Karl Harrison Landscapes Ltd
Forum Replies Created
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Ivan, you are always welcome to post anytime, no matter how often.
It is easy for suppliers to offer advice on gapping and you can read as many books as you dare but I recon in reality for the absolute best gap is determined by a combination of the various texts out there and experience by decking installers.
Advice is usually best by TRADA but I do disagree on anything over 6mm, so there 10mm is a little excessive, even though for certain attributes of deck construction.
I do agree with your statement “But surely the most relevant factor is at what time of the year are the boards being laid, and are they kiln-dried timber, or seasoned, or unseasoned.†And of course your choice will be enhanced by using a moisture content metre. Again this detailed analysis is only useful on a surface fixed deck and not good what so ever on a side fixed deck as with the latter the gap is determined by the clip. Some clips allow movement for this to a certain extent but not all.
E.g., i’m specifying English larch from a saw mill less then 10 miles from the site in question. The boards will be reasonably freshly sawn, and it’s fair to assume, stored outside in the English autumn. So, it’s highly likely that when laid, the boards will be at their most swollen, and will likely contract in the summer. Presumably, a 4mm gap with wet, fresh sawn boards, could open up to as much as 8mm in the summer sun, and a kiln-dried board laid in August with a 4mm gap could possibly see those gaps close up entirely in a long wet winter. I’m using 145x32mm boards. Any thoughts?
May I ask why you have specified a timber not usually used for decking? One can’t be sure of its “best use†and I do know that this timber is rather soft – to the extent similar to pine and shouldn’t be confused with Siberian Larch which is more than double the hardness and completely different animal altogether. I would be certain this would be a lively product and I would recommend tight joist centres and also massive screws 5.5 x 75 mm (minimum)
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 Designâ€. 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…
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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…
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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.
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
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?
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:Â
“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
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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.
That looks a great job, stylish and neat… I can see the boards lifting, what a shame… be great to see what advice the manufacturer does to help
Phillip Wigg said:
The whole deck is already “picture framed” so unfortunately not an option
I’d like to see an image… first thoughts, would it be possible to rip down a deck board width around the perimeter and lift the off cuts – this would then have full length deck board laid in the gap…?
Great faceted Coping with the use of the L-shape profiled Ipe… A difficult job executed well.
#nicework #exterpark #hardwooddecking #DeckingNetwork
Cupping is due mainly to 2 factors.
1. The way the board has been cut from the log, tangentially or flat sawn (99% of decking)Â is where you will see the most movement and 1/4 cut has almost no movement. The cost of 1/4 creates lots of waste and is very, very expensive.
2. The equilibrium of moisture on the upper side of the board should be the same as the underside. If you have a wet underneath and a dry top as the moisture transpirates out of the timber it will cause movement in that side, so if the outer side has no movement then the board will cup.
Great, how much are the “cheapy” screws from B&Q, are these the green coloured screws. I guess they would be ok as such for soft wood deck but never on a hardwood.
I use the Fastenmaster 50mm at 4.74 pence per screw or 63mm exterior wood screw CMDS-63-500 at 3.93 pence per screw, these are the super cheap pro screws on the market
Like this Mark, drill the head off, seem pretty simple and a good idea. Have you ever used any of the screw extraction kits available, these work for the larger units but struggle with the small screws (decking) and the high torque required to overcome the initial friction or perhaps the “Easy out” for example…
You could consider manufacturing yourself, purchase local smooth timber, route out the desired channel and fix in Magma anti slip strips.
You may find its worth using the product from Russwoods or consider an alternative product.
What would the delivery costs be in any case?
I have dabbled but without any real success.
I see that some products are only warranted in the tin because:
- Did the tin get correctly agitated prior to application
- How thick was the application
- What was the temperature upon application
- What was the moisture content of the timber
- What is the intended use, amount of foot traffic
- What is the weather like post application
All of these attributes and many other will contribute to the function of the anti slip capability of the product…
In short, unless otherwise convinced I don’t use it.
