds of projects (if not much more) where money is so much of a driving factor that just about every corner is cut and no one seems to care until it’s too late.  I have yet to meet a contractor that has even heard about the span tables and in a high number of cases simply apply what knowledge they think is right at the time… I don’t argue anymore I simply walk away.

Secondly, and less related to Ivan’s query, is the use of tables.  Tables (by their nature) take a simplified approach and consider the worst case loading / design scenario which leads to conservative designs.

I agree, Span Tables save time and the result of using them is fit for purpose without over engineering a project.

Designers have a responsibility to understand what they are doing and prepare a safe design.  Use of “approved” tables simplifies the design and minimises design costs but consequently member sections tend to be overestimated because they have to consider all design scenarios.  There is a balance of using less economic sections against the additional cost of undertaking structural calculations / employing a structural engineer to validate the design.

I think this is best for a separate discussion maybe “where are the boundaries for Decking Design”. What are the limitations of a garden designer, should they specify the job of a structural engineer or have to understand span tables at all? One could argue that as there are no official codes for deck construction so is there two levels of design, one being a beautiful garden design that has deck as part of the scheme and then, two where the structure is drawn out in accordance with…

 

Whilst I cant comment on the TRADA tables, those that I was able to view only provided limited information and were not particularly appropriate for use as a design tool.  They tend to concentrate on design solutions with maximum spans as opposed to providing a range of section sizes for different spans / loading scenarios / site constraints etc.  I assume the use of a 1200 post grid in Ivan’s design is to coordinate with the 400 joist spacing for the decking boards.  If this is a common design scenario then tables that provide section sizes for 1200, 1500, and 1800 module designs, in addition to the maximum spans, may provide more flexibility for designers to select more efficient designs.

The reason for limited open information is that this field of structural design is far more complicated than at first glance, of course there are basic elements to decking structures and they are common knowledge, if you look at the codes under EC5 for timber structures they will blow your socks off…

 

Ideally, tables should be prepared to meet the needs of the users as opposed to those preparing the Codes whose primary aim is to ensure safety is not compromised.  Unfortunately, the codes are more likely to be influenced by the interests of the larger organisations in the industry (who tend to be represented on the committees responsible for drafting the codes) as opposed to the smaller practises that (I suspect) undertake the majority of the work in the industry.

As far as I am aware they are. If you want to get deeper into timber structure and its design, join TRADA. Timber structure design has been over looked by too many for too long, now it’s a specialist field and at Napier in Edinburgh you can study a masters in Wood siences where the entrance level is a post grad in structural engineering…

Perhaps the above is food for thought and apologies if I have wandered away from Ivan’s query and will be pleased to hear TRADA’s feedback  regarding the higher loading now being put forward by TDA.

There is always more than meets the eye in the world of timber.

Good discussion points. Personally I found UK contractors so bad I ended up building my own deck. Price was not an issue, 

With the UK weather I don't know why anyone would want to concrete posts directly into the ground, except of course to save money, which it doesn't in the end. Contractors are long gone by the time problems start showing up.

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