Help with ply selection - garden building

[Chef40]

Hello there,

I'd really appreciate any advice - I'm building a garden workshop/shed/storage room to a very high standard so it will last. I'm also insulating it.

I've build a plinth-based base that's off the ground with frame and joists etc. + reinforced membrane (to keep pests off my PIR) and then PIR.

I want to finish the top with ply but my question is should I buy Marine Ply BS1088 to ensure I'm safe with the exposed edges around the perimeter and also in case the main floor gets wet, or is this a waste?

I guess the alternative is to use EN314 Class 3 (or maybe 2?) and epoxy the edges?

If anyone has any experience, I'd be very grateful.

Thanks, Ben

[Onoff]

Got any pictures of the base so far?

 

The "wet" shouldn't be getting anywhere near the ply floor edge if you've got a good wall detail.

 

Worst thing you can do it make it up as you go along.

 

Post some pictures and people here will chip in with good ideas.

[ProDave]

If the design is right, the edges of the base sheet will not be "exposed"  the wall cladding will come down over the edges of the base.

[Chef40]

This is the general idea but I’m not building the building itself and the firm that is has said the edges of the floor will be exposed because it sits on top of the ply. They’ve said I need to take account of that…

thanks!

[Onoff]

2 hours ago, Chef40 said:

This is the general idea but I’m not building the building itself and the firm that is has said the edges of the floor will be exposed because it sits on top of the ply. They’ve said I need to take account of that…

thanks!

 

Then sorry but they haven't got a clue and sound like cowboys. All they're interested in is dumping a building on a frame/base and walking away. 

 

You say you're building to a very high standard. That means then properly insulated (walls, floor and roof), draught proof with a vapour barrier and with consideration given to cold bridging. Then you need to think about heating AND ventilation. 

 

Anything else is a poor imitation of a dry, warm workspace where your tools won't rot/rust. Do it right and it'll be a pleasure to work in with little in the way of ongoing maintenance and low heating bills. 

 

An exposed ply floor edge is wrong on so many levels.

[Onoff]

Have a look here:

 

 

[ProDave]

3 hours ago, Chef40 said:

This is the general idea but I’m not building the building itself and the firm that is has said the edges of the floor will be exposed because it sits on top of the ply. They’ve said I need to take account of that…

thanks!

 

Consider a different manufacturer or build it yourself.

 

Once you have your foundation beams laid and your floor, I would want the wall cladding extending down over the floor including the frame and extending a little lower than the floor framing so water that runs down drips off under the building without wetting the floor structure in any way.

[PeterW]

3 hours ago, Chef40 said:

This is the general idea but I’m not building the building itself and the firm that is has said the edges of the floor will be exposed because it sits on top of the ply. They’ve said I need to take account of that…

thanks!

in that case the whole of the base needs to be 25mm smaller all round than the building and your building then overhangs the floor structure. You can then put some sort of DPC around the edges and a trim panel too to cover the ends and keep water out. 

[Temp]

8 hours ago, Chef40 said:

This is the general idea but I’m not building the building itself and the firm that is has said the edges of the floor will be exposed because it sits on top of the ply. They’ve said I need to take account of that…

 

You never really want a building sitting on a larger flat surface as rain will pool on the edge and run under the walls or at least they will be sitting in water.  If the building is clad with weather boards on battens then I'd size the floor so the battens and cladding can extend down over the edges of the floor. 

 

If you really can't do that I think I would put something  like DPM or flat roofing membrane on top of the floor and wrap it down over the edge. Whatever it needs to be UV stable or covered with something to prevent UV degradation. Still gives you a problem sealing the walls to the top.

 

Be careful if you decide to use wood preserver because I think it is difficult to glue to so presumably hard to get any sealers to stick?

 

My builders merchant stocks at least 4 grades of WBP. If its going to be exposed I'd want the best they can give me. I've previously had WBP from one of the DiY  sheds delaminate in a few weeks outdoors.

[Chef40]

5 hours ago, ProDave said:

Consider a different manufacturer or build it yourself.

 

Once you have your foundation beams laid and your floor, I would want the wall cladding extending down over the floor including the frame and extending a little lower than the floor framing so water that runs down drips off under the building without wetting the floor structure in any way.

Thanks ProDave... I take your point and will press them to make it a little larger and extend it down but I think it's going to be a tricky project for me to take on in entirety on my own at the moment.

[Chef40]

5 hours ago, PeterW said:

in that case the whole of the base needs to be 25mm smaller all round than the building and your building then overhangs the floor structure. You can then put some sort of DPC around the edges and a trim panel too to cover the ends and keep water out. 

Thanks Peter. I'm putting membrane (reinforced to try and stop pests chewing) underneath the 100mm PIR I'll use. Would you suggest a DPM above the PIR and under the ply, wrapping up and over the floor then trim it back when the building is in place? I was thinking of using 25mm ply.

Thanks!

[Chef40]

28 minutes ago, Temp said:

 

You never really want a building sitting on a larger flat surface as rain will pool on the edge and run under the walls or at least they will be sitting in water.  If the building is clad with weather boards on battens then I'd size the floor so the battens and cladding can extend down over the edges of the floor. 

 

If you really can't do that I think I would put something  like DPM or flat roofing membrane on top of the floor and wrap it down over the edge. Whatever it needs to be UV stable or covered with something to prevent UV degradation. Still gives you a problem sealing the walls to the top.

 

Be careful if you decide to use wood preserver because I think it is difficult to glue to so presumably hard to get any sealers to stick?

 

My builders merchant stocks at least 4 grades of WBP. If its going to be exposed I'd want the best they can give me. I've previously had WBP from one of the DiY  sheds delaminate in a few weeks outdoors.

That's really helpful, cheers. When you say 'sealing the walls to the top', do you mean stopping any ingress between the floor and the bottom of the wall? I imagine the building provider would have done this before but I think they're used to installing on a concrete base.

 

I also guess the short answer is if they won't change the design and I don't want to cancel my order, then I need to wrap in membrane under the floor and around the edge and think about sealing that - maybe epoxy? Also, I suppose the short answer is also to use expensive (proper) marine ply.

 

Will it be OK to have membrane under the PIR and then membrane on top?

 

Thanks!

[Chef40]

6 hours ago, Onoff said:

 

Then sorry but they haven't got a clue and sound like cowboys. All they're interested in is dumping a building on a frame/base and walking away. 

 

You say you're building to a very high standard. That means then properly insulated (walls, floor and roof), draught proof with a vapour barrier and with consideration given to cold bridging. Then you need to think about heating AND ventilation. 

 

Anything else is a poor imitation of a dry, warm workspace where your tools won't rot/rust. Do it right and it'll be a pleasure to work in with little in the way of ongoing maintenance and low heating bills. 

 

An exposed ply floor edge is wrong on so many levels.

I take your point - I think 'dumping a building' is pretty much what they do.

 

The building isn't cheap and seems pretty solid (25mm cladding etc.) I'm insulating the floor with 100mm PIR and reinforced membrane underneath and I've asked them to put a Vapour Barrier around the walls. I'm then planning to install PIR between the studs and add something to finish it. 

 

I do want somewhere to safely store tools but don't think I'll often want to actually heat it - if I did, I'd probably just have an oil rad or something.

[Temp]

1 hour ago, Chef40 said:

That's really helpful, cheers. When you say 'sealing the walls to the top', do you mean stopping any ingress between the floor and the bottom of the wall?

 

Yes exactly that.

 

I designed and built my own outbuilding on a concrete slab. Put a lot of thought into keeping water out. What I did was arrange for the slab to be the size of the frame but smaller than the clading. I laid two courses of engineering blocks on the slab then a DPC and the frame sits on top. The cladding extends down over the brick. Any water running down the cladding drips onto gravel surrounding the slab. Any water blown under the  bottom edge of the cladding is stopped by the brick courses. This one doesn't have an insulated floor but i could lay a sand blind on the slab, DPM lapped up the wall then PIR topped with OSB flooring. 

[Chef40]

38 minutes ago, Temp said:

 

Yes exactly that.

 

I designed and built my own outbuilding on a concrete slab. Put a lot of thought into keeping water out. What I did was arrange for the slab to be the size of the frame but smaller than the clading. I laid two courses of engineering blocks on the slab then a DPC and the frame sits on top. The cladding extends down over the brick. Any water running down the cladding drips onto gravel surrounding the slab. Any water blown under the  bottom edge of the cladding is stopped by the brick courses. This one doesn't have an insulated floor but i could lay a sand blind on the slab, DPM lapped up the wall then PIR topped with OSB flooring. 

Amazing - t hat sounds like a really awesome project. I don’t think mine’s going to be that good but I don’t intend to use it like a room.

really helpful though, thank you. I’m going to ask the supplier to make the frame the same size as the base and then put DPC over the PIR and up the outside of the frame (and the ply) under the cladding. This can be overlapped by the vapour barrier on the top, so nothing can run down, or I guess it could wrap under the frame and just be cut, which may be better.

they may not be happy but we’ll see!

 

out of interest, would you suggest anything remedial around the 50mm hole for ducting to bring services in, or just some expanding foam to keep it as sealed as poss?

 

very best, Ben

[Chef40]

I've gone back to the supplier this morning and asked the following - does this all sound correct and reasonable please? (They're being a bit difficult and I'm now worried about my considerable deposit) but I'd rather get it right!!!

 

Extract from my email with attached note:

I’d be really grateful if you could take a look and revise our project accordingly and let me know if there are any problems with this please?

 

I think, in essence, it just involves the following:

 

1. Me to add a DPM running above the PIR insulation
2. A flexible flashing to be added around the perimeter at the time of construction
3. The frame of the building to be made to the full width of the base
4. The vapor barrier to be brought down to cover the ply and the joist
5. The DPM to be wrapped up to protect the ply edge
6. Cladding to be extended downward to cover the joists and frame beneath the building and any exposed end-grain

Detail of workshop construction.pdf

[Temp]

Looks good to me.

 

Perhaps be prepared to adjust the size of the floor a bit in case their tolerances are off.

Edited June 21, 2021 by Temp

[Moonshine]

19 hours ago, Temp said:

I designed and built my own outbuilding on a concrete slab. Put a lot of thought into keeping water out.

 

have you got any drawings / detailing, i am struggling with my floor detail and getting it to work, as below

 

 

[Temp]

I can't find my old sketchup drawings but made this sketch. Think the construction is similar to yours.

 

 

Some details...

 

* When I laid the slab I pushed 12mm stainless steel threaded rods (with nuts on) into the slab so I could bolt down the wall plate later. Mostly because we are in a windy location near top of hill. (Caution: These are very dangerous until covered up. I nearly fell onto one!) 

 

* I extended the vertical battens down until they are about 30mm off the top of the slab so the lowest cladding plank hides most of the brick. I used gravel around the slab to reduce splashing and haven't had a problem with the bottom planks (but they are oak).  

 

* I didn't bother with rebar mesh in the slab but probably should have done. Not been a problem though.

 

* I used 12mm WBP rather than 9mm OSB because mine has a pitched tiled roof. Some of the frame (eg corners) was also made from 4x4 rather than 2x4. 

 

* Where there are joints in the cladding boards I put strips of DPM behind to deflect water to the outside. These were trimmed back flush with the bottom of the planks later and can't be seen unless you peer into a joint. Not sure if this is essential or not. To support the joints you can either use wide battens (so nails aren't too close to the end of a board) or fit short extra vertical battens so there are two, one either side of the joint.

 

* At the corners of the building I fitted (mostly decorative) posts like this (Plan view). These corner posts were also supported 30mm off the slab using stainless steel bolts in holes drilled into the ends. They were also screwed to vertical battens and the screw heads plugged to simulate pegged construction. 

 

 

[Onoff]

My unfinished (that's an in joke here ?) attempt at a garden room section. As this is an infilled stud wall I'd plan to put a continuous insulation layer inside to mitigate cold bridging. Similar detail at the ceiling and floor detail 

 

One big thing is you want your cladding to stop 150mm above the "ground". Then pay attention to whatever's below that to protect it. Missing from my drawing is "cladding"  over the sole plate and thermoblock. I'd likely use aluminium. Some have used black uPVC cladding which looks great. 

 

The age old premise is that an average raindrop hits the ground and bounces up about 6". 

 

 

I'd maybe look to move the OSB to the inside of the studs, better for screwing things on walls. 

 

I started to cost this up...but haven't finished...

[Chef40]

17 hours ago, Temp said:

Looks good to me.

 

Perhaps be prepared to adjust the size of the floor a bit in case their tolerances are off.

Thanks very much indeed! Working on it!

 

Very best, Ben