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Question DetailsAsked on 1/3/2014

How much will it cost to do attic flooring approx 250 sq ft? I am planning to use the space for storage

Currently, there is no flooring in my attic, looking for basic flooring costs - Material + Labour for approx 250 sq ft
Location : Ellicott City, MD

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2 Answers


Is the space (attic floor) designed or able to accomodate additional weight?

Most are not and you will need to do some additional framing at minimum to support the additional weight.

What are you planning on storing up there?

Answered 6 years ago by WoWHomeSolutions


Assuming you are talking just basic 3/4 inch plywood to stack stuff on, probably in the ballpark of $4-10/SF - the lower end of you have an open attic with exposed joists and no obstructions, highest end if you have trusses so supporting beams will have to be put in, and the flooring pieced around the truss members. I am assuming the attic is not part of your building envelope - i.e. is not heated and air conditioned.

Things to consider, as WOWHomeSolutions indicated:

1) weight - if you are planning on putting any weight up there (as opposed to empty boxes and maybe a light piece of furniture or two and such) you need an assessment of load capacity by an architect or structural engineer - most attics are not designed for any joist loading from storage, unless the attic was designed as a future living space. This is particularly true in areas where your joists might have mid-span walls under them like over bedrooms and upstairs hallway, but free-span the entire house width over living room and dining room - do not load those free span areas if at all possible.

2) moisture - attics get moist in hot humid summer conditions and during rainstorms, and frequently condense moisture and form frost in winter, so anything organic risks mildew and mold and water damage - and organic DEFINITELY includes cardboard boxes. You are quite close to Cheasapeake Bay, so your moisture levels tend to be high pretty puch year around, and of course Washington/Baltimore area summer humidity ...

3) ventilation - you have to be sure to maintain ntural airflow for ventilation around the items or they will get musty smelling and can start mildewing. Also, it is vital to not sit the flooring directly on insulation because it will act as a vapor barrier, and household air moving up under it will be trapped by the wood and can cause mold in the insulation and rot in the plywood and joists. Before putting the flooring in, the existing insulation should be pulled up and any penetrations or gap to the house sealed to prevent airflow into the attic under the flooring, if this has not previously been done as part of an energy upgrade and attic insulation project. THis is likely to cost $1-2/SF additional under the flooring area - and go about 3 feet outside that area also. I have seen several homes with partial roof collapses because people put a nice plywood floor in the attic to make a storage or play area, it trapped moist household air coming up from below and rotted the joists to the point they failed, causing partial roof collapse and about ten thousand dollars damage - in one case the walls even splayed out under snow load, totalling the house. Therefore, you should leave no less than 3-6 inches air space, open at opposite sides, for airflow under the flooring. My recommendation - build it like a deck - using at least 2x4's and preferably 2x6's on edge with deck joist brackets holding them to the existing joists, leaving the on-edge new flooring joists "floating" in the brackets to avoid shrinkage and expansion forces on the roof joists, then put the plywood flooring or gapped 2x4's or 2x6's (better for airflow) crosswise across those joists for your supporting surface - this leaves a healthy air gap under the flooring at least, and if gapped boards like a deck, continuous air flow all along - much better. You can use cheap economy 2x4's for the decking to reduce cost. Some people go the other route, use plywood but drill 1/2" holes every 6 inches or so both ways all over it for ventilation. I also recommend not nailing down the flooring, or screwing down in only a few places so it does not flop around, so it can easily be taken up to inspect underneath for mold, and to do any necessary electrical or HVAC duct repairs. Do NOT cover over bathroom or kitchen fans - block out for them and their ducting. Check manufacturer instructions on required airgap around them. Ditto for recessed can lighting and trougher flourescent fixtures - you have to leave specified air gaps and free airflow over them - if covered over, put in a LABELLED access square. It also helps to use paint or HEAVY felt tip marker to indicate where underlying ducting and electrical runs, and any junction boxes are required by code to have a labelled access panel too.

4) vermin - make sure your eave screening is intact, so you do not create a nesting site for insects or rodents. If you want double protection, you can use metal window screening to enclose the storage area with a screen wall. I also recommend putting bug screening under your ridge vent if it is the louvered rather than mesh or screened type, to block insect access.

5) if you enclose the storage area with walls, make sure it has a ceiling BELOW the rafters and is ventilated - do not take the walls all the way up to the roof sheathing, as you need to maintain the air space between rafters from eaves to ridge vent, and to avoid trapping roof heat btween your storage area and the roof sheathing.

6) if you have gable vents on the ends of the house instead of ridge vents, do not block more than half the width of the area, or you will risk moisture buildup because the air willnot be able to flow from end to end of the attic between the gable vents. I would really recommned adding ridge vents if your storage area will be taking up much of the attic space or coming within 2 feet of the roof, to ensure ventilation.

As you can see, what starts as a very simple concept can get complex in a hurry, but do it wrong and you can cause some real problems.

One cheap alternative if just stashing a few pieces of light furniture like chairs and some boxes is to put no solid flooring at all - just some 2x4's or 2x6's on edge on joist brackets fastened to the existing joists, then greenhouse/privacy lattice to support the boxes - like this -

just laid over your new stringers - will supoort light weights, and does not block airflow. You can put the stringers and lattice in about a 4 foot strip lengthwise down the attic on both sides of a central 3-4' aisle to get to them, which gives you access and also keeps them away from the airflow from the eaves.

A hint - when building this, it is commonly easier to remove a gable vent, if you have them, for materials access, rather than trying to twist and maneuver pieces up through a ceiling hatch.

And watch out for stepping on the drywall ceilings - you can go right through, or pop it off its nails and cause a lot of pricey damage REAL QUICK, and avoid stepping on electrical wires and HVAC ducting.

I would suggest you start using your storage space gradually - don't put a lot of stuff up there or anything of any value until you see if you are going to have moisture problems that could damage the material - attics are really not a very heirloom-friendly place.

Answered 6 years ago by LCD

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