How to build a solid wood cover for a fire pit kiln

[JT101]

Hi everyone

I want a cover for my fire pit/kiln on my allotment. It is quite big, an 800mm wide x 600mm deep hole. I need to protect it from the elements, and more importantly needs to be strong enough to be stood on as I'm certain it will get walked on. It has a clay edge of 100mm for the cover to sit on.

At first I looked at old hardwood round garden tables, but as these get old, they weaken significantly, plus they have small gaps to let the rain through. If this happens I end up with a swamp in the pit.

My current thinking is some large 9" x 2" joists, cut to size, joined on edge and then cut into a circle.
Possibly braced from below, but at present the braces would have to be much less than 2", more like 15mm max as i currently have a corrugated sheet right underneath which is used during the firing to choke out the fire towards the end. I could build up the 100mm clay edge to accept thicker bracing if necessary.

I am hoping a sharp edge, and good wood glue will provide the sealing of the joints, and the timber is treated, so it's more the structural strength in the joints.

And the simpler the better. Ideally I'd make this within a few hours, not over a couple of weekends.
This is only an allotment, doesn't need to be high end carpentry. Just strong and tight.

I only have an electric plane and router, no biscuit joiner. Was assuming T&G, lap joint, dowels, pocket screws & glue etc.

Any ideas welcomed

 

[AndyT]

I think your idea would work.
Scaffold planks would be a bit thinner but still strong enough.
One suggestion - if you put the bracing on the top, you could cut handholes in it to make it easier to lift off. The boards will expand and contract a bit so when fixing the cross pieces, use oversized holes and screws, slots and screws, or clenched nails, which have a bit of give in them.
I'd paint it to help it last a bit.

 

[Inspector]

A sheet of pressure treated plywood and the appropriate screws. Cut two discs and screw them together.

Pete

 

[AndyT]

That sounds a much better idea.
An offcut of roofing felt would finish it off nicely.

 

[JT101]

Thanks Inspector and Andy.

Two reasons I won't use ply. Delamination and appearance. I appreciate that pressure treated timber isn't exactly rustic, but although it is only an allotment, I'm trying to make it look a bit more wood like and natural with everything I build there. This is a little social area. Just a bit of fun.
And I'm certain that would delaminate over time.

So I'm pretty much fixed on doing it with solid lumps of timber. The reason I didn't suggest scaffold boards is their longevity. I've been using them for years as edging, tables etc. But they barely make it past 2 years before they crumble to bits.

So I'm inclined to stick to hardwood or treated wood. Sorry if i didn't make that clear. Good point about bracing from above. Hadn't thought of that.

But what about the joints. Would you even join them along their edge? Or just butt them up and rely on the bracing? At the very least I would have to have square edges so I can create a seal to prevent water seepage.

Cheers

 

[SteveF]

I am not sure that you will not get that sealed enough that a heavy downpoor will not just fill your hole with water
maybe consider a thick sheet (or 2) of plywood covered with edpm rubber roof fabric

Steve

 

[basssound]

I'd be more inclined to drill two or three small holes in the bottom to allow water to drain out then you don't have to worry about sealing from above.
For that rustic look you could cut out a circle from a pallet and not worry about rain water entering with the drain holes.

 

[Inspector]

SteveF he just said he doesn't believe in plywood even though boats made of marine plywood last for many decades and the pressure treated wood foundation of my house has a lifespan of at least 50 years that's not good enough. He has a vision of boards for his cover so that's what he wants advice on. I don't know what you guys can get but we have recycled plastic boards used for decks that could also be used for a cover but that doesn't fit with the rustic vision he has for the area so I won't suggest it. I wouldn't try to make a lid that big out of thick boards and expect it to remain leak free (seasonal shrinkage is in the neighbourhood of an inch) and I feel that even if it was watertight the hole will likely fill up from seepage as the water table rises after a heavy rain. I hope he finds a solution and wish him well.
Pete

 

[Woody2Shoes]

Looking at your drawing, I think you'll find it hard to keep the rain out:

- the lid needs to oversail the hole in the ground all the way round;
- as has been said upthread you need to find some way to stop the rain getting between the boards.

Also, are you sure that the water table won't rise in winter and fill the pit from below?

I'd be tempted to try a lid made like a pitched roof with timber shingles - something a bit like this: https://www.youtube.com/watch?v=jGakPHwi7js ??!?!?!

Or maybe something simpler with a more conventional ridge - sort of like this: https://www.youtube.com/watch?v=Qz5Nhmwx2mc

Cheers, W2S

 

[JT101]

You only have to dig 1ft down, and you hit solid clay round here. I used some of this dug out clay to line the walls, so there is a fairly impermeable layer all round.

So hopefully no water from the water table can get in. So I can't just drill holes in the bottom.

However, once rain gets in, it can't escape.

I can tell you over many years of experience, that I have used construction grade WBP ply and it always delaminates within a short space of time, disintegrating into pieces I then have to remove from my plot. Maybe high end marine grade ply might be different.

In any case this is a creative project, and yes, I want it to look a certain way. Like real wood. If you think I'm being difficult, so be it.

I did throw the question out there to see if you guys came up with something I hadn't thought of. Ok, so I've set my heart on solid wood, but I take on board what you guys have said about shrinkage, ensuring it is much bigger than the hole, and putting bracing on top.

No-one has mentioned edge joints of any kind, so I'm assuming the bracing will suffice. Though I am tempted to do some kind of lap joint or T&G and glue it. That way the water has to get through a glued joint.

If the water still gets through the sides, I can probably fill gaps with clay.

If it still gets though the water table, then yes, I'm buggared.

 

[Woody2Shoes]

I'm on heavy weald clay and a hole like that would fill up in winter - no question. Of course YMMV!

Have you discounted my suggestion of making a lid with sloping sides (sort of like a little roof)?

Cheers, W2S

PS I also think it's worth considering that any timber (treated or not, 'durable' or not) in contact with the ground will rot - surprisingly fast IME.

 

[JT101]

This is kind of a dual purpose thing. More of a general burning pit / kiln below ground, and a fire pit bbq above. Since the kiln part won't be used most of the time, on top of this lid will sit a cast iron firepit. So I can't put a sloped lid there. I suppose I could then build a removable sloped roof above it.

Ok, well this could be a failed project. Maybe that's why pit kilns tend to be temporary. I have seen some deep ones, but maybe they get filled in afterwards.

I should also say that this was a little foray into carpentry to learn about edge jointing. I was interested in learning what was the strongest way to put this thing together with real timber so that it could take a humans weight.

I've no doubt I will need something similar in the future for other projects.

 

[samhay]

Treated softwood will last ok if you keep it off the ground and don't cut too much of the treated wood out (unless you have a better DIY treatment than I have). For this reason I wouldn't recommend you cut your own tongue and grooves as water will collect in the joint, which is now largely un-treated... I'd edge joint by butting with a good weatherproof adhesive and brace as others have suggested to allow for considerable movement, including cupping. It won't keep 100% of the rain out, but neither will your clay lining. If you could build enough slope into it to prevent rain pooling on the top, this would be helpful.

If I wanted it to last, I'd use cedar or a durable hardwood. Treated softwood is cheaper and easier to find though, so I'd use this and be prepared to replace it when it fails.

 

[JT101]

Thanks guys. Plenty to go on