How best to construct summer room floor

longwalks1

I’m planning a corner summer room, with each long side being 2.6m long.  Currently have a solid concrete base poured that’s level.  
One colleague has said construct the walls first, place them on a thick dpm on the concrete base, then once fixed together, build the floor inside the walls, also resting on the dpm.

another has said to build the floor shape and joist/noggin it first, on the dpm on the concrete.  Then insulate and fix flooring boards, then fix the walls on top of the completed base.

whats the correct way please?  

casper_gutman

What will the walls of this "summer room" be made from? If it's a timber outbuilding like a summerhouse, then both ways can be normal/correct.

A summerhouse built like a log cabin will be built the first way. The walls are made from as solid timber lengths slotted together one on top of another and are supported directly on the slab. The floor is put in afterwards and is effectively a floating floor, built on the slab inside the perimeter of the walls. 

A summerhouse can also be built the second way, like a conventional shed. This way the floor is put down first, then walls made from framed panels with cladding attached to the outside are supported on the edges of the floor structure.

longwalks1

Sorry Casper I should of said, framed studwork walls and noggins, same as an internal stud wall. The battened and clad outside 

Jeepers_Creepers

I personally wouldn't build anything directly off that concrete base, DPM or no. I'd raise the whole caboodle up, even if it's just by 2 inches - the height of a single brick.

I'd lay a series of brick 'pillars' - even if they are made from just one brick each. These will support the main joists at their required spacings (usually 600mm to 1m depending on joist size) and a layer of DPC on top of each brick will protect the joists from 'rising'.

If the concrete base is actually flat and level, then the bricks don't even need to be mortared down - they can just sit there. Any tiny variations in level - up to, say, 5mm, can be taken care off by adding multiple pieces of DPC.

If there's more than a slight unevenness in the base, then bed down corner /'datum' bricks first on a half-inch bed of mortar, and use a long straight edge and a level to get these level with eachother.

Once these are set, lay as many bricks as you need between them to support the joists - every 600mm or 1m depending on joist size - and use the straight edge between each 'fixed' datum brick to get the others level. You should end up with a perfectly level series of bricks on which to lay the joists. 

Complete the base frame using noggins as before.

Then build the walls up from this.

When you clad the outside, make sure the lowest board hangs down beyond the lowest part of the joist by about an inch - all then rain will then run off on to the slab and away, and the joists under the shed will remain ventilated and bone dry with the small ~1" gap remaining.

Add a layer of insulation over the joists before laying the inside floor.

longwalks1

Thanks Jeepers - I had thought about using a load of old, clean pavers I’ve got laying around, think they’re 35mm thick and placing them evenly and building onto them.

if I done it thistle way, would I wrap the dpm up the sides of the floor joists and fasten all round ( and thrn cover with the lowest cladding board?  What about airflow around the joists/lowest side of the insulation? 

Insulation on top of the joists, then floor might limit me with the 2.5m max height as it’s less that 1m from our boundary

Jeepers_Creepers

Pavers will do. In theory a DPM layer just laid flat between the top of the paver and the contact area of the joist (with a nice overlap to be sure) will block any 'rising'. You can 'wrap', but then run the remote risk of trapped moisture. The only other source of water is, well, rain, so that's the main reason for having these pillars - get everything off the slab.

You keep timber rot-free by (a) preventing them from getting damp (D'oh), and (b) keeping them ventilated so's they dry out if the first doesn't always work.

(Personally - as I've done with my own 'Wendy House' - once the joists are laid on their DPC and on the bricks, I then gives them a good splash of timber preservative.)

Only a tiny gap is required between the bottom cladding board and the ground/slab since it's all the way around = plenty ventilation.

By insulation, I mean the thinnest layer, and then only if you'll have heating in there. If not, don't bother! And by thinnest, I mean - ooh - even just half inch thick, or an inch tops, a complete unbroken layer laid on top of the joists (so they remain ventilated from underneath), and then your choice of flooring on top.

But, if no inside heating, then insulation is relatively pointless. Mind you, heating from solar gain can be trapped better, so - personally, for such little extra cost and hassle - I think I'd fit a layer.

FreeBear

britishboy said: I’m planning a corner summer room, with each long side being 2.6m long.

I'd recommend doing multiples of 1.2m - Saves having to cut thin strips to fill in those few inches at either end as your standard lengths of timber/insulation/boards are going to be 1.2m, 2.4m, or 4.8m.