Birch ply or structural ply

[Neil S]

Hi All,
I am refitting my bathroom at the moment. The new cast resin shower tray instructions say it must be laid on an 18mm thick birch plywood base. Only not that many merchants state what the wood is anymore. Wickes sells a normal plywood and a softwood structural ply.
Would the softwood structural ply be Ok under a shower tray?
I note that Birch is classed as a hardwood (non evergreen) and is actually supposed to be quite hard physically as well.
Or should I seek out the genuine birch ply?

TIA

-Neil

 

[Jake]

Birch ply is not very rot resistant, but it is pretty stiff (because the layers are thin and many) which is why they probably recommend it. In your situation with that choice I'd use the "normal" by which I take it you mean the WBP hardwood ply that Wickes sell, but add a second layer between the joists underneath the main bit of ply (sitting on some noggins screwed to the side of the joists) to stiffen everything up. But then I over engineer everything by default.

 

[Marineboy]

I just used 18mm ply from one of the sheds as a base for the concrete bedding layer. Plenty stiff enough.

 

[MikeG.]

The ply itself is pretty irrelevant. It's the rigidity of the base the ply is fixed to which is going to determine the success of mounting your shower tray.

 

[Woody2Shoes]

Hi All,
I am refitting my bathroom at the moment. The new cast resin shower tray instructions say it must be laid on an 18mm thick birch plywood base. Only not that many merchants state what the wood is anymore. Wickes sells a normal plywood and a softwood structural ply.
Would the softwood structural ply be Ok under a shower tray?
I note that Birch is classed as a hardwood (non evergreen) and is actually supposed to be quite hard physically as well.
Or should I seek out the genuine birch ply?

TIA

-Neil

Birch is deciduous and a softwood. Birch is no more durable than most other species used for making plywood, including most tropical hardwoods (eg malaysian hardwood ply) I think you need to consider two aspects here:
1 Structural strength. Minimum 18mm thick, structural board eg spruce plywood or OSB3 properly supported at all edges using extra noggings if necessary. You want a board stated to be suitable for roofing/flooring and marked as such.
2 protection from water ingress, an the rot that inevitably follows.

 

[garethharvey]

Not able to help with the Ply but before laying the tray look for a product called Aqua Strap, this product is awesome. If your tray does not have an upstanding, then this is essential

Here is the link: https://aquastrap.com

 

[SammyQ]

Plus one for Aqua Strap, absolutely first class product.

Sam

 

[Just4Fun]

Birch is deciduous and a softwood.

Are you sure birch is a softwood? I always thought it is a hardwood.

 

[MikeG.]

It is a hardwood, but the timber is like that of softwoods. Balsa is a hardwood.....

 

[Greenthumb]

Any marine grade ply will do, 18mm is perfect, i would also suggest reinforcing the floor joists to save any future movement, i am a bathroom fitter, so i do this often, use a liquid tanking kit on the shower walls to save damage to walls from water penetration through grout

 

[Sgian Dubh]

Birch is deciduous and a softwood.

I suspect a slip of the tongue there, which can happen to anyone.

But ... unfortunately, birch can't be both; it's either a hardwood or a softwood.

Birch is, in truth, an angiosperm (covered seeds), so it's therefore a hardwood by definition, aka deciduous or a broad leaved species of tree. The timber it produces is a hard wood unlike, for example, Balsa (Ochroma pyramidale) which is also a hardwood but its timber is a soft wood, the softest of all hardwoods in fact.

Yew (Taxus baccata) on the other hand is a gymnosperm (naked seeds), aka softwood or evergreen tree species and the timber from it is a hard wood.

It's important to be precise when using terms such as hardwood and hard wood, and softwood and soft wood. As I have just illustrated, not all hard woods come from a hardwood tree species, and similarly, not all softwoods produce soft wood.

As to the question about what to put under a shower tray, it looks like there are enough useful answers already meaning I don't need to add anything, ha, ha. Slainte.

 

[Woody2Shoes]

Birch is deciduous and a softwood.

I suspect a slip of the tongue there, which can happen to anyone.

But ... unfortunately, birch can't be both; it's either a hardwood or a softwood.

Birch is, in truth, an angiosperm (covered seeds), so it's therefore a hardwood by definition, aka deciduous or a broad leaved species of tree. The timber it produces is a hard wood unlike, for example, Balsa (Ochroma pyramidale) which is also a hardwood but its timber is a soft wood, the softest of all hardwoods in fact.

Yew (Taxus baccata) on the other hand is a gymnosperm (naked seeds), aka softwood or evergreen tree species and the timber from it is a hard wood.

It's important to be precise when using terms such as hardwood and hard wood, and softwood and soft wood. As I have just illustrated, not all hard woods come from a hardwood tree species, and similarly, not all softwoods produce soft wood.

As to the question about what to put under a shower tray, it looks like there are enough useful answers already meaning I don't need to add anything, ha, ha. Slainte.

Sorry! Yes, brainfart triggered by seeing something not quite right in the op! Deciduous and therefore a hardwood, but physically pretty soft and not durable.

 

[Just4Fun]

a hardwood, but physically pretty soft and not durable.

I seem to be in nit-picking mode today. Sorry about that, but I would disagree with this description also. I use a lot of birch as it is the best locally-sourced wood available to me. Whilst it is not the hardest wood in the world it is a lot harder than softwoods like pine & spruce and the like and I would certainly not describe it as "pretty soft and not durable". I enjoy working with birch, apart from when switch-back grain makes it a pain to plane.

 

[Sgian Dubh]

a hardwood, but physically pretty soft and not durable.

I seem to be in nit-picking mode today. Sorry about that, but I would disagree with this description also. I use a lot of birch as it is the best locally-sourced wood available to me. Whilst it is not the hardest wood in the world it is a lot harder than softwoods like pine & spruce and the like and I would certainly not describe it as "pretty soft and not durable". I enjoy working with birch, apart from when switch-back grain makes it a pain to plane.

Well, just to add to the general pedantry, I agree that it's not "pretty soft", because birch is relatively hard. But I do partially agree with Woody2Shoes's statement that it's "not durable", although note the rather pedantic durability clarification that follows. Durability in timber tech terms refers specifically to a wood's ability to resist fungal decay in ground contact, e.g., the foot of a post buried in the ground. All the birch species, except one, are in class 5, i.e., perishable, meaning they can be expected to rot away within five years. The exception is paper birch (Betula papyrifera) which just about sneaks into class 4, being slightly or non-durable with a 5 to 10 year life expectancy in ground contact. Having said, paper birch isn't much used for large items likely to be in ground contact it being, traditionally, mostly used for small turnings, hoops, bobbins, toys, dowels, treen, and so on.

Luckily I am happy to restrict exposing my nit-picking anal qualities to woody and woodworking subjects, and find it very easy to resist displaying them in the the threads that get all heated and bothered up, such as those that expose the worst of human nature in, for example, sharpening threads (boring), and those that wander into politics, particularly in the General Chat area, that mostly just amuse me because of the never ending point scoring, occasional snippiness, and the inflexibility of some participants. Slainte.

 

[Just4Fun]

Well, just to add to the general pedantry, I agree that it's not "pretty soft", because birch is relatively hard. But I do partially agree with Woody2Shoes's statement that it's "not durable", although note the rather pedantic durability clarification that follows.

Not pedantic at all, it is useful information. I have never used birch for outside / ground work so never faced that issue but it is good information to bear in mind. I have noticed that old birch trees (we have quite a few growing around the house) can rot from the middle of the trunk outwards and I wonder if this is another manifestation of the same issue.

Coming back to the original theme of this topic, would the perishability of birch have any potential impact on the use of birch plywood in the floor under a shower where it is conceivable that at some point it might get wet (if there were a water leak for example)?

 

[Neil S]

Thanks everyone, it looks like any reasonably hard ply will do. But as recommended, I think I'm going to reinforce the base with either a double layer of ply or extra noggins to increase the support under the ply.
I've also ordered the elements tile backing boards kit which comes with some water tanking/sealing tape and waterproofing solution.
Just ordered the Aquastrap as well.

-Neil

 

[Woody2Shoes]

Well, just to add to the general pedantry, I agree that it's not "pretty soft", because birch is relatively hard. But I do partially agree with Woody2Shoes's statement that it's "not durable", although note the rather pedantic durability clarification that follows.

Not pedantic at all, it is useful information. I have never used birch for outside / ground work so never faced that issue but it is good information to bear in mind. I have noticed that old birch trees (we have quite a few growing around the house) can rot from the middle of the trunk outwards and I wonder if this is another manifestation of the same issue.

Coming back to the original theme of this topic, would the perishability of birch have any potential impact on the use of birch plywood in the floor under a shower where it is conceivable that at some point it might get wet (if there were a water leak for example)?

I think that the physical hardness of birch depends on where it has grown. Harder will equate to further north where the growing season is shorter and the climate is drier for a larger part of the year - the grain structure is tighter. The birch that I cut for firewood in SE England is soft by comparison with the stuff from the Baltic. In my part of the world, birch is not a longlived species because of our damp maritime climate which engenders rot - I think in most climates it's considered a 'pioneer species' as it establishes a canopy and then gives way to pine or oak. From the point of view of rot under a shower tray, all wood - unless you're planning to use greenheart or something - is pretty much the same. The key is to use defensive detailing - take great care to prevent moisture getting where it shouldn't and to minimise the risk of damage when it inevitably does - especially to floor joists.

 

[Woody2Shoes]

Just to add that physical hardness and structural strength are not strongly correlated.

 

[Just4Fun]

I think that the physical hardness of birch depends on where it has grown. Harder will equate to further north where the growing season is shorter and the climate is drier for a larger part of the year - the grain structure is tighter.

Could be true. The birch I use is grown locally to me here in Finland and is close grained. It is also quite heavy, which I guess would support your theory. I don't know the density - I could weigh some I suppose - but it is a lot heavier than any softwood I have worked with.

 

[Sgian Dubh]

I have noticed that old birch trees (we have quite a few growing around the house) can rot from the middle of the trunk outwards and I wonder if this is another manifestation of the same issue.

Not really I'd say. If you look at mature to old trees it's quite common to find the centre has partially rotted away, or even hollow from fungal attack. Mature to old varies from one tree species to another, and one indicator of longevity is the 'root to shoot' ratio. Oaks have a large ratio with the root system in some species (not European oak) being up to ten times greater in volume than the shoots above ground. Birch has a significantly smaller root to shoot ratio, although I can't recall the typical ratio off the top of my head. In other words, the oak has greater resilience built into it through its 'foundations' with reserves of energy and the ability to regenerate after during or after stress or potentially life threatening events. One saying has it, and I paraphrase, that an oak grows for 300 years, rests for 300, and spends 300 years dying, although many oaks never reach that sort of age and some exceed it. The lifespan of a typical birch tree is in the region of one hundred to about 150 years.

As Woody2Shoes mentioned, birch, particularly in warmer southern climates, is frequently a pioneer species through its seeds sprouting quickly, the sapling quick to maturity, followed by reproducing, and dying leaving space and better soil conditions for the understory of longer lived trees, such as ash, beech, oak, maples, etc to take over and become the dominant species. In the further northern, colder and harsher climates of, for example, northern Russia and Scandinavia (where you appear to be) the conditions aren't so good for those ashes, oaks, beeches, and it's better suited to the birch trees, as evidenced by the vast forests of the stuff there (fast being depleted by a mixture of legal and a great deal of illegal, or barely legal logging in certain areas - I'll not point the finger at any one country with a significant area of northern lands, ha, ha.)

Harsher conditions I believe do lead to a harder birch end product, i.e., the wood. I think it's generally the case, as Woody2Shoes mentions, that birch wood from the warmer more southerly parts of its range tends to be softer than the slower grown stuff from the chilly boreal north. I'm not one hundred percent sure of that, but I think that tends to be the case.

Coming back to the original theme of this topic, would the perishability of birch have any potential impact on the use of birch plywood in the floor under a shower where it is conceivable that at some point it might get wet (if there were a water leak for example)?

A one-off or rare intermittent wetting wouldn't be a problem, as long as the wood immediately has a chance to dry. The problem with a leak that develops under a shower is that such leaks, once they develop, tend to be regular, e.g., daily, or even multiple wettings per day in a household with several residents, and the wood base is essentially enclosed with no real chance for decent air movement to dry it. Essentially, if it stays wet pretty much all time for an extended period the wood will rot out eventually, no matter what the species might be. It's just that some wood species will hold out better and longer than others, but in reality, the leak needs to be fixed ... if the shower user/s notice the problem soon enough to effect a repair that is. Slainte.