Plywood standards - EN 636

[IanT]

For anyone who needs to know the 'quality' of plywood they are using for baseboard construction - the technical information provided below should prove useful.

In general terms, plywood for structural uses is typically produced from softwood species such as Douglas fir, Spruce, Pine, Fir etc and hardwoods such as Birch. Plywood for decorative use utilises hardwood species from temperate and tropical sources. Various coatings or treatments can be applied to plywood to enhance its physical characteristics and performance, including durability, fire resistance, abrasion resistance etc.

For many years, there have been relatively few changes to the terminology applied to plywood, however, there have recently been some fundamental changes which specifiers and users of plywood products should be aware.

In terms of the use of plywood products in the construction sector, the harmonised standard for wood based panels, EN 13986, requires that plywood must comply with one of the three performance classes within EN636 and suppliers must be able to provide evidence to substantiate such claims for their products.

The key changes are;
• If you have previously specified or purchased WBP (weather and boil-proof) or Exterior quality plywood for construction purposes, you should now specify plywood to comply with EN636, Class 3.
• If you have previously purchased Interior or MR (moisture resistant) plywood for construction purposes, you should now specify plywood to comply with EN 636, Class 1.
• A new intermediate category has been introduced, EN 636 Class 2, which is suitable for construction uses in protected exterior or humid service environments.

The requirements for each Class can be summarised as follows. (Note: This summary is provided for information only, reference should be made to the Standard for further and more detailed information).

Plywood for use in dry conditions: EN 636-1: Plywood to be used in conditions characterised by a moisture content in the material corresponding to a temperature of 20oC and a relative humidity of the surrounding air only exceeding 65% for a few weeks per year. These conditions correspond with Service Class 1 according to EN 1995-1. Boards of this type are suitable for use in Biological Hazard Class 1 of EN 335-3. End uses include warm roofs, intermediate floors, timber frame internal and party walls.

Plywood for use in humid conditions: EN 636-2: Plywood to be used in conditions characterised by a moisture content in the material corresponding to a temperature of 20oC and a relative humidity of the surrounding air only exceeding 85% for a few weeks per year. These conditions correspond with Service Class 2 according to EN 1995-1. Boards of this type are suitable for use in Biological Hazard Class 1 and 2 of EN 335-3. Note: Plywood of this type is appropriate for protected external applications, but is also capable of resisting exposure to weather for short periods, i.e. when exposed during construction. It is also suitable for interior applications where the service moisture condition is raised above the humidity level of dry conditions. End uses include cold roofs, ground floors and timber frame external walls.

Plywood for use in exterior conditions: EN 636-3: Plywood to be used in climatic conditions leading to higher moisture contents than in service Class 2. These conditions correspond with Service Class 3 according to EN 1995-1-1. Boards of this type are suitable for use in Biological Hazard Classes 1, 2 and 3 of EN 335-3. Note: Plywood of this type is capable of withstanding exposure to weathering conditions and liquid water, or water vapour in a damp, but ventilated location, under consideration of Section 8.2 of EN 636. End uses include fully exposed service conditions.

There have also been changes to the bond quality classifications of plywood. Plywood bond quality is divided into three classes, according to EN 636-1, EN 636-2 and EN 636-3, based upon moisture resistance.
• Class 1: Dry conditions: This bonding class is appropriate for normal interior climate.
• Class 2: Humid conditions: This bonding class is appropriate for protected external applications, but is capable of resisting weather exposure for short periods, e.g. when exposed during construction. It is also suitable for interior situations where the service moisture content is raised above the Class 1 level.
• Class 3: Exterior conditions: This bonding class is designed for exposure to weather over sustained periods.

It must be noted that the durability of plywood (i.e. its resistance to decay etc) depends not only upon the bonding performance level, but also on other factors, especially the durability rating of the species used in its manufacture.

It is a matter of considerable importance that where appropriately specified plywood (in common with other wood based panels) is used in humid or exterior conditions, it is imperative that the product receives an appropriate protective finish, with particular attention being paid to the sealing of all faces and edges, to minimise moisture induced movement in service.

(Source - Scottish Timber Trade Association)

Regards,

Ian T

[IanT]

And for reference, the best definition of BS 1088 (Marine) ply I can find is from Wikipedia, and is as follows;

"In materials, the BS 1088 specification is a marine plywood specification that applies to plywood produced with untreated tropical hardwood veneers that have a set level of resistance to fungal attack. The plies are bonded with WBP glue. Although the initials BS are for "British Standard", the finished product does not have to be "British made". The standard is associated with Lloyd's of London since it performs testing of products to this standard.

WBP Glue Line -- BS 1088 plywood must use an adhesive, which has been proven to be highly resistant to weather, micro-organisms, cold and boiling water, steam and dry heat. The product's bonding must pass a series of British Standard tests.

Face Veneers -- These must present a solid surface that is free from open defects. Face veneers must be free of knots other than "sound pin" knots, of which there shall be no more than six(6) in any area of one(1) square foot, and there can be no more than an average of two(2) such knots per square foot area over the entire surface of the plywood sheet. The veneers must be reasonably free from irregular grain. The use of edge joints is limited, and end joints are not allowed.

Core Veneers -- Core veneers have the same basic requirements as face veneers, except that small splits are allowed, and there is no limit on the number of pin knots or edge joints. However, end joints are not permitted.

Limits of Manufacturing Defects -- Defective bonds, pleats and overlaps, and gaps in faces are not permitted. Occasional gaps may be repaired using veneer inserts bonded with the proper adhesive.
Moisture Content -- BS 1088 plywood must have a moisture content between 6% and 14% when it leaves the factory.

Finishing -- Boards will be sanded on both sides equally.
Length & Width -- The length or width of a board produced as a standard size shall not be less than the specified size nor more than 6.3 mm (0.25") greater than the specified size.
Squareness -- The lengths of the diagonals of a board shall not differ by more than 0.25% of the length of the diagonal.

Thickness Tolerances -- Tolerances vary as follows.
•   4 mm +.02/-0.6 6 mm +.04/-0.65 9 mm +.06/-0.75 12 mm +.09/-0.82
•   15 mm +.1/-0.9 18 mm +.12/-0.98 22 mm +.16/-1.08 25 mm +1.8/-1.16

From the above we can assume that 6 mm material will arrive at thickness' between 6.04 mm and 5.35 mm.

Face Veneer thickness -- For any three-ply construction, which applies to 3 and 4 mm material, each face veneer shall be not thinner than 1/8 of the total thickness of veneers assembled dry. Since the dry thicknesses of the boards are 3.6 and 4.6 respectively, we can assume that for these thicknesses only the face veneers will be as follows:

•   3.6 mm dry x 12.5% (1/8) = 0.45 mm 4.6 mm dry x 12.5% (1/8) = 0.575 mm
Multi-Ply Construction-- This applies to boards thicker than 4.8 mm (3/16")
•   Each face veneer shall be a minimum of 1.3 mm and not thicker than 3.8 mm.
•   Each core veneer shall be no thicker than 4.8 mm"

Regards,

Ian T

[IanT]

Well, I was not a very Happy Bunny earlier today (still not completely happy now to be honest).

You will recall (it is after all here below) the discussion on the benefits of using 'Marine Ply' and where best to source it from. In the end I ordered 2 x 8' x 4' sheets of "BS1088 Marine Ply" from Jewsons and (with my inflatable roof-rack) collected it a few day afterwards. It was about twice the price of Wickes WBP Ply in the same size. You may also recall that what was actually delivered had no BS kite mark – but was stamped EN636-3 –although I later found a "BS1088" stamp on the edge of the board but still no BS kite mark.

I've had quite a lot going on recently but had managed to get the odd (cold but dry) day outside the workshop to cut the boards up. With a bit of scheming I was very pleased with myself, as I managed to get enough 'components' to build 2 x 1350mm x 600mm and 2 x 900mm x 600mm units – equal to five 900mm long standard modules. I spent some time setting each cut up to get them accurate which took quite a while given there were thirty pieces in total. I finished them all last week and stacked them flat in the workshop – which is dry and not particularly damp (I also have machine tools in there).

Today I went down to start assembly (the AGM is looming!) and was dismayed to find that some of the longer lengths had delaminated. Now I had found a few voids when cutting but had decided to just get on regardless – but this was not what I was expecting. I drove the split lengths down to Jewsons but the Manager was not around.

Well, he's just called me and we've had an 'interesting' discussion.

We agreed that the sheets were 8' x 4' but that I had "cut them up" (?). In doing so I had broken the "edge sealing" and clearly had not had the good sense to immediately re-seal the edges with an oil based sealant. So basically this was all my fault!

It was unfortunate for him perhaps that I had spent a little time looking into this subject – so had the documents to hand. I fired back with "Plywood of this type is capable of withstanding exposure to weathering conditions and liquid water, or water vapour in a damp, but ventilated location, under consideration of Section 8.2 of EN 636. End uses include fully exposed service conditions."

He then tried to blame the manufacturer – but as (I hope) we all know that is not an acceptable defense from any retailer under UK sale of goods legislation.

Eventually he offered to halve the price (and I kept the ply) or refund it completely (and I return him the ply in 30 parts). I decided to keep my work and hope that once assembled and treated the modules will be OK. I will of course seal all edges before I get too much older. Only time will tell if this has been a good decision.

So I'm not sure what I will do in the future – or what to recommend to others.

You can order "BS1088" quality ply but you still may not get the quality you expect for paying twice the price. Perhaps a specialist company that supplies marine ply to boat builders will have a better quality "Marine" product but then the expense will most likely be very high indeed.

I probably won't be buying ply at Jewsons any time soon though.

Regards,

Ian T

[MikeWilliams]

Oh dear.  And I think it was me who suggested Marine Ply in the first place.  errr umm, sorry!

The piece I have in the garden wasn't sealed at all, just cut on a circular saw and stood outside for about ten years now.  It has warped, but that's all.

I can understand your concerns and think you've been very calm in the circumstances.

Mike

[bambuko]

I don't know how much you have paid for this pile of dung... but if you really wanted decent stuff, here is what I would use:
http://www.aircraftplywoodandtimber.co.uk/gl1_plywood.html
I wouldn't risk my bacon using anything else, so you can trust me - there will be no voids, and it will perfect for what you are doing.
I have tried before getting half decent stuff from variety of local builder merchants and it was all as bad as your stuff.

Chris

[bambuko]

And just as an addendum:
http://www.aircraftspruce.com/catalog/pdf/Germanischer%20Lloyd.pdf
Makes interesting reading if you are into your plywood 

Chris

[IanT]

Well Chris, that 'GL1' aircraft ply really does sound like the Bee's Knees, even though I don't intend to immerse my Modules in water for 24 hours at a time!

Unfortunately, at nearly 100 GBP for a 6mm board just over 4' x 4' it makes my (so-called) Marine sound relatively cheap. To buy the same area of board as my two 4' x 8' boards (which are 9mm thick!) it would cost me about 400 GBP to buy the GL1, four times the cost of the 'Marine' and more than eight times the cost of the same ammount of WBP board. That's 100 GBP per Module (sans track etc)

So I don't think that's going to be a viable solution - at least for me.

I think it's back to WBP and good old fashioned paint in the future - unless anyone has any other suggestions?

Regards,

Ian T

[bambuko]

Not quite...
Germanisher Lloyd stuff was just for illustration (and you only need GL1 for aircraft, we can easily get away with GL2 or GL3 stuff).
What you want (and yes I know, for some reason it's more difficult to get it here in UK, than on the continent) is proper birch ply.
Do search for the suppliers of Wisa plywood, for example:
Wisa Spruce Plywood Softwood WBP sheets
What you do not want is far eastern stuff claiming to be "marine ply" that is being flogged by all the builder's merchants - insist on proper birch stuff. It can be bought, just its rather difficult to find in UK 

Chris
ps Wisa website is source of very good info - lot of useful downloads.

[IanT]

Hi Chris,

Yes, that looks a very much better ply product - and at 24.77 GBP for a 1200 x 2400 (9mm) sheet for what appears to be a 7-ply - is very affordable too (that's from the online company so it may not include VAT & delivery)

But I also see that the 'Wisa' home site lists a company in Maidenhead as their UK distributor - so (for me) that might be useful too (only about 20 odd miles away from here).

Well worth investigating when I look at my next Modules (maybe some curves for 2012 I hope) - with Modules you can grow a bit at a time...

In the meantime - I am going to try to salvage the defective ply I already have. I think I will start a new thread on penetrating/wicking wood glues.

Thanks for your suggestion - very useful.

Regards,

Ian T