Decking

[MikeG]

Ive read that 5mm gap should be sufficient for movement and drainage.

Goes off to google 'pressure impregnated timber'.

Thanks for your comment.

I have designed the frame using 4 x 2" timber. Does these seem sufficient?

4x2's can only span a maximum of 4 feet or so, if at 400 (16") centres. If the support for them (ie your concrete blocks) is any further apart than this, then the timbers will need to be bigger.

Tips:
-lay them with a very slight fall so that water runs off.
-use treated timber for the joists, and apply a "cut-end" treatment wherever you saw them to length. Avoid exposed end-grain as much as possible.
-don't, whatever you do, varnish them!! Use a proprietry decking oil.
-fire-pit? This wants careful thought
-drill pilot holes for screws near the ends of boards to prevent splitting.
-use the narrower decking boards to reduce cupping
-store all loose timber out of the sun prior to construction. Sun is far more damaging than rain.
-if you intend using the decking in autumn and spring, then consider buying (much more expensive) decking boards with non-slip inserts. Decking without this can be lethally slippery after winter.
-build in a DPC between the conc blocks and the joists (not the membrane on the ground, which will only suppress weeds)
-use solid blocking between joists to prevent them twisting
-have a good think as to whether decking is actually the suitable surface in this location, and consider using stone instead.

Mike

 

[swee'pea99]

Don't mention it to 'er indores, cos she'll insist on getting in 'someone who knows what they're doing', and it'll cost you an arm & a leg.

 

[Archie_tect]

Are fire pits compatible with timber decking?

As with internal fireplaces you need to have a continuous separation air gap of 40mm between it and any any timber, and if it's a masonry fire pit the sides need to be at least 200mm thick so that the heat from the fire can dissipate within the masonry so that radiation from the fire pit can't cause the timbers to spontaneously burn. Metal fire pits should have their own installation recommendations, which must be followed to the letter. Download and have a look at The Building Regs, Approved Document Part J

 

[davdandy]

I have a chimenea on mine but it sits on top of a large paving stone.Works a treat.

 

[akb]

Thanks for all the comments and suggestions; very helpful. The decking is planned to be constructed to the corner of the garden with the most sun but will consider the non slip boards.

@MikeG

To to clarify my understanding of the joist sizes and supports. The longest lenght of the area is approx 5.3m (the largest board lengths I have found are 5.4m so luckily not too much faffing!) I was going to level the ground, lay a membrane and then lay slabs at centres to support the joist. Can these centres be 4ft to allow me to use 4x2"? The cross supports will be at 400mm ctrs, approx 2.4 metres (approx 8ft) in length; again could I place a paving slab in the middle of each 4x2" joist for support?

What is the purpose of the DPC between the timber and concrete?

The cut end treatment; is this a paint applied treatment I can apply to each cut once constructed?

Cupping? I was going to use boards 125mm wide?

Ok, not a fire pit; log burner raised from the timber. Was going to build a slab into the decking where this will sit as I dont like the idea of it sitting on timber. This is the pit we have: http://www.bmstores.co.uk/products/decorative-leaf-log-burner-286995

Thanks again for the help.

 

[Ganymede]

Thanks for all the comments and suggestions; very helpful. The decking is planned to be constructed to the corner of the garden with the most sun but will consider the non slip boards.

@MikeG

To to clarify my understanding of the joist sizes and supports. The longest lenght of the area is approx 5.3m (the largest board lengths I have found are 5.4m so luckily not too much faffing!) I was going to level the ground, lay a membrane and then lay slabs at centres to support the joist. Can these centres be 4ft to allow me to use 4x2"? The cross supports will be at 400mm ctrs, approx 2.4 metres (approx 8ft) in length; again could I place a paving slab in the middle of each 4x2" joist for support?

What is the purpose of the DPC between the timber and concrete?

The cut end treatment; is this a paint applied treatment I can apply to each cut once constructed?

Cupping? I was going to use boards 125mm wide?

Ok, not a fire pit; log burner raised from the timber. Was going to build a slab into the decking where this will sit as I dont like the idea of it sitting on timber. This is the pit we have: http://www.bmstores.co.uk/products/decorative-leaf-log-burner-286995

Thanks again for the help.

Re placing your log burner: consider the direction of the prevailing wind in the summer months. I was at a party a couple of years ago where we all had to sit in a stream of smoke as the chiminea was to the south-west of the patio on a beautiful sunny breezy evening.

 

[MikeG]

If I have this right, your finished area of decking will be 5.3m x 2.4m, max. Is that right? You seem to be proposing 2 long joists at either end of 2.4m joists, which will be at 400 C's. If I have your plans right in my mind, then each individual 2.4 m joist will need supporting at each end and in the middle, but the ends are taken care of by hanging them off the long joists. These long joists will need a support under them at 1200 C's.......in other words, you'll need a support under either end, and 4 intermediate supports under each long joist. By the way, you can't just screw or nail into the end grain of the intermediate (shorter) joists to fix them to the long ones. They should be on jiffy hangers, or be notched over a bearer. I would be uncomfortable with the latter given how little timber there would be left if you were to do this, so I suggest you use hangers.

Protim do a good cut-end treatment, funnily enough called "Protim End Cut". This is a thin but coloured liquid, which you tip into a tray or bucket, and stand the cut ends of the timber in for 30 seconds or so.

Cupping? Personally, I'd go for the deck boards that finish about 90mm wide, which you then screw near one edge, and then near the opposite edge on the adjacent joist. in other words, you screw down the LH side on one joist, the RH side on the next one, and so on. If you go for the wider board, I suggest you use 2 screws per board per joist, to prevent cupping (curling) of the wood. However, this increases the chances of the timber splitting as it shrinks, which is why I always advocate the narrower boards.

The DPC? Well, water will sit on top of the concrete blocks, and will be retained by the blocks. Keeping timber off this, even if by a little, will increase the life of the timber.

I just don't like the idea of a fire-anything on the deck. I don't like the idea of anything sitting permanently on top of the timber either, such as a hearth, because this retains moisture and will lead to rotting and failure, eventually, however well treated the timber is. I would design this feature out if I were you, perhaps placing it adjacent to the decking, but sitting on the ground, and keeping the clearance distances that Archie-Tect gave as a bare minimum.

 

[akb]

Excellent, thanks for your detailed response. Particularly thanks for the info about joist hangers; wasnt aware this was required. Shall post photos once I start for progress. Wish me luck!